ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-3573-2017HEPPA-II model–measurement intercomparison project: EPP indirect effects during the
dynamically perturbed NH winter 2008–2009FunkeBerndbernd@iaa.eshttps://orcid.org/0000-0003-0462-4702BallWilliamhttps://orcid.org/0000-0002-1005-3670BenderStefanhttps://orcid.org/0000-0002-8728-053XGardiniAngelaHarveyV. LynnLambertAlynLópez-PuertasManuelhttps://orcid.org/0000-0003-2941-7734MarshDaniel R.MeranerKatharinahttps://orcid.org/0000-0002-5316-4987NiederHolgerPäivärintaSanna-MariPérotKristellhttps://orcid.org/0000-0002-4267-8560RandallCora E.ReddmannThomasRozanovEugenehttps://orcid.org/0000-0003-0479-4488SchmidtHaukehttps://orcid.org/0000-0001-7977-5041SeppäläAnnikahttps://orcid.org/0000-0002-5028-8220SinnhuberMiriamhttps://orcid.org/0000-0002-3527-9051SukhodolovTimofeiStillerGabriele P.https://orcid.org/0000-0003-2883-6873TsvetkovaNatalia D.VerronenPekka T.https://orcid.org/0000-0002-3479-9071VersickStefanvon ClarmannThomasWalkerKaley A.https://orcid.org/0000-0003-3420-9454YushkovVladimirInstituto de Astrofísica de Andalucía, CSIC, Apdo. 3004, 18008 Granada, SpainPhysikalisch-Meteorologisches Observatorium, World Radiation Center, Davos, SwitzerlandEarth Observation Unit, Finnish Meteorological Institute, Helsinki, FinlandKarlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research (IMK-ASF), P.O. Box 3640, 76021 Karlsruhe, GermanyLaboratory for Atmospheric and Space Physics, University of Colorado, Boulder, USAJet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USANational Center for Atmospheric Research, Boulder, Colorado, USAMax Planck Institute for Meteorology, Hamburg, GermanyDepartment of Physics, University of Helsinki, Helsinki, FinlandChalmers University of Technology, Göteborg, SwedenInstitute for Atmospheric and Climate Science ETH, Zurich, SwitzerlandCentral Aerological Observatory, Moscow, RussiaDepartment of Physics, University of Toronto, Toronto, Ontario, CanadaKarlsruhe Institute of Technology (KIT), Steinbuch Centre for Computing (SCC), Karlsruhe, GermanyBernd Funke (bernd@iaa.es)14March2017175357336042November20169December201614February201722February2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/3573/2017/acp-17-3573-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/3573/2017/acp-17-3573-2017.pdf
We compare simulations from three high-top (with upper lid
above 120 km) and five medium-top (with upper lid around 80 km) atmospheric
models with observations of odd nitrogen
(NOx=NO + NO2), temperature, and carbon
monoxide from seven satellite instruments (ACE-FTS on SciSat, GOMOS, MIPAS,
and SCIAMACHY on Envisat, MLS on Aura, SABER on TIMED, and SMR on Odin)
during the Northern Hemisphere (NH) polar winter 2008/2009. The models
included in the comparison are the 3-D chemistry transport model 3dCTM, the
ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, FinROSE, the Hamburg Model
of the Neutral and Ionized Atmosphere (HAMMONIA), the Karlsruhe Simulation
Model of the Middle Atmosphere (KASIMA), the modelling tools for SOlar Climate
Ozone Links studies (SOCOL and CAO-SOCOL), and the Whole Atmosphere Community
Climate Model (WACCM4). The comparison focuses on the energetic particle
precipitation (EPP) indirect effect, that is, the polar winter descent of
NOx largely produced by EPP in the mesosphere and lower thermosphere.
A particular emphasis is given to the impact of the sudden stratospheric
warming (SSW) in January 2009 and the subsequent elevated stratopause (ES)
event associated with enhanced descent of mesospheric air. The chemistry
climate model simulations have been nudged toward reanalysis data in the
troposphere and stratosphere while being unconstrained above. An odd nitrogen
upper boundary condition obtained from MIPAS observations has further been
applied to medium-top models. Most models provide a good representation of
the mesospheric tracer descent in general, and the EPP indirect effect in
particular, during the unperturbed (pre-SSW) period of the NH winter
2008/2009. The observed NOx descent into the lower mesosphere and
stratosphere is generally reproduced within 20 %. Larger discrepancies of a
few model simulations could be traced back either to the impact of the
models' gravity wave drag scheme on the polar wintertime meridional
circulation or to a combination of prescribed NOx mixing ratio at the
uppermost model layer and low vertical resolution. In March–April, after the
ES event, however, modelled mesospheric and stratospheric NOx
distributions deviate significantly from the observations. The too-fast and
early downward propagation of the NOx tongue, encountered in most
simulations, coincides with a temperature high bias in the lower mesosphere
(0.2–0.05 hPa), likely caused by an overestimation of descent
velocities. In contrast, upper-mesospheric temperatures (at
0.05–0.001 hPa) are generally underestimated by the high-top models after
the onset of the ES event, being indicative for too-slow descent and hence
too-low NOx fluxes. As a consequence, the magnitude of the simulated
NOx tongue is generally underestimated by these models. Descending
NOx amounts simulated with medium-top models are on average closer to
the observations but show a large spread of up to several hundred percent.
This is primarily attributed to the different vertical model domains in which
the NOx upper boundary condition is applied. In general, the
intercomparison demonstrates the ability of state-of-the-art atmospheric
models to reproduce the EPP indirect effect in dynamically and
geomagnetically quiescent NH winter conditions. The encountered differences
between observed and simulated NOx, CO, and temperature distributions
during the perturbed phase of the 2009 NH winter, however, emphasize the need
for model improvements in the dynamical representation of elevated
stratopause events in order to allow for a better description of the EPP
indirect effect under these particular conditions.
Introduction
The potential impact of energetic particle precipitation (EPP) on regional
climate is nowadays becoming recognized. Solar forcing recommendations for
the recently launched Climate Model Intercomparison Project Phase 6
include, for the first time, the consideration of
energetic particle effects . EPP is strongly linked to
solar activity and hence to the solar cycle, either directly by coronal mass
ejections producing sporadically large fluxes of solar energetic particles or
indirectly by the quasi-continuous impact of the solar wind on the Earth's
magnetosphere. In the mesosphere and lower thermosphere (MLT), EPP-induced
ionization initiates the production of odd nitrogen and odd hydrogen (the
latter below ∼ 85 km), both of them destroying ozone via catalytic
cycles. Odd nitrogen (NOx=NO + NO2) is long-lived
during polar winter and is then regularly transported down from its source
region into the stratosphere to altitudes well below 30 km
e.g.. This so-called EPP indirect effect
contributes significant amounts of NOx to the polar middle atmosphere
during each winter. EPP-induced ozone changes are thought to modify the
thermal structure and winds in the stratosphere which, in turn, modulate the
strength of the Arctic polar vortex. The introduced signal could then
propagate down to the surface, introducing significant variations of regional
climate, particularly in the Northern Hemisphere (NH)
.
At present, many chemistry climate models account for EPP-induced ionization
and its chemical impact on the neutral atmosphere, which is required for the
simulation of atmospheric EPP effects and ultimately for the investigation of
potential EPP–climate links. A comprehensive evaluation of these models'
capacity to reproduce observed EPP effects by means of coordinated
intercomparison studies is a necessary step towards this goal. The High
Energy Particle Precipitation in the Atmosphere (HEPPA) model vs. data
intercomparison initiative evaluated the chemical response
of nitrogen and chlorine species in nine atmospheric models to the
“Halloween” solar proton event in late October 2003 with observations taken
by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on
Envisat. Reasonable agreement of observed and modelled reactive nitrogen and
ozone changes was found, demonstrating the models' overall ability to
reproduce the direct EPP effect by solar protons. However, most models failed
to adequately describe the repartitioning of nitrogen compounds in the
aftermath of the event which could be attributed to deficiencies in the
representation of the D-region ion chemistry and motivated recent model
developments .
The observation-based evaluation of the simulated atmospheric effects of
magnetospheric particles, which are thought to be of higher relevance for
climate, is more challenging because of the quasi-continuous flux of
electrons compared to protons and the difficulty in separating between local
production and downward transport of NOx during polar winter.
Although a pronounced dependence of reactive nitrogen enhancements in the
polar winter stratosphere and mesosphere on the geomagnetic activity levels
has been demonstrated , dynamical variability, particularly
in the NH, can mask out this effect. In particular, the occurrence of
elevated stratopause (ES) events following sudden stratospheric warmings
(SSWs) during Arctic winters often causes much larger mesospheric NOx
enhancements than expected from the actual geomagnetic activity level, after
a brief NOx depletion related to the weakened vortex during the SSW.
The ability of climate models to adequately simulate tracer transport in
Arctic winters, including perturbed winters characterized by SSW and ES events,
is therefore crucial to accurately model EPP effects and their possible NH
regional climate impacts.
Simulations of mesospheric tracer descent during dynamically perturbed NH
winters have been compared with observations in several studies. Using the
KArlsruhe SImulation Model of the middle Atmosphere (KASIMA) with specified
dynamics below 48 km and prescribed NOx concentrations from MIPAS
night-time NO2 observations above 55 km,
calculated the amount of EPP-NOx entering the stratosphere from July
2002 to March 2004. KASIMA reproduced the MIPAS observations of NOx
entering the stratosphere reasonably well, even during the SSW winter
2003/2004. However, the ability of the model to adequately simulate
mesospheric tracer transport could not be tested because of the constrained
NOx in the mesosphere. and
, in turn, used FinROSE with constrained NOx
at the upper boundary (∼ 80 km) for both early 2009 and 2012. Their
results show that FinROSE is able to qualitatively reproduce the downward
descent of NOx from the MLT region into the stratosphere, but the
actual NOx amounts can differ significantly from the observations. In
the case of chemical transport models (CTMs), the results are strongly affected by the meteorological
data, i.e. a source of uncertainty, used to drive the model.
used a version of the Canadian Middle Atmosphere Model
(CMAM) that was nudged toward reanalysis data up to 1 hPa to examine the
impacts of parameterized orographic and non-orographic gravity wave drag (GWD)
on the zonal mean circulation of the mesosphere during the perturbed NH
winters 2006 and 2009 in comparison with temperature and carbon monoxide (CO)
observations from the Microwave Limb Sounder (MLS) on Aura. They found that
non-orographic GWD is primarily responsible for driving the circulation that
results in the descent of CO from the thermosphere following the warmings.
investigated the NOx descent during the Arctic
winter/spring of 2004 with Whole Atmosphere Community Climate Model (WACCM)
simulations that were nudged to Modern-Era Retrospective Analysis for
Research and Applications (MERRA) data. They found that their simulated
NOx, although qualitatively reproducing the enhanced descent after
the ES event, was up to a factor of 5 too low compared with satellite
observations. This underestimation was attributed to missing NO production by
high-energy electrons in the mesosphere in combination with an
underestimation of mesospheric descent during the recovery phase after the
SSW. compared simulations of mesospheric tracer descent
in the winter and spring of 2009 with two versions of WACCM, one constrained
with data from MERRA, which extends up to 50 km, and the other constrained to
the Navy Operational Global Atmospheric Prediction System-Advanced Level
Physics High Altitude (NOGAPS-ALPHA), which extends up to 92 km. By
comparison with Solar Occultation for Ice Experiment (SOFIE) data they showed
that constraining WACCM to NOGAPS-ALPHA yields a dramatic improvement in the
simulated descent of enhanced NOx and very low methane.
