Dehydration in the Antarctic winter stratosphere is a well-known phenomenon
that is annually observed by satellites and occasionally observed by
balloon-borne measurements. However, in situ measurements of dehydrated air
masses in the Antarctic vortex are very rare. Here, we present detailed
observations with the in situ and GLORIA remote sensing instrument payload
aboard the German aircraft HALO. Strongly dehydrated air masses down to
1.6 ppmv of water vapor were observed as far north as 47
Antarctic stratospheric dehydration occurs regularly every winter and
spring in the very isolated and very cold southern hemispheric polar vortex
Dehydration of Antarctic vortex air masses is observed frequently by satellite
measurements
In general, the process of dehydration is well understood. Relatively
little is known about the fate of the dehydrated air masses at the bottom
of the stable vortex.
Nevertheless, Antarctic stratospheric air masses can also be exchanged with tropospheric
air masses (stratosphere–troposphere exchange (STE)) and can be transported
deep into the troposphere creating a dry tongue reaching down to the Earth's surface.
This air is rich in ozone and reactive nitrogen due to the stratospheric
origin and can influence the chemical composition of the Antarctic
troposphere
The following research questions will be answered, based on the observations of the ESMVal flight (13 September 2012): (1) can we confirm dehydration in the lowermost stratosphere and at the bottom of the Antarctic vortex? (2) How deep do dehydrated air masses from the vortex descend into the troposphere? (3) Where and how have Antarctic dehydrated stratospheric air masses been transported?
To answer the questions above, the study presented here is structured as follows: in
Sect.
The German research aircraft HALO, deployed during TACTS and ESMVal, has
a long flight endurance of up to 12
The airborne Lyman-
The Hygrometer for Atmospheric Investigation (HAI)
The TRIHOP instrument is a three-channel quantum cascade laser
infrared absorption spectrometer capable of the subsequent measurement
of CO,
FAIRO is a new accurate ozone instrument developed for use onboard
the HALO aircraft. It combines two techniques, the UV photometry
(light absorption of
The Gimballed Limb Observer for Radiance Imaging of the Atmosphere
(GLORIA) combines a two-dimensional focal plane detector array with
a Fourier transform spectrometer to capture about 6000 infrared limb
spectra simultaneously. This enables remote sensing observations with
high vertical and horizontal resolution to resolve small-scale
structures
Dynamical situation with color-coded PV from ECMWF data on
13 September 2012 at 12:00 UTC on two different potential temperature
levels:
The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation
(CALIPSO) satellite is one of five satellites in the NASA A-train
constellation. CALIPSO completes 14.55 orbits per day with an
inclination of 98.2
Global meteorological reanalysis ERA-Interim data
The general dynamical situation of the Antarctic vortex is shown in
Fig.
Flight pattern of the ESMVal flight on 13 September 2012. The black
contours illustrate the horizontal westerly wind from ECMWF data. Yellow dots
represent the vortex edge derived from the Nash criterion
The latitudinal cross section of PV from ECMWF along the flight path is shown in
Fig.
The time series of the in situ measurements are shown in
Fig.
Time series of ESMVal Antarctica flight on 13 September 2012: the upper panel shows FISH (blue), HAI (green) and GLORIA (red) water vapor measurements, water vapor saturation mixing ratio with respect to ice (cyan, derived from HALO temperature), pressure (black), and altitude (gray) at flight level. The lower panel shows the tracer observation ozone (red), methane (blue) and interpolated ECMWF PV values (orange). Time ranges within the vortex are marked with light-blue shadows according to the tracer measurements. Time ranges leg one and leg two are marked for hygrometer intercomparison of FISH and HAI (see text).
The saturation mixing ratio with respect to ice within the vortex is
3 to 4 times larger than the measured water vapor mixing
ratio. Thus, the sampled air masses were clearly subsaturated with
relative humidities of 25–33 % and it is unlikely that
ice particles remained in the probed air masses. In addition, HAI and
FISH show no peaks in the total water vapor time series (based on
1
In order to show the fairly good agreement of both FISH and HAI based on 1
The GLORIA instrument measures quasi-vertical profiles along the
flight path by viewing the atmosphere on the right side of HALO. Here,
we focus on the
The thermal tropopause is also derived from the GLORIA-retrieved
temperature profiles and is marked with black dots. Interestingly, low
water vapor mixing ratios around and below 2
Embedded in the dry vortex air masses, small filaments (marked in
Fig.