Most of these studies suggest that the model representation of the perturbed
dynamics during NH winters with SSWs and ES events has a crucial impact on
the simulated amount of NOx transported into the stratosphere and
that a proper parameterization of unresolved GWD is key to achieving agreement
with observations. However, previous studies focused on individual models,
making it difficult to assess the overall ability of state-of-the-art
atmospheric models to reproduce the EPP indirect effect in NH winters.
Comprehensive multi-model intercomparisons addressing dynamically perturbed
NH winters, however, have so far been restricted to the assessment of the
temperature zonal mean, planetary wave, and tidal variability during the 2009
SSW event in the middle and upper atmosphere , as well
as to the impacts on the ionosphere variability .
Further, although our knowledge of temperature and tracer distributions in
polar winters has dramatically increased with the advent of atmospheric
satellite observations, specific intercomparisons and validation efforts
focussing on such conditions are sparse. A systematic assessment of this
knowledge is therefore essential to quantitatively diagnose the model
performance with respect to mesospheric tracer transport under perturbed (and
unperturbed) polar winter conditions.
A coordinated intercomparison project focussing on tracer descent and the EPP
indirect effect during such a winter was therefore initiated in the frame of
the SPARC/WCRP's SOLARIS-HEPPA activity. In this so-called HEPPA-II project,
simulations of the NH polar winter 2008/2009 from eight atmospheric models
have been compared with observations of temperature and concentrations of
NOx and CO from seven satellite instruments including the Atmospheric
Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) on SciSat, the
Envisat instruments Global Ozone Monitoring by Occultation of Stars (GOMOS),
MIPAS, and the SCanning Imaging Absorption spectroMeter for Atmospheric
CHartographY (SCIAMACHY), as well as MLS on Aura, the Sounding of the
Atmosphere using Broadband Emission Radiometry (SABER) instrument on the
Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED)
satellite, and the Sub-Millimetre Radiometer (SMR) on Odin. The 2008/2009
winter was chosen for this intercomparison exercise not only because of its
peculiar dynamical conditions, characterized by the pronounced SSW in January
and the unusually strong descent of odd nitrogen despite the low geomagnetic
activity level around solar minimum, but also because of the availability of
a large number of observations from different satellite instruments that
allowed for a detailed evaluation of the model simulations. The models
included in the comparison are the 3-D chemistry transport model (3dCTM), the
ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, FinROSE, the Hamburg Model
of the Neutral and Ionized Atmosphere (HAMMONIA), KASIMA, the modelling tools
for SOlar Climate Ozone Links studies (SOCOL and CAO-SOCOL), and WACCM
(Version 4). Only three of these models (3dCTM, HAMMONIA, and WACCM) extend
up into the lower thermosphere where a large fraction of EPP-induced odd
nitrogen production occurs. All other models have their upper lid in the
mesosphere and require an odd nitrogen upper boundary condition (UBC), accounting
for EPP production higher up, for simulating the introduced EPP indirect
effect in the model domain. This UBC has been
constructed from NOx observations of the MIPAS instrument taken
during the Arctic winter 2008–2009. The chemistry climate model simulations
have been nudged toward reanalysis data below 1 hPa while being
unconstrained above. The restriction of specified dynamics to the
stratosphere is a compromise that is hoped to provide a realistic evolution
of mesospheric meteorology by upward control, while still allowing for the
assessment of self-generated tracer descent in the models.
In this study we report results from the HEPPA-II intercomparison project. A
major aim is the identification and characterization of model biases and
their uncertainties in the simulations of the perturbed 2008/2009 NH winter
by systematically comparing to the suite of satellite observations. For this
purpose, common diagnostics are applied in all comparisons, and the sampling
characteristics of the instruments are taken into account. Since the study
focusses on the evaluation of the ability of the models to simulate the
source and transport of MLT tracers by means of observed quantities (i.e.
temperature and trace gas abundances), any more sophisticated analysis, e.g.
qualifying the different GW drag parameterizations, is outside the scope
of this comparison. However, our analysis should motivate such
studies to identify the deficits in key processes of this vertical coupling.
The paper is organized as follows: Sect. gives an
overview on the satellite observations and data products used in this study.
Section describes the participating chemistry climate and
transport models. The NOx UBC employed in the medium-top models is
described in Sect. , and Sect. introduces
the intercomparison method. Results of the intercomparisons are discussed in
Sect. with focus on the representation of the EPP indirect
effect by the high-top models in the upper mesosphere and lower thermosphere
and, in Sect. , with focus on the upper-stratospheric and
mesospheric representation in all models.
Satellite observationsACE-FTS/SciSat
The ACE-FTS has performed infrared solar
occultation measurements from the SciSat satellite since February 2004
. The SciSat satellite is in a highly inclined circular
orbit (74∘) and thus provides measurements from 85∘ N to
85∘ S over each year with a significant focus on polar
measurements. Up to 30 measurements are made each day by ACE-FTS, extending
from the cloud tops to ∼ 150 km. ACE-FTS observations of temperature,
CO, and NOx during Arctic ES winters have been analysed in several
previous studies e.g.. Here, version 3.0
of the ACE-FTS dataset was used, which covers 21 February 2004 to 30
September 2010. The ACE-FTS retrieval algorithm is described in
and the specific details of version 3.0/3.5 are provided
in . NOx is provided from ACE-FTS using the
retrieved NO (6–107 km) and NO2 (7–52 km) profiles. Above
52 km, where both sunset and sunrise NO2 concentrations are very
small and hence not detectable, the scaled a priori NO2 profile has
been used to extend the NOx profiles to the higher altitudes. The
CO profiles extend from 5 to 110 km and temperature is retrieved from
15 to 125 km. The vertical resolution of the ACE-FTS measurements is
∼ 3 km, based on the instrument field of view .
The version 3.5 NO profiles differ from HALOE by -15 to +6 % between 27
and 53 km and from summertime MIPAS measurements by -9 to +2 % between
36 and 52 km . For NO2, the bias found between
ACE-FTS and a suite of other limb and occultation sounders is better than
18 % from 17 to 27 km and -15 % from 28 to 41 km .
For both of these species, a box model was used to apply a diurnal scaling to
the ACE-FTS profiles before the comparisons. ACE-FTS CO has been
compared with MIPAS and MLS by . On average, there is a
-11 % bias between 28 and 50 km with respect to MIPAS and a bias of
±10 %. Based on comparisons with coincident satellite observations
(within 350 km and 3 h), it has been found that ACE-FTS v3.5 temperatures
agree to within ±2 K between 15 and 40 km, within ±7 K between 40
and 80 km, and within ±12 K between 80 and 100 km (P. Sheese, personal
communication, 2016).
GOMOS/Envisat
GOMOS was a stellar occultation instrument on the polar orbiting Envisat
satellite, operating between 2002 and 2012 . This satellite
has been flying in a sun-synchronous orbit at approximately 800 km altitude.
GOMOS consisted of a UV–visible (VIS) spectrometer, two IR channels, and two
photometers, measuring the stellar flux through the atmosphere at high
sampling frequency. GOMOS measured vertical profiles of O3,
NO2, NO3, H2, O, O2, and aerosols in
the middle atmosphere. Here, we have used GOMOS NO2 profiles (version
GOPR_6.0c_6.0f) measured in night-time conditions (solar zenith angle at
tangent point location >107∘; solar zenith angle at spacecraft
location >90∘ to avoid stray light). The altitude range for
NO2 in non-polar conditions is 20–50 km and extends up to 70 km in
polar winter when enhanced amounts of NO2 are present in the
atmosphere . The typical precision
of the NO2 measurements is 5–20 % while the systematic error of
the NO2 observations is estimated to be of the order of few percent
(1–5 %) . Vertical resolution is 4 km
. As NO is quickly converted into NO2
by reaction with O3 after sunset, the night-time GOMOS NO2
measurements used here are a reasonable representation of stratospheric and
lower mesospheric NOx.
Because stars are used as the light source, the locations of the observations
change with time. A representative distribution of the latitudes sampled
during the course of a year can be seen in Figs. 7–9 of
. Due to this sampling, for the NH polar region in winter
2008–2009, GOMOS night-time NO2 observations were available for the
period of December 2008–January 2009. GOMOS measurements provide the
constituent profiles as number densities. For the purpose of this study these
were converted to volume mixing ratios (VMRs) using temperature and pressure
profiles from the WACCM model (see below).
MIPAS/Envisat
The MIPAS instrument on Envisat provided global
stratospheric and mesospheric measurements of temperature
, NO and NO2,
CO, as well as numerous other trace species
e.g. during 2002–2012. Here, we use
observations taken in the nearly continuous nominal observation mode
(scanning range 6–70 km, hereinafter referred to as MIPAS-NOM), as well as
occasional special mode observations (middle and upper-atmospheric
observation modes covering 20–100 and 40–170 km, respectively, hereinafter
referred to as MIPAS-UA), the latter taken with a frequency of about 1 out of
5 days. We also use special mode UA observations which include three orbits
per day passing the 20∘ W–70∘ E and
160∘ E–110∘ W sectors during 14–18 and 21–27 January
2009 and which were taken as support for the Dynamics and Energetics of the
Lower Thermosphere in Aurora 2 (DELTA-2) campaign .
MIPAS-NOM NOx data have been built from NO and NO2
data versions V5r_NO_220 and V5r_NO2_220, respectively. MIPAS-UA
NOx data are based on data versions V4o_NO_501/611 and
V4o_NO2_501/600. In the middle- to high-latitude polar winters, typical
vertical resolutions are 4–6 km below 50 km and 6–9 km above, while the
single measurement precision is on the order of 5–15 %. Systematic errors,
dominated by non-local thermodynamic equilibrium (non-LTE) uncertainties of NO and NO2, have
been estimated to be less than 10 %. CO data (version V5r_CO_220)
used here have a single measurement precision ranging from 20–30 % above
45 km to 70–80 % in the lower stratosphere. The vertical resolution is
6–12 km. The single measurement precision of temperature data (versions
v5r_T_220 and v5r_T_521/621 for MIPAS-NOM and MIPAS-UA, respectively) is
0.5–2 K below 70 km and 2–7 K above. The systematic error is typically
1–3 K below 85 km and 3–11 K above. The average vertical resolution is
3–6 km below 90 km and 6–10 km above.
MLS/Aura
The MLS instrument was launched
on 15 July 2004 and measures thermal microwave emission from Earth's limb. On
each day MLS provides ∼ 3500 vertical profiles of temperature and trace
gases between 82∘ S and 82∘ N spaced
∼ 1.5∘ apart along great circles following the orbit track.
employed MLS data version 3.2 to analyse tracer transport
during the Arctic ES winter 2006. Here, we use version 4.2 temperature and
CO. Temperature is deemed useful for scientific studies between 316 and
0.001 hPa. The vertical resolution is 5 km near 40 km and increases to
∼ 10 km near 90 km . In the mesosphere, systematic
and random errors are 2.5 K and comparisons with correlative measurements
show a 0–7 K cold bias . CO is recommended for
scientific use from 215 to 0.0046 hPa . The vertical
resolution is 4–5 km in the stratosphere and 6–7 km in the mesosphere.
indicate that the CO data have a 25–50 % positive
bias in the mesosphere. Estimates of absolute accuracy are 10 %
. For this work, temperature and CO data have been
filtered using the precision, status, quality, and convergence values
provided by the MLS science team .
SABER/TIMED
The SABER instrument is a 10 channel limb scanning radiometer
, launched in December 2001 on board the NASA TIMED
mission. SABER was measuring in the “northward” viewing mode
(83∘ N to 52∘ S) during the subperiods: 1 October–17
November in 2008; 11 January–15 March and 18–31 May in 2009. The rest of
the days, i.e. 17 November 2008–15 January 2009 and 15 March–19 May in
2009, it was observing in the “southward” viewing mode (52∘ N to
83∘ S). There is a rich literature dealing with the analysis of
SABER temperature version 1.07 in the context of NH
polar winter dynamics e.g.. Our study
uses data from the Level 2A files of version 2.0. Typical single measurement
random errors are < 0.5 K below 55 km, 1–2 K in the mesosphere, and
∼ 7 K above. The systematic errors are < 1.5 K below 75 km, 4 K at
85 km, and 5 K at 100 km . The
vertical resolution is about 2 km. A thorough comparison of these
temperatures with those measured by other satellites, MIPAS, ACE-FTS, MLS,
OSIRIS, SOFIE, and by lidar measurements has been recently carried out by
in a study about the validation of MIPAS vM21
temperatures. The comparison of SABER v2.0 with MIPAS vM21 is remarkably
good, with differences smaller than 2 K at all altitudes and seasons, except
for high-latitude summers above 65 km where they are 3–4 K at 65–80 km
(MIPAS colder) and 5–7 K around the mesopause (MIPAS warmer).
SMR/Odin
The SMR instrument is a limb emission sounder
aboard Odin, a Swedish-led satellite launched in 2001 in cooperation with the
Canadian, French, and Finnish space agencies . Odin is
orbiting the Earth in a sun-synchronous orbit at an initial altitude of
580 km and at Equator-crossing times varying between 06:00 and
07:00 local time for the morning overpass (18:00 and 19:00 local time fore the evening overpass). These parameters are slightly changing with time due to the
drifting orbit. SMR is measuring globally a variety of trace gases and the
temperature from the upper troposphere to the lower thermosphere
.
Nitric oxide is retrieved from the observation of thermal emission lines in a
band centred around 551.7 GHz. The version 2.1 of NO data is used in this
study. The overall vertical coverage is from 7 to 115 km, and in the
altitude range considered here the vertical resolution is about 7 km
. NO data are available approximately 4 days per month
after 2007, on an irregular basis of 2 observation days in a 14-day cycle.
Systematic errors amount to 3 % from spectroscopic parameters, 2 % from
calibration, and 3–6 % from sideband suppression . The
single measurement retrieval error amounts to 44–48 %, in the case of
Antarctic night-time mesosphere–lower thermosphere, as studied by
. A comparison study performed by showed
that SMR NO measurements were consistent with NO measurements by SCIAMACHY,
MIPAS, and ACE-FTS despite the different measurement methods and retrieval
strategies used for these four instruments.
SCIAMACHY/Envisat
The SCIAMACHY
see is a limb-sounding
UV–VIS–NIR spectrometer on Envisat. Among the main measurement modes, the
nominal limb mode carried out limb measurements from ground to 105 km until
mid-October 2003, and after 15 October 2003 up to 91 km. From July 2008
until April 2012, SCIAMACHY carried out a special mesosphere–lower
thermosphere mode (MLT), scanning from 50 to 150 km for 1 day every 2
weeks.
Nitric oxide is retrieved from the NO gamma bands (UV channel 1,
230–314 nm) in the 60–160 km range using a
tomographic approach. The retrieval from the MLT mode yields the NO number
densities with a vertical resolution of 5–10 km between 70 and 150 km.
With the nominal mode, the same resolution is achieved between 65 and 80 km.
The average single orbit measurement error amounts to about 30 %.
Systematic errors amount to 7 % from uncertain spectroscopic data, 3 %
from uncertainties in the solar spectrum , and about 10 %
from temperature uncertainties. Because the NO gamma bands are fluorescent
emissions, the retrieval of NO is restricted to daylight observations. Polar
winter data are therefore restricted to latitudes equatorward of the polar
night terminator (around 70∘ in the mesosphere–lower thermosphere at
winter solstice).
The retrieved NO number densities from the MLT mode have been compared to
ACE-FTS, MIPAS, and SMR . The measurements were found to be
consistent among all instruments with SCIAMACHY retrieving slightly lower
densities compared to the other instruments during polar winter but higher
values in mesospheric polar summer and mid-to-low latitudes.
Chemistry climate models
In the following, the participating atmospheric models are described and
details on the set-up of the simulations are provided. Since the dynamical
evolution in the mesosphere is strongly constrained by the behaviour of the
lower atmosphere, particularly during a perturbed NH winter, model
simulations have been either nudged to or rely entirely on meteorological
reanalysis data in order to allow for comparisons to observations. High-top
models, having their upper lid above 120 km and including explicit schemes
for consideration of NOx production by particle-induced ionization,
are described in Sect. . Medium-top models, having their
upper lid around 80 km, are described in Sect. . These
models applied a common odd nitrogen UBC in order to account for EPP
production above the model domain (see Sect. ). A summary of the
different model settings and characteristics is given in
Table .
High-top models3dCTM
3dCTM is a global 3-D chemistry transport model developed based on
the chemistry scheme of the SLIMCAT model and the
transport scheme of the CTM-B for use in the middle
atmosphere up to the lower thermosphere. Temperature as well as horizontal
and vertical wind fields are prescribed by data from the LIMA general
circulation model , and the model upper boundary is defined
by the availability of these data. For the version used here, LIMA is nudged
to (1∘× 1∘) ECMWF operational data with a constant
nudging of temperature, zonal and meridional winds between the surface and
35 km, and a linear decrease in nudging strength to 45 km, the upper limit
of the nudging area. No parameterization of the GWD is
implemented either in LIMA or in 3dCTM. Only waves with horizontal scales of
≥ 500 km and a temporal resolution of 2–12 h are represented
. A comparison of momentum flux climatologies provided in
Fig. 7 of with common GWD schemes as shown,
e.g. in Fig. 5 of , shows that the gravity wave momentum
flux in the mesosphere is underestimated by LIMA by about a factor of 2–3 in
both the summer and winter hemisphere. In the winter hemisphere, the
vertical structure of the GW momentum flux is also somehow different; while
essentially show one broad peak at ∼ 65–95 km
altitude, varying in strength from -80 to 120 ms-1 d-1, the LIMA
profile shows a double peak structure with a broad peak of
-40–60 ms-1 d-1 at ∼ 70–90 km altitude, a minimum in
90–100 km, and a secondary peak above 100 km. This means that the vertical
downward motion throughout the mesosphere will be underestimated during
winter.
The model chemistry scheme has been adapted from the original SLIMCAT code
for use in the mesosphere and lower thermosphere as described in
: the model considers the photolysis of O2,
CO2, CH4, and H2O in the far-UV wavelength range down
to the Lyman α line. Also, in the mesosphere and lower thermosphere,
chemical families are not considered for NOx and Ox species,
and H2O, O2, and H2 are now integrated as active
chemical species in the model. Additionally, parameterizations for the impact
of atmospheric ionization from particle impact and photoionization are
considered based on ion-chemistry model studies . The
photoionization rate is based on the parameterization of
; particle impact ionization rates are prescribed using
the four-dimensional field provided by the AIMOS model
version 1.2. Model data are output every 15 min and interpolated onto the
satellite geolocations from this.
Summarized description of the atmospheric models
involved in this study.
a See model descriptions in Sect. for details.b S11: ; S03: ; S06:
HAMMONIA
HAMMONIA is an
upward extension of the ECHAM5 atmospheric general circulation model
. The model's dynamics and radiation are fully coupled to
the chemical Model of Ozone and Related Tracers
MOZART,. A detailed description of the model is given
by . To simulate the effects of EPP, HAMMONIA is modified
to incorporate the ion chemistry of the E and F region as described in
and . The ion chemistry treats 5
ion–electron recombinations and 12 ion-neutral reactions including 50 neutral
and 6 charged (O+, O2+, N+, N2+,
NO+, e-) components. Additionally, five reactions
directly involving energetic particles are considered. The corresponding
reaction rates are calculated using the particle-induced ionization rates
provided by Atmospheric Ionization Module Osnabrück (AIMOS version 1.6)
. The explicit simulation of energetic particle effects on
chemistry is limited to above 10-3 hPa, whereas below this altitude the
production of N(2D), N(4S), and HOx is
parameterized following . Photochemistry includes six
reactions involving radiation at wavelengths shorter than Lyman-α.
Therefore the parameterization of and the observed
10.7 cm solar radio flux is used. Orographic gravity waves are parameterized
according to , while non-orographic gravity waves are
parameterized according to the Doppler-spread theory from . A
geographically uniform isotropic gravity wave source spectrum with a constant
root-mean-square (RMS) wave wind speed of 0.8 m s-1 launched at 830 hPa is
used. Additional to the homogeneous source of gravity waves, HAMMONIA
considers the generation of gravity waves from tropospheric fronts following
. At locations where frontogenesis occurs the gravity wave
spectrum is launched with an RMS wave wind speed of 2 m s-1 instead of
0.8 m s-1. A more detailed description of the gravity wave scheme used in
HAMMONIA is given in . Note also that this setting of the
gravity wave parameters differs from the simulation of the same winter
analysed in where the waves were launched at about 650 hPa
and no frontal sources were used. Sea surface temperature and sea ice
cover are taken from the Atmospheric Model Intercomparison Project 2 (AMIP2)
climatology. Output is provided every 2 h and afterwards interpolated to
the satellite geolocations. The model is nudged from 850 to 1 hPa with
an upper and lower transition zone to the 6-hourly values of the ERA-Interim
reanalysis data . The “spin-up” time is 1 year starting on
1 January 2008.
WACCM
For the simulations presented here, the NCAR Community Earth System Model
(http://www.cesm.ucar.edu/, ) is used with the
Whole Atmosphere Community Climate Model as its atmospheric component
(hereinafter referred to as WACCM4). The model is forced
with meteorological fields from the Modern Era Retrospective Analysis for
Research and Applications (MERRA), a NASA reanalysis using the Goddard Earth
Observing System Data Assimilation System Version 5 .
The forcing is achieved by relaxing temperature, zonal and meridional winds,
and surface pressure with a time constant of 50 h from the surface to
40 km. Above that level the forcing is reduced linearly, so that the model
is free-running between 50 km and the model top at approximately 140 km
(4.5 × 10-6 hPa). Heating rates and photolysis are calculated
using observed daily solar spectral irradiance based on the empirical model
of and geomagnetic activity effects in the auroral region
are parameterized in terms of the Kp index . The standard
WACCM chemistry is described and evaluated extensively in .
Reaction rates are from . For these simulations we have
modified the N+N2 reaction to include two additional
pathways as described in . It should be noted that both
WACCM and HAMMONIA use the same chemical solver based on the MOZART3
chemistry , include the same set of ionized species, and
use the parameterized EUV ionization rates from . For
these simulations the latter parameterization has been extended to include the
photoionization of CO2 in the EUV. Above 5 × 10-4 hPa
(∼ 100 km) ionization from electrons is calculated by the WACCM
parameterized aurora. It is assumed that 1.25 N atoms are produced per
ion pair and divide the N atom production between ground state,
N(4S), at 0.55 per ion pair and excited state, N(2D), at 0.7
per ion pair . This simulation followed the
“REFC1D” protocol of the Chemistry Climate Model Initiative
for the specification of time-dependent greenhouse gases
and ozone-depleting substances. WACCM constituent and temperature profiles
were saved at the model grid point and time step (model time step is
30 min) closest to each of the MIPAS observation locations. Eddy
diffusion created by the dissipation of parameterized gravity waves in WACCM
depends on the value assumed for the Prandtl number, Pr, which
describes the ratio of the eddy momentum flux to the eddy flux of potential
temperature or chemical species. In these simulations Pr=4, as in the
study of .
Medium-top modelsCAO-SOCOL
Since HEPPA-I the CCM SOCOL (modelling tool for studies
SOlar Climate Ozone Links) has been upgraded to version 3 with substantial
changes related to the advection of the species. These changes and the
detailed evaluation of the new version performance were documented by
. The CCM SOCOL v.3 consists of the MEZON chemistry
transport model and MA-ECHAM5, the middle atmosphere
version of the ECHAM general circulation model .
Dynamical and physical processes in SOCOL are calculated every 15 min
within the model, while full radiative and chemical calculations are
performed every 2 h. Chemical constituents are transported using a
flux-form semi-Lagrangian scheme , and the chemical solver
is based on a Newton–Raphson iterative method taking into account 41 chemical
species, 140 gas-phase reactions, 46 photolysis reactions, and 16
heterogeneous reactions. The CCM SOCOL v.3 was installed in CAO (Central
Aerological Observatory, Moscow, Russian Federation) and modified to use
assimilation of the meteorological fields from the ERA-I reanalysis, which is
necessary to reproduce the considered SSW and ES events in January 2009. The
model is nudged from 850 to 1 hPa using the
approach. Orographic gravity waves are parameterized according to
. Non-orographic gravity waves are parameterized using Hines
(1997) scheme implemented to ECHAM5 with a constant RMS wave
wind speed of 1.0 m s-1 introduced at 830 hPa for all geographical locations.
The daily mean NOx mixing ratio at 0.01 hPa from MIPAS measurements
(see Sect. ) was used as the UBC at the
uppermost model layer. The NOx mixing ratio was divided between
NO and NO2 according to their ratio in the model for any
particular time step at the second layer from the model top. Model output was
interpolated in time and space to the provided satellite geolocations.
EMAC
The EMAC model is a numerical chemistry
and climate simulation system that includes submodels describing
tropospheric and middle atmosphere processes and their interaction with
oceans, land, and human influences . It uses the second
version of the Modular Earth Submodel System (MESSy2) to link
multi-institutional computer codes. The core atmospheric model is the fifth-generation European Centre Hamburg general circulation model
ECHAM5,. For the present study we applied EMAC
(ECHAM5 version 5.3.02, MESSy version 2.50) in the T42L90MA resolution. The
model is nudged to ERA-Interim reanalysis data from the surface to 0.2 hPa
(with decreasing nudging strength in the transition region in the five levels
above) using the nudging coefficients suggested in . The
UBC for NOx is prescribed in the top four layers
(0.01 to 0.09 hPa) of the model. For gravity waves we used the submodel
GWAVE which contains the original Hines non-orographic gravity wave routines
from ECHAM5 in a modularized structure. We tuned the
parameter rmscon (RMS wind speed at bottom launch level of
642.9 hPa), which controls the dissipation of gravity waves, to
0.8 m s-1. For gas-phase reactions we used the submodel MECCA
and for photolysis the submodel JVAL .
Included were 110 gas-phase reactions and 44 photolysis reactions. The
NOx family was reduced to NO and NO2. The chemical
tracers were initialized from a multi-annual EMAC model run. Model output was
done for each time step (10 min) which afterwards was interpolated to the
satellite geolocations.
FinROSE
FinROSE is a global 3-D CTM (further developed model version of the
one described by ). The model dynamics for the whole
model domain is forced with external meteorological data, whereas the
vertical wind is calculated inside the model by using the continuity
equation. In this study FinROSE is nudged with ECMWF operational analysis
data. This means that changes in the atmospheric composition do not affect
the model dynamics, and gravity wave parameterization is included already in
the meteorological forcing data. FinROSE reproduces the distributions of 41
species from the stratosphere up to the mesosphere and lower thermosphere
and also includes about 120 homogeneous reactions and 30 photodissociation
processes. Photodissociation frequencies are calculated using a radiative
transfer model . In addition to homogeneous chemistry, the
model also includes heterogeneous chemistry, i.e. formation and
sedimentation of polar stratospheric clouds (PSCs) and reactions on PSCs. The
model is designed for middle atmospheric studies and thus the chemistry is
not defined in the troposphere, but the tropospheric abundances are given as
boundary conditions. For this study, the UBC for NOx (i.e.
NO + NO2) was implemented in the MLT region at about
0.03–0.01 hPa (the top two model layers). Output in the satellite
geolocations was composed already during the model run by finding the closest
model grid point and time step to every geolocation.
Upper panel: daily averaged NOx mixing ratios from satellite
observations (open squares) at 0.022 hPa within 60–90∘ N
(black is MIPAS-NOM, blue is MIPAS-UA, red is SMR/Odin,
green is ACE-FTS) and those of the upper boundary condition (filled
diamonds) sampled at the respective observations' time and location. Lower
panel: mean latitude averaged over all observations of the individual
instruments within 60–90∘ N. All averages are area-weighted.
KASIMA
The KASIMA model is a 3-D mechanistic model of the middle atmosphere including
full middle atmosphere chemistry . The model can be coupled
to specific meteorological situations by using analysed lower boundary
conditions and nudging terms for vorticity, divergence, and temperature. Here
the version used for the HEPPA-I experiment has been applied
but with a horizontal resolution of about
2.8∘× 2.8∘ (T42). The frequency of output is every
6 h. The model is nudged to ERA-Interim analyses below 1 hPa. A numerical
time step of 6 min was used in the experiments. The model uses a
Lindzen-type parameterization to include the effect of
breaking gravity waves, but no specific parameterization of orographic gravity
waves. Further details of the model are found in . The UBC
for NOx was set at the 0.3 hPa level, and not above. This
occasionally causes deviations between the observations and the model above
this level.
SOCOL
The applied version of the CCM SOCOL improves upon CAO-SOCOL and was
prepared for participation in the IGBP/SPARC CCMI project. The tropospheric
chemistry component was extended by adding the Mainz Isoprene Mechanism
(MIM-1), which comprises 16 organic species and a further 44 chemical
reactions . The cloud influence on photolysis rates was
introduced using a cloud modification factor . Interactive
lightning source of NOx was introduced following the
approach and adopting local scaling factors based on
satellite measurements. The kinetic constants and absorption cross sections
were updated following . The new parameterization of the
UV heating rates as well as NOx and
HOx production by energetic particles was
adopted. For HEPPA-II the model was run with T42 horizontal resolution, which
corresponds approximately to 2.8∘× 2.8∘, and 39
vertical levels between the ground and 0.01 hPa. The nudging set-up and UBC
for NOx are the same as in CAO-SOCOL.
Observed and modelled NOx VMRs of MIPAS and ACE (upper two
rows) and NO of SMR (lower row) in NH polar MLT region during November
2008–March 2009. Model output of the “high-top” models 3dCTM, HAMMONIA,
and WACCM has been sampled at the locations and times of the observations
(MPAS-UA, ACE-FTS, and SMR) for comparison. Pink lines indicate the observed
VMR levels of 0.1, 1, and 10 ppmv. White regions reflect missing or not
meaningful data.
NOx UBC for medium-top models
The UBC for NOx mixing ratio has been constructed from MIPAS-NOM
observation data versions v4o_NO_200 and v4o_NO2_200 by projecting
individual observations onto a regular grid in longitude, latitude, pressure
level, and time with daily cadence using a distance-weighting algorithm. All
observations taken within ±12 h time difference, ±10∘ latitude, and ±25∘ longitude have been considered at each grid
point (weighted by the inverse distance squared) and have been vertically
interpolated to a fixed pressure grid. Data gaps in space and time have been
filled by linear interpolation. Note that in the model–measurement
intercomparisons a newer version of MIPAS NOx is used, which was not
available when the UBC was generated prior to the model
runs. The horizontal resolution of the NOx UBC is
1.25∘× 2.5∘ (latitude × longitude).
Thirteen vertical pressure levels within 1–0.01 hPa are covered to allow
for interpolation to the respective upper lid of the models. The NOx
UBC has been evaluated by comparing with available satellite observations
(see Fig. ). To avoid sampling errors in the comparisons, the
UBC field has been sampled at the measurements' locations of each day before
averaging over the polar cap region. In general, there is very good agreement
(within 10–20 %) with independent NOx observations. However,
larger differences up to 20–50 % occur sporadically for observations close
to the vortex edge (e.g. when comparing to ACT-FTS at the end of February)
where horizontal gradients are very pronounced.
Intercomparison strategy
The discrete horizontal sampling of satellite observations can cause large
uncertainties in intercomparisons of observed and modelled averaged
quantities, particularly if the sampling is sparse, irregular, or variable in
time . To reduce the impact of sampling errors, we follow
the same approach that was successfully applied in the first HEPPA
intercomparison study : the model output has been sampled at
the locations and times of the individual observations and has been
vertically interpolated to the observed pressure levels. If available (i.e.
in the case of MIPAS and MLS), averaging kernels have been applied to the
model results as described in . Profiles have only been
considered in the vertical range where the instruments' sensitivity is high
enough to provide meaningful data; the remaining profile regions have been
excluded in both observations and model results.
Model–measurement comparisons were performed on basis of daily and/or
quasi-monthly averaged zonal mean data, which have been calculated in the
same way for both observations and simulations. For most comparisons, data
have been further binned within 70–90∘ N, applying area-conserving
(cos(θ)) weights. Note, however, that the sampled portion of this
latitude bin varies from instrument to instrument, making a direct comparison
of the observational results difficult. However, the comparison of model
biases with respect to different observational datasets is mostly unaffected.
The binning has been extended to 60–90∘ N in the comparisons to
ACE-FTS data in order to allow for evaluations prior to February 2009. We
recall that ACE-FTS has a discrete but time-varying latitude coverage (see
Fig. ) such that the resulting averages represent only a small
fraction of the entire bin.
Upper mesosphere and lower thermosphere
In this section NOx, CO, and temperature fields of the high-top
models 3dCTM, HAMMONIA, and WACCM are compared to the observations in the
MLT, the source region of odd nitrogen produced by EPP. Although, strictly
speaking, temperature is not a tracer of vertical motion, the adiabatic
warming during periods of strong descent introduces observable changes of the
thermal structure of this region which can be used as diagnostics of vertical
transport in the models. The simultaneous evaluation of modelled NOx,
CO, and temperature distributions allows then to attribute model biases to
deficiencies in the simulation of either particle-induced NOx
production or of dynamics.
Comparison of observed polar mid-winter NOx mean profiles
(thick black lines) a to 3dCTM (blue), HAMMONIA (green), and WACCM (red).
Right panel: ratio of model results and MIPAS-UA (solid), SMR/Odin (dashed),
and ACE-FTS (dotted) observations. The grey shaded area indicates the
±25 % range. Data have been averaged over 70–90∘ N and 5
December 2008–12 January 2009 (60–90∘ N and 5 November 2008–12
January 2009 in the case of ACE-FTS).
Figure shows the vertical distribution of NH polar NOx
over time in the simulations and MIPAS-UA, ACE-FTS, and ODIN-SMR observations
at 0.1 to 2×10-4 hPa. SCIAMACHY observations of NO
densities have not been included in this figure because NH polar observations
are only available after the beginning of February. Note that MIPAS-UA and
ACE-FTS provided NOx VMRs, while SMR observed
NO VMR only. This, however, introduces differences only below
approximately 0.01 hPa since NOx is entirely in the form of
NO above. The comparisons with the three instruments provide a
consistent picture of model biases. While WACCM and HAMMONIA reproduce the
observations fairly well during the whole time period in the upper mesosphere
and lower thermosphere (above the 0.01 hPa level), 3dCTM exhibits too small
NOx abundances in this vertical region. Below the 0.01 hPa level and
during the pre-SSW phase of the winter (November–January), WACCM and
HAMMONIA agree well with the observations while 3dCTM overestimates
NOx in this vertical region during most of the pre-SSW phase.
The SSW event starts with the breakdown of the polar vortex, and the dilution
of the mesospheric NOx by upwelling and increased horizontal
mixing. This is clearly observed by MIPAS and SMR as a decrease of
NOx between roughly 0.01 and 0.001 hPa. This initial NOx
decrease is captured well by WACCM and 3dCTM, though it is too weak in the
HAMMONIA simulation. The initial decrease of NOx during the SSW is
followed by strong downwelling of NOx leading to a pronounced
increase of mesospheric NOx and the development of the characteristic
NOx “tongue”. This is qualitatively captured by all models,
however, the amount of NOx transported into the lower mesosphere
(below 0.01 hPa) is significantly underestimated. The timing of the onset of
the enhanced descent varies considerably among the models and, compared to
the observations, occurs slightly too early in HAMMONIA and too late in
3dCTM. The onset of ES-related NOx increases in WACCM coincides with
the observed onset, however, the modelled increases appear to last for a
shorter time.
Comparison of observed polar mid-winter CO mean profiles (thick black
lines) to 3dCTM (blue), HAMMONIA (green), and WACCM (red). Right panel: ratio
of model results and MIPAS-UA (solid), MLS (dashed), and ACE-FTS (dotted)
observations. The grey shaded area indicates the ±25 % range. Data have
been averaged over 70–90∘ N and 5 December 2008–12 January 2009
(60–90∘ N and 5 November 2008–12 January 2009 in the case of
ACE-FTS).
Unperturbed early (pre-SSW) phase
In the following, the observed and modelled vertical structure of
NOx, CO, and temperature during mid-winter (pre-SSW phase) is
analysed in more detail to evaluate the models' ability to reproduce the EPP
indirect effect for unperturbed conditions. Figure compares
the observed and modelled NOx mid-winter mean profiles averaged over
70–90∘ N and 5 December 2008–15 January 2009 (60–90∘ N
and 5 November 2008–15 January 2009 in the case of ACE-FTS) above the
altitude of 0.05 hPa. The observed vertical structure of NOx is
reasonably well reproduced by HAMMONIA and WACCM during this period.
Differences with respect to the observations are mostly within 20–50 %,
with WACCM being overall more on the high side and HAMMONIA more on the low
side (particularly at altitudes below 0.002 hPa). As discussed earlier, the
3dCTM simulations show a much less pronounced vertical gradient resulting in
a significant (in terms of the observational spread) NOx
underestimation (up to a factor of 8) at altitudes above 10-2 hPa and
overestimation (up to a factor of 3) below. Figure compares
the corresponding mean profiles of CO, observed by MIPAS-UA, MLS, and ACE-FTS
above the altitude of 0.5 hPa. Again, WACCM and HAMMONIA show a vertical
gradient that is roughly in agreement with the observations. In contrast, the absolute CO values of WACCM are slightly (up to 40 %) higher
while HAMMONIA underestimates the CO abundances by a factor of 2–3. The
latter can be explained by missing thermospheric production mechanisms in the
model, specifically the CO2 photolysis in the extreme ultraviolet (at
wavelengths < 121 nm) and the reaction of CO2 with the atomic
oxygen ion , that act in addition to the photolysis of
CO2 in Lyman-alpha and the Schumann–Runge continuum. The 3dCTM
simulations, similarly as for NOx, show a gradient in the
mesosphere that is too weak compared to the observations, resulting in an underestimation
above 0.03 hPa and an overestimation below. The corresponding temperature
profiles (see Fig. ), observed by MIPAS-UA, MLS, and ACE-FTS
(note that SABER is not included because the observations in December cover
only up to 52∘ N) indicate good agreement with the observations for
HAMMONIA and a slight warm bias of 5–10 K for WACCM. Mesospheric 3dCTM
temperatures are systematically too cold by 10–30 K in the middle and lower
mesosphere.
Comparison of observed polar mid-winter temperature mean profiles
(thick black lines) to 3dCTM (blue), HAMMONIA (green), and WACCM (red). Right
panel: temperature difference of the simulations and MIPAS-UA (solid), MLS
(dashed), and ACE-FTS (dotted) observations. The grey shaded area indicates
the ±5 K range. Data have been averaged over 70–90∘ N and 5
December 2008–12 January 2009 (60–90∘ N and 5 November 2008–12
January 2009 in the case of ACE-FTS).
Comparison of observed NOx mean profiles (thick black lines)
for February 2009 (during the ES event) and 70–90∘ N to 3dCTM
(blue), HAMMONIA (green), and WACCM (red). Right panel: ratio of model
results and MIPAS-UA (solid), SMR/Odin (dashed), ACE-FTS (dotted), and
SCIAMACHY (dash-dotted) observations. The grey shaded area indicates the
±25 % range. Data have been averaged over 70–90∘ N and 1
February–1 March 2009 (60–90∘ N and 1 February 2008–15 March 2009
in the case of ACE-FTS).
The good overall agreement of NOx, CO, and temperature from HAMMONIA
and WACCM with the observations in December suggests that both NOx
sources and dynamical conditions are well represented by these models,
allowing for an adequate description of the EPP indirect effect in the MLT
during unperturbed conditions early in NH winters. Interestingly, the
consideration of ionization induced by mid-energy electron in HAMMONIA
(via AIMOS) does not introduce noticeable differences in the NO distribution
with respect to WACCM, the latter only accounting for auroral electrons. This
suggests that the impact of mid-energy electron during the solar minimum 2008/2009 NH winter
was rather small. 3dCTM simulations, in contrast, show significant
discrepancies with the observations. The similarity of the model bias in the
vertical gradients of NOx and CO suggests that these differences with
respect to the observations are due to the representation of dynamics in
3dCTM rather than to the EPP source. The vertical gradient of the 3dCTM CO
and NOx profiles both show values in the lower thermosphere that are too low
and values in the upper to mid-mesosphere that are too high. The underestimation of
lower thermospheric CO is likely due to the model chemistry as, like in
HAMMONIA, neither the EUV photolysis of CO2 nor the production of CO
by positive ion chemistry in the lower thermosphere are considered in 3dCTM.
The underestimation of thermospheric NOx could be caused by a too-weak NO production or too-fast transport out of the (polar) source region,
either by horizontal mixing or across the mesopause. The high values of both
CO and NOx in the mesosphere, however, are likely due to the
representation of mesospheric dynamics in 3dCTM, which is driven by
temperatures and wind fields from the LIMA model. A likely reason seems the
neglect of subscale (≤500 km) gravity waves in the LIMA model, leading
to an underestimation of the GW drag throughout the mesosphere but to an
overestimation in the lowermost thermosphere (see Sect. 2.7). This leads to a
suppression of vertical motion in the mesosphere, which is also reflected in a
negative bias in temperatures, and, consequently, to an accumulation of CO and
NOx.
Perturbed late (post-SSW) phase
Figure compares the observed and modelled NOx
February mean profiles corresponding to the perturbed post-SSW phase of this
winter, characterized by enhanced descent of NOx. This comparison
includes also SCIAMACHY NO density averages. Above 0.005 hPa, a larger
spread of model–measurement differences compared to December is found, likely
related to the enhanced spatial and temporal variability. On average,
however, these differences are very similar to those encountered during
mid-winter. Below 0.005 hPa, all models systematically underestimate the
observed NOx increases associated with the ES event by a factor of
2–3.
Temporal evolution of daily averaged polar cap temperatures at
4–0.0005 hPa from SABER observations and simulations of 3dCTM, HAMMONIA,
and WACCM (from top left to bottom right). The white contours correspond to
the observed temperatures of 220 and 240 K.
Top: temporal evolution of daily averaged polar cap temperatures at
4–0.0005 hPa observed by MIPAS-UA, MLS/Aura, ACE-FTS, and SABER (from left
to right). Bottom: corresponding differences between temperatures simulated
with the “high-top” models (3dCTM, HAMMONIA, and WACCM) and the
observations.
Adiabatic heating associated with the enhanced mesospheric descent is
responsible for the reformation of the stratopause at a pressure level as
high as 0.005 hPa. Figure shows the temporal evolution of the
vertical temperature structure at 70–90∘ N in January–March as
observed by SABER and simulated by 3dCTM (LIMA), HAMMONIA, and WACCM. We have
chosen this observational dataset for the comparison to the models because of
its full temporal coverage in this period and the high vertical resolution in
the entire vertical range. The observed elevated stratopause started to
develop at the beginning of February and remained at around 0.01 hPa for a
month before it descended to its climatological height in the course of
March. The highest stratopause temperatures during the elevated phase were
reached around 20 February. Although all models simulate an elevated
stratopause, its temporal evolution differs significantly from the
observations. HAMMONIA and WACCM show an ES onset and formation level similar
to the observed ones, but highest temperatures at this level are reached
immediately after the onset, about 20 days earlier than in the observations.
In both models, the ES level starts to descend immediately after its
formation, more quickly than observed and faster in HAMMONIA than in WACCM.
During the descent, the modelled stratopauses become increasingly warmer.
3dCTM, in contrast, simulates a much later onset (about 2 weeks after the
observed one) and the ES temperatures are much colder than in the
observations. However, the modelled ES remains at an elevated level for a
longer time (although slightly lower than the observed ES) and the time delay
until reaching the maximum ES temperatures is comparable to the observed
temperature evolution. These differences between 3dCTM on the one hand and
WACCM, HAMMONIA, and mostly also the observations on the other hand
highlight the role of subscale gravity waves for the temporal evolution of
the ES event. The onset of the SSW event is driven mainly by large-scale
planetary waves breaking down the horizontal circulation and is captured
comparatively well by all three models. However, the reformation of the
stratopause at upper-mesospheric altitudes is driven by small-scale gravity
waves reaching up to the upper mesosphere after the event. As these smaller
gravity waves are essentially missing in the LIMA data, the build-up of the
elevated stratopause is delayed in 3dCTM, and its strength is weaker.
To investigate whether the encountered differences between the models and
SABER data are robust with respect to instrumental uncertainties, we extend
the analysis to MIPAS-UA, ACE, and MLS temperature observations and compare
the model differences to all observations (see Fig. ). Despite
minor changes related to the different latitude range covered by the
instruments, the encountered model biases are consistent for all instruments,
indicating a too-cold mesosphere of 3dCTM and a dipole-type pattern in
HAMMONIA and, less pronounced, in WACCM with colder temperatures after the ES
onset in the upper mesosphere and warmer temperatures below.
A similar analysis of NH polar temperature evolution in early 2009 in several
whole atmosphere models (including HAMMONIA) and MLS observations has been
performed by . Their Fig. 1 can be directly compared to
our Fig. . In agreement with our results, most of the
investigated models in the study of did not maintain
the stratopause height near 0.01 hPa until the end of February as in the
observations, except WACCM-X, which was nudged to NOGAPS-ALPHA reanalysis
data (assimilating observed temperatures) up to 92 km.
further showed with WACCM simulations of the same NH winter that nudging to
a more realistic meteorology (with an ES evolution closer to the
observations) up to 92 km dramatically improves the simulated NO descent
during this event compared to SOFIE observations.
Unresolved non-orographic GWD is thought to play a crucial role in the
strengthening of mesospheric descent in the vicinity of the NO source region
during ES events by providing enhanced westward momentum, which forces a
poleward and downward residual circulation
. Motivated by the results of
our analysis, investigated the sensitivity of the
HAMMONIA model to changes in the parameterization of non-orographic gravity
waves. By weakening the amplitude of the gravity waves at the source level,
they could substantially improve the modelled temperature and NOx
increases (both in terms of timing and amount) compared to the MIPAS
observations. They found that the amount of transported NOx depends
strongly on the altitude at which momentum is deposited in the mesosphere.
Smaller gravity wave amplitudes favour the wave breaking and momentum
deposition at higher altitudes, closer to the NO source region. The
structural similarities of HAMMONIA and WACCM temperature biases suggest that
changes in the non-orographic GWD parameterization might also improve the
representation of NOx descent during ES events in WACCM.
Upper stratosphere and mesosphere (USM)
MIPAS-NOM and modeled
temporal evolutions of CO at 4–0.02 hPa within 70–90∘ N. White
lines indicate the observed VMR levels of 0.3, 1, 3, and 10 ppmv.
MIPAS-NOM (top) and MLS/Aura (bottom) temporal evolutions of CO VMR
in comparison with the model results within 70–90∘ N at 0.02 hPa.
In this section CO, NOx, and temperature fields of all involved
models are compared to the observations in the USM. The aim is to evaluate the models' ability to reproduce
NOx transport into the stratosphere during both the unperturbed
pre-SSW phase and the ES event and to identify whether discrepancies with
respect to the observations are related to dynamics or chemistry. The latter
is of particular concern for the medium-top models applying the NOx
UBC.
CO
CO is an excellent tracer of vertical motion in the USM during polar winter
because of its pronounced vertical gradient in this region and the long
chemical lifetime under dark conditions. Further, the relatively less
pronounced gradient at higher altitudes (compared to NOx) results in
a weaker sensitivity to dynamical variability in the MLT, hence allowing us to
study the descent in the USM separately. In addition, the very low
stratospheric CO background concentrations allow us to trace mesospheric descent
down to altitudes below 30 km without the need to invoke tracer correlations
as in the case of odd nitrogen .
MIPAS-NOM (top) and MLS/Aura (bottom) temporal evolutions of CO VMR
in comparison with the model results within 70–90∘ N at 0.5 hPa.
CO observations are available from MLS, ACE, and MIPAS. As an example,
Fig. compares the MIPAS-NOM CO temporal evolution with the
models. At a first glance, the observed evolution of the CO vertical
distribution is qualitatively well reproduced by most models except for
FinROSE, which exhibits a very weak vertical gradient all over the winter.
This behaviour is caused by a simplified CO2 representation leading
to overestimation of CO production and a largely enhanced CO background in
the middle and upper atmosphere. All other models capture the observed polar
winter descent down to pressure levels around 3 hPa in the first part of the
winter, the sudden reduction of CO during the SSW caused by meridional mixing
and upwelling, as well as the enhanced descent during the ES event.
A more quantitative analysis is provided by Figs.
and , comparing the modelled CO evolutions at 0.02 and 0.5 hPa,
respectively, to MIPAS-NOM and MLS observations (note that FinROSE is not
included here because of the unrealistically high mixing ratios). The
comparisons to both instruments provide a very similar picture, hence
confirming the robustness of the encountered model biases. Observed CO
abundances at 0.02 hPa are around 6–8 ppmv during the pre-SSW phase,
decrease to 4 ppmv during the SSW, and show a pronounced peak of
12–14 ppmv in February related to the ES event. Medium-top models exhibit
slightly lower CO abundances (around 5 ppmv) that do not vary significantly
over the winter. This behaviour is expected since transport of lower
thermospheric CO into the model domain is typically not considered and, as
consequence, dynamically induced variations are mostly absent at this
pressure level close to the models' upper lid. As an exception, tracers are
transported in KASIMA above the chemical domain at 90 km which causes
accumulation effects, resulting in slightly increased abundances during early
winter. Further, minor differences in the late-winter abundances simulated by
KASIMA and CAO on the one hand and EMAC and SOCOL on the other hand can be
attributed to the use of different kinetic data in the chemistry schemes,
primarily affecting OH involved in the CO loss reaction. The observed CO
evolution at 0.02 hPa is qualitatively well captured by WACCM, although the
abundances during the pre-SSW phase of about 10 ppmv are overestimated by
∼ 40 % compared to the observations and the ES-related peak occurs
earlier than in the observations. HAMMONIA CO abundances are underestimated
due to missing thermospheric CO production mechanisms (see previous section)
and are very close to the CO amount simulated by the medium-top models
(∼ 5 ppmv). 3dCTM simulates early-winter CO abundances that are
roughly in agreement with the observations. ES-related CO enhancements in the
post-SSW phase, however, are delayed and persist for a longer period than
observed.
The observed CO evolution at 0.5 hPa is well reproduced by most medium-top
models and WACCM in the pre-SSW phase. KASIMA and 3dCTM overestimate the CO
abundances by a factor of ∼ 2.5 and ∼ 1.5, respectively, while
HAMMONIA simulates about 50 % lower than observed CO abundances. The
ES-related CO increases peak in most models too early (around mid-March)
compared to the observed peak occurrence around 1 April, although the peak
magnitude is reasonably well simulated (with exception of HAMMONIA). The CO
peak in HAMMONIA occurs even 2 weeks earlier than in the other models. In
3dCTM, the CO tongue does not reach the 0.5 hPa level (see
Fig. ), likely because of the too-late formation of the elevated
stratopause discussed in the previous section. The high CO abundances of this
model in February, immediately after the SSW, seem to be caused by horizontal
mixing, after a short period of localized upwelling during the sudden
warming.
MIPAS-NOM and modelled temporal evolutions of NOx in the
pre-SSW phase of the 2008/09 NH winter at 1–0.02 hPa within
70–90∘ N. White lines indicate the observed VMR levels of 10, 20,
50, 70, 100, and 150 ppbv. White regions reflect missing or not meaningful
data.
The individual impacts of orographic and non-orographic GWD on
the mesospheric CO evolution in the CMAM model has been evaluated by
comparing with the same MLS observations during the 2008–2009 NH winter by
. Our Fig. can be qualitatively compared
to their Fig. 8 (although the latter shows the CO evolution at a slightly
higher pressure level). The CO evolution in the CMAM simulation, including
all gravity wave sources, is very similar to that obtained by most of the
models included in our study (note that the apparently smaller time lag of
the ES-related peak in the study is related to the
higher pressure level of their comparison). However, there are
similarities between their simulation without orographic GWD and the KASIMA
simulation presented here, particularly regarding the CO overestimation in
the pre-SSW phase and the relatively broad CO peak after the ES event. Note
that KASIMA does not employ a specific parameterization for orographic
GWD which may be justified as KASIMA is nudged up to 1 hPa but
seems not to be sufficient near the stratopause. This is also seen in the low
bias of the stratopause temperature in the pre-SSW phase (see
Fig. ). Further, our 3dCTM results share some characteristics of the
CMAM simulation without any GWD. In particular, both simulations exhibit a
steady (though fluctuating) increase of CO until the SSW, a short recovery
time after the warming, and the absence of an ES-related peak in March/April.
This again highlights the importance of the proportion of the gravity wave
spectrum not considered in the LIMA model – the subscale (≤ 500 km)
waves for the mesospheric meridional wintertime circulation, in particular
during the recovery phase of the elevated stratopause event as discussed in
the previous section, but also for the “undisturbed” pre-event period.
NOx in the early (pre-SSW) phase
In the following, the observed and modelled vertical structure of NOx
in the USM during mid-winter (pre-SSW phase) is analysed in more detail to
evaluate how well the models reproduce the EPP indirect effect in this region
for unperturbed conditions. Figure compares the NOx
evolution of all models at 1–0.02 hPa with the MIPAS data. All models
capture the observed early-winter NOx descent characterized by a
quasi-continuous increase of NOx until the SSW-related disruption in
mid-January. The magnitude of the observed NOx enhancements is well
reproduced by EMAC, FinROSE, KASIMA, HAMMONIA, and WACCM. Descending
NOx can be distinguished from the background in these simulations and
in the observations down to pressure levels of 0.3–0.5 hPa. Further descent
below this level cannot be traced because NOx is converted to other
reservoir nitrogen species (principally HNO3) below approximately
45 km. Descent of EPP-generated total reactive nitrogen has been observed
down to altitudes as low as 30 km during the pre-SSW phase of Arctic winter
2009 .
As discussed in Sect. , 3dCTM overestimates the observed
NOx increasingly towards lower altitudes and shows a double peak
structure (with a NOx depletion around mid-December) that is not seen
in the MIPAS NOx data, though a similar feature is also observed in
3dCTM CO, and at least indicated in MIPAS CO, at the same time. Also SOCOL
and CAO overestimate substantially the descending NOx amounts. Since
the CO descent is well described by the latter two models, the NOx
overestimation is likely related to the prescription of NOx at the
upper model lid. The NOx abundances at the upper model level
(0.01 hPa) are in agreement with the values specified by the UBC. However,
in contrast to the observations and other models, which show a rapid decrease
towards lower altitudes, the abundances remain nearly constant in the entire
vertical range above 0.03 hPa. This behaviour is caused by a model boundary
artefact introducing unrealistically fast vertical propagation of the
NOx caused either by too-high vertical velocities at the model lid or
low vertical model resolution. Indeed, the descending NOx amounts are
substantially reduced in a test simulation with NOx prescribed at the
second layer from the top (not shown), making the SOCOL results similar to
those of EMAC.
Left: MIPAS-NOM and modelled mean NOx profile for the period
15 December 2008–12 January 2009 within 70–90∘ N. Right: GOMOS and
modelled mean night-time NO2 for the same period within
75–85∘ N. The error bars indicate random retrieval errors of the
averaged observational data.
A more quantitative view of the modelled mid-winter NOx profiles in
comparison with observations of the MIPAS and GOMOS instruments (the latter
measuring night-time NO2) is provided in Fig. . Other
instruments measuring NOx species could not be included in this
comparison: SMR because they measured only NO but most of NOx is in
the form of NO2 below 0.1 hPa in dark conditions, SCIAMACHY because
it is not sensitive to NO below ∼ 65 km, and ACE-FTS because it did
not sample latitudes polewards of 70∘ N in mid-winter. Both MIPAS and
GOMOS consistently show VMRs of about 20 ppbv at 0.05 hPa, decreasing to
the background values of 5 ppbv at 0.8 hPa. The observed profile is
reproduced within 20 % by EMAC, FinROSE, HAMMONIA, and WACCM. The KASIMA
results are about 50 % higher than the observations. 3dCTM, CAO, and SOCOL
overestimate the observations by a factor of 2–3.
Overall, most atmospheric models are capable of providing a realistic and
consistent picture of NOx descent in dynamically and geomagnetically
unperturbed NH early winters as in 2008/2009. This is the case for high-top
models explicitly considering odd nitrogen production by EPP in the MLT
region, as well as for medium-top models employing a NOx UBC. However, some individual models show significant biases
in the simulated early-winter NOx descent which could be traced back
to deficiencies in either the dynamical or chemical schemes.
MIPAS-NOM and modelled temporal evolutions of NOx during the
ES event at 1–0.02 hPa within 70–90∘ N. White lines indicate the
observed VMR levels of 10, 20, 50, 70, 100, 150, and 200 ppbv.
Same as Fig. , but for ACE-FTS.
NOx in the perturbed late (post-SSW) phase
Limitations of high-top models to reproduce quantitatively the observed
NOx descent from the upper mesosphere during the perturbed part of
the 2008/09 NH winter (post-SSW phase) have already been discussed in
Sect. . An important question is whether medium-top models,
prescribing realistic NOx distributions at the model's upper lid,
could provide a better description of ES-induced odd nitrogen transport by
bypassing the problem of underestimated descent in the region above 80 km,
as encountered in the high-top models. Figures
and show the temporal evolutions of modelled NOx
during the ES event in comparison with MIPAS-NOM and ACE-FTS observations,
respectively. Despite the sampling-related differences, both instruments
provide a very consistent picture of model biases. In particular, the time
shift (earlier occurrence) of the modelled NOx tongue (except 3dCTM),
also identified in the CO comparisons, is clearly visible in the comparisons
with both instruments.
Again, SOCOL and CAO overestimate significantly the observed NOx
(about a factor of 5) in the descending tongue (for the reasons already
identified in the mid-winter comparisons). This overestimation is even more
pronounced than in the pre-SSW phase. In the case of HAMMONIA, related to the
fast downward propagation of the ES (see Sect. ), the
NOx peak occurs earlier and the tongue descends faster, merging with
the background already in mid-February. In 3dCTM, the NOx tongue
reaches the lower mesosphere (0.02 hPa) later than in the other models and
in observations due to the too-slow descent rates throughout the mesosphere.
Thus, the development of the NOx tongue in the lower mesosphere is
delayed, and it does not reach to stratospheric altitudes.
The NOx tongue observed by MIPAS reaches the 1 hPa level by the end
of April. The reversal of the residual circulation in spring disabled further
downward propagation of the tongue. ACE-FTS observed polar latitudes until 25
March, when the tongue reached the 0.3 hPa level in agreement with MIPAS
observations at the same time. Compared to the observations, the NOx
tongue in the model simulations (except HAMMONIA and 3dCTM) penetrates
deeper, reaching the 2–3 hPa pressure levels at the end of April.
Left: MIPAS-NOM and modelled time evolution of the occurrence of the
NOx peak as function of pressure after the ES event. Right: observed
and modelled NOx peak values, averaged over 70–90∘ N.
Figure shows more quantitatively the observed and modelled
occurrence time and magnitude of the NOx peak as a function of
pressure level. The similar peak timing simulated by all models (except 3dCTM
and HAMMONIA), about 2 weeks earlier than the observed peak below the
0.2 hPa level, is surprising. In the WACCM simulation, this time shift with
respect to the observations is present over the whole vertical range.
Interestingly, the peak occurrence time in the medium-top models, all
prescribing the observed NOx evolution at their upper lid, converges
with the descent to the same occurrence time as simulated by WACCM at lower
altitudes, i.e. earlier than in the observations. It is worth noting that a
HAMMONIA simulation (not shown) with reduced non-orographic gravity wave
amplitude exhibits both a NOx peak occurrence
time and magnitude in very good agreement with the observations down to
pressure levels around 0.3 hPa. Below, however, the peak occurrence time in
this particular HAMMONIA simulation converges again to that of most of the
other simulations.
Despite the consistency of the models with respect to the timing of the
NOx descent in the lower mesosphere, indicating similar dynamical
representations, the spread of the magnitude of the modelled NOx
peaks (right panel of Fig. ) is very large (within 0.2–3 times
the observed magnitude), even when excluding the CAO and SOCOL results. This
is particularly surprising in the case of the medium-top models, all of them
prescribing the same NOx obtained from observations, and will be
discussed in more detail at the end of this section.
MIPAS-NOM and modelled temporal evolutions of NOx at
0.5 hPa (top) and of temperature at 0.2 hPa (bottom) within
70–90∘ N during the ES event.
Top: temporal evolution of daily averaged polar cap temperatures at
4–0.02 hPa observed by MIPAS-NOM, MLS/Aura, ACE-FTS, and SABER (from left
to right). Bottom: corresponding differences between temperatures simulated
with the “medium-top” models (CAO, EMAC, FinROSE, KASIMA, and SOCOL) and
the observations.
Figure shows the temporal evolution of the MIPAS observations and
modelled NOx at 0.5 hPa together with the temperature evolution
slightly above, at 0.2 hPa. There is a clear link between the earlier
occurrence of the modelled NOx peaks and the time shift of the
modelled temperature increases after the SSW, occurring systematically about
2 weeks earlier than in the observations (with the exceptions of HAMMONIA and
3dCTM). In order to check whether the temperature bias of the simulations with
respect to MIPAS is consistent with the other measurements, we show in
Fig. the vertical structure of the temperature differences between
the medium-top models and MIPAS-NOM, MLS, ACE-FTS, and SABER observations,
similarly as done for the high-top models in Sect. . All
medium-top models show a warm bias of 15–25 K around 0.2 hPa in February
and early March, and a cold bias of 5–10 K around 1 hPa during the same
period (though slightly less pronounced in KASIMA). Similar biases have been
detected in the WACCM simulations (see Fig. ).
MIPAS-UA and modelled NH zonal mean temperature distribution on 15 February 2009.
The systematic, dipole-type temperature bias of the high-top model WACCM and
all medium-top models, with similar amplitudes and time evolutions, explains
the consistently too-early occurrence of the NOx descent encountered
in these models. It also hints at a common origin. One plausible reason for
the temperature bias could be the meteorological data nudged in most models
below 1 hPa. Around this pressure level, a cold bias of these models is
observed, including FinROSE, which relies entirely on ECMWF operational
analysis data, and EMAC, which applies the nudging to ERA-Interim reanalysis
data up to the altitude of 0.2 hPa. This indicates that the cold bias is
present already in the ECMWF operational analysis and ERA-Interim data. This
bias might then likely influence the model dynamics extending above the
nudged region. The cold bias around 1 hPa in February is also seen in the
WACCM simulation (see Fig. ), suggesting that it is also present
in the MERRA reanalysis. This is confirmed by comparison of MERRA and MLS
temperatures (not shown). Only in the HAMMONIA simulation, which shows a
pronounced warm bias in the entire 2–0.1 hPa region, the local influence of
the nudged meteorology at the edge of the nudging region seems to be
outweighed by the internal model dynamics. It is beyond the scope of this
paper to investigate in detail the possible mechanisms for the vertical
propagation of dynamical biases, introduced by the nudging, resulting in a
descent of mesospheric NOx that is too early. However, since the cold bias
encountered at 1 hPa is restricted to latitudes northward of 60∘ (see Fig. ) and hence implies a strengthening of the meridional
temperature gradient, it is likely to accelerate zonal winds at this level
and above, which in turn would lead to changed filtering conditions for the
propagation of gravity waves. Another important question which needs to be
addressed in upcoming studies is the causes of the cold bias in the employed
reanalysis datasets that have been found here.
The spread of the magnitude of the ES-related NOx tongue encountered
below 0.1 hPa in the medium-top models, despite the prescription of a common
odd nitrogen upper boundary above, deserves some further discussion. The
consistency of simulated temperature evolutions indicates that vertical
transport is represented in these models in a similar way. It is therefore
unlikely that differences in the descent velocities are the main cause for
the spread. Differences in meridional transport and mixing above the vortex
edge and subsequent enhanced photochemical loss could also contribute to the
differences but would not explain overestimation. A most plausible
explanation is the detailed treatment of the UBC.
Prescribing at an altitude with too fast vertical transport, as indicated
here at 0.2 hPa, will unavoidably cause a too-strong flux of NOx
into the domain below. Therefore, models that use a UBC definition extending
to lower pressure levels likely overestimate the NOx flux. This is,
for example, the case of EMAC, which prescribes NOx in the entire
vertical domain above 0.1 hPa: the peak magnitude of the tongue is, as
expected, close to the observations in the UBC domain. However, it becomes
increasingly larger than the observed magnitude during the descent down to
0.7 hPa, where it is overestimated by a factor of 3. This highlights the
importance of a realistic dynamical representation in the UBC domain in
models prescribing NOx concentrations.
Conclusions
We have presented the results of the HEPPA-II intercomparison project,
conducted in the framework of SPARC/WCRP's SOLARIS-HEPPA activity, which aims
at evaluating the simulations of the NH polar winter 2008/2009 from eight
atmospheric models by comparison with observations of temperature and
concentrations of NOx and CO from seven satellite instruments. The
large number of participating models allowed for a comprehensive assessment
of the ability of state-of-the-art chemistry climate models to reproduce the
observed EPP indirect effect in a dynamically perturbed NH winter under
conditions of very low geomagnetic activity. The use of multi-instrument data
for model evaluation allowed for not only the assessment of the significance
of identified model biases but also the estimation of the uncertainty range of our
current knowledge on tracer and temperature distributions in Arctic winters.
It has been shown that the appropriate consideration of the
instrument-specific sampling patterns is key to a meaningful multi-instrument
analysis, particularly during perturbed dynamical conditions. The high degree
of consistency between the comparisons of the models to individual
observations has proven the reliability of the currently available satellite
record during polar winter conditions.
Most models provide a good representation of the mesospheric tracer descent
in general, and the EPP indirect effect in particular, during the unperturbed
(pre-SSW) period of the NH winter 2008/2009. Observed NOx descent
into the lower mesosphere and stratosphere is generally reproduced within
20 %. Larger discrepancies of a few model simulations, resulting in
overestimated NOx enhancements, could be traced back either to an
unrealistic representation of the polar winter dynamics or to an inadequate
prescription of the NOx partitioning at the uppermost model layer
leading to boundary artefacts.
In March–April, after the ES event, however, modelled mesospheric and
stratospheric NOx distributions deviate significantly from the
observations. The too-fast and early downward propagation of the NOx
tongue, encountered in most simulations, coincides with a warm bias in the
lower mesosphere (0.2–0.05 hPa) likely caused by an overestimation of
descent velocities. In contrast, upper-mesospheric temperatures at
0.05–0.001 hPa are in general underestimated by the high-top models after
the onset of the ES event, being indicative of a too-slow descent and hence
too small NOx fluxes. As a consequence, the magnitude of the
simulated NOx tongue is generally underestimated by these models.
Descending NOx amounts simulated by the medium-top models with
prescribed NOx are on average closer to the observations but show a
large spread of up to several hundred percent. This is primarily attributed
to the different vertical model regimes where the NOx upper boundary
condition is applied.
In general, the intercomparison demonstrates the ability of state-of-the-art
atmospheric models to reproduce the observed EPP indirect effect in
dynamically and geomagnetically quiescent early NH winter conditions as
present in November 2008–January 2009. It should be noted, however, that the
extrapolation of this result to high geomagnetic activity conditions should
be done with caution since mid-energy electron impact in the mesosphere,
which was of minor importance during this particular winter, could lead to
additional complications. Further, to obtain good agreement between simulated
and observed mesospheric tracer descent it is necessary to constrain
stratospheric dynamics in the models by (re-)analysed meteorology.
The differences encountered between observed and simulated NOx, CO,
and temperature distributions during the perturbed phase of the 2009 NH
winter (i.e. February–April), however, emphasise the need for model
improvements in the dynamical representation of ES events in order to allow
for a better description of the EPP indirect effect under these particular
conditions. Our results reinforce the findings from previous studies that the
adequate parameterization of unresolved GWD, particularly of its
non-orographic component, is crucial for achieving such improvements. They
also demonstrate that the dynamical boundary condition at 1 hPa, employed in
our models, is not sufficient to fully determine the mesospheric circulation
yet is crucial for the tracer transport into the stratosphere. Even when the
winds are constrained in the stratosphere by observations, the calculated GWD
in the mesosphere by different parameterizations can differ strongly. As
discussed by , such differences are related more to the
characteristics of the launch spectra rather than to the treatment of the
dissipation mechanisms in the parameterizations used. Indeed,
have shown that by modifying the launch characteristics
of the gravity waves it is possible to tune the simulated NOx descent
towards the observations. Depending on the model, heating rates are
calculated from coupled fields of radiative active gases or climatologies
are used, adding further to differences and uncertainties. In addition,
despite the similar definition of the nudging regime (< 1 hPa) in all
model simulations, the vertical extent of the transition region between fully
constrained and free-running mode varies among the models, which could
introduce additional model spread. Finally, have shown that
model dynamical fields are prone to errors due to the nudging approach
itself, even when data and forcing terms are known exactly and there are no
model biases. Interestingly, these intrinsic errors tend to grow with the
complexity of the GWD representation employed in the model.
Many of the model-specific issues identified in the course of this project
are currently being solved e.g.. Lessons learned are
hoped to also be of use for future model developments, particularly with
respect to the consideration of EPP effects in upcoming coordinated model
intercomparison projects. However, the bias encountered in the
meteorological reanalysis data in the post-SSW upper stratosphere and lower
mesosphere potentially triggered the common tendency of the models to produce
a descent in the lower mesosphere that is too early. These results imply the need to
improve data assimilation systems for producing reanalysis data, especially
with respect to the representation of the polar winter USM. This is particularly important because the use of specified
dynamics in atmospheric models is a necessary step to allow for meaningful
comparisons to observations on seasonal and shorter timescales.
All the model and observational data supporting the
analysis and conclusions have been archived and are available upon request
from the corresponding author.
The authors declare that they have no conflict of
interest.
Acknowledgements
This work has been conducted in the frame of the WCRP/SPARC SOLARIS-HEPPA
activity. The IAA team was supported by the Spanish MCINN under grant
ESP2014-54362-P and EC FEDER funds. The MPI-MET team was supported by the
Max Planck Gesellschaft (MPG), and computational resources were made
available by Deutsches Klimarechenzentrum (DKRZ) through support from
Bundesministerium für Bildung und Forschung (BMBF). The FMI team was
supported by the Academy of Finland through the projects 276926 (SECTIC:
Sun-Earth Connection Through Ion Chemistry), 258165, and 265005 (CLASP:
Climate and Solar Particle Forcing). CAO team was supported by the Russian
Science Foundation under grant 15-17-10024. SOCOL team was funded by Swiss
National Science Foundation (SNSF) grants 200021-149182 (SILA), 200020-163206
(SIMA), and CRSII2-147659 (FUPSOL-II). S. Bender, M. Sinnhuber, and H. Nieder
(all KIT) gratefully acknowledge funding by the Helmholtz Association of
German Research Centres (HGF), grant VH-NG-624. NCAR is sponsored by the
National Science Foundation (NSF). Computing resources for WACCM simulations
were provided by the Climate Simulation Laboratory at NCAR's Computational
and Information Systems Laboratory, sponsored by the NSF and other agencies.
Work at the Jet Propulsion Laboratory, California Institute of Technology,
was carried out under a contract with the National Aeronautics and Space
Administration. The Atmospheric Chemistry Experiment (ACE), also known as
SciSat, is a Canadian-led mission mainly supported by the Canadian Space
Agency. Odin is a Swedish-led satellite project funded jointly by Sweden
(SNSB), Canada (CSA), Finland (TEKES), and France (CNES) and is part of
European Space Agency's (ESA) third-party mission program. We thank two
anonymous reviewers for helpful suggestions that led to improvements in the
quality of the present work.
Edited by: F.-J. Lübken
Reviewed by: two anonymous referees
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