GLORIA time series of the
Detailed investigation of the observed Antarctic dehydration is
performed using air mass trajectories calculated with the tool
CLaMS-traj, which is part of the Chemical Lagrangian Model for the
Stratosphere (CLaMS)
In the following we will use the trajectory analysis to investigate the timescale and location of potential dehydration events. In a second step we will analyze the trajectory data for possible exchange into the Antarctic troposphere.
In this section, we investigate the history of the in situ-measured
dehydrated air masses. For that purpose, the trajectories are
calculated every 10
To show the presence of ice particles and subsequent dehydration,
CALIPSO data (see Sect.
Figure
The recurring low saturation mixing ratios and corresponding low
temperatures are primarily caused by gravity waves, which are induced
by the high topography of the Antarctic continent
Calculated frequency distribution (number, gray shadow) and median (orange line) of ice saturation mixing ratio along trajectories 50 days back in time starting at the flight path on 13 September 2012 (12:20 to 12:29 UTC, 54 trajectories in total). If more than 50 % of trajectories have a corresponding CALIPSO observations of ice or no ice, the time is marked with blue or white, respectively. Otherwise, it is marked in black, indicating no CALIPSO match.
After freezing, the ice particles can even reach the troposphere due
to sedimentation, especially if dehydration takes place at low
altitudes (11.5–12.5
The trajectories for analyzing the transport of dehydrated air masses observed by GLORIA are calculated 70 days backward in time from each tangent point (in total 30 000 single trajectories). In addition, the trajectories are also calculated 14 days forward to show where the air masses are further transported. With this set of trajectories, the GLORIA measurements can be interpreted and the associated transport pathways can be investigated.
In the first subsection hereafter, the dehydrated air masses observed with the in situ instruments and GLORIA in the lowermost stratosphere are analyzed and the origin of moist filaments is investigated. The second subsection contains the trajectory analysis of dry air masses found to be below the thermal tropopause and investigates the further transport.
The time each GLORIA tangent point trajectory has spent in the
vortex before the measurement. The green boxes mark two of the filaments (6,
7) observed by GLORIA (see Fig.
From the in situ ozone and methane measurements, it is visible that
the HALO aircraft has penetrated the Antarctic vortex (see
Sect.
RDF (reverse domain filling) of each GLORIA tangent point trajectory
with ECMWF water vapor of 5 days prior to the measurement time. Red boxes
show the location of the observed filaments. The flight path, tropopause
height, potential temperature, and PV isolines are the same as in
Fig.
The color code denotes the time since trajectories passed the
thermal tropopause from stratosphere to troposphere. Bluish colors denote
trajectories which recently passed the thermal tropopause, reddish colors
indicate tropopause crossings longer ago. The green-framed areas mark the
trajectories where GLORIA observations show water vapor below 3
In contrast, the air masses that spent between 5 and 40 days
in the vortex (marked by light-blue and reddish colors) indicate
mixing of midlatitude air into the vortex. Even at the vortex edge and in
the core of the vortex itself, small filaments are apparent. As
indicated in Sect.
To show that all observed filaments (red boxes in
Fig.
Air parcel properties based on trajectory analyses for all
green-framed trajectories from Fig.
Figure
The weak vertical temperature gradient and the poor definition of
the thermal tropopause is implied by the tropopause heights
derived from GLORIA, which have a broad scatter (
The stratospheric and vortex air masses did not cross the thermal tropopause
(dark blue colors). The air masses below the measured thermal tropopause are
obviously more patchy and contain more trajectories originating from
the troposphere (reddish colors). However, some freshly transported
stratospheric air masses are also discernible below the thermal
tropopause (light-blue colors), down to 7
Figure
On 10 September, on the point furthest south at
latitudes around 80
Thus, RWB events drive the
mixing process together with isentropic transport through the thermal
tropopause. Additionally, RWB events detach air masses from the PV structure and can facilitate
a wave-driven, secondary circulation that transports warmer air masses
from around the vortex edge to the colder inner vortex. The change in PV is most likely
not caused by cloud processes due to the dryness of these air masses. However, despite the
dryness, the slight enhancement of ozone and the
residual water vapor enable radiative cooling and a reduction in
potential temperature, i.e., descent of air masses.
Once transported into the troposphere, air masses will be
transported down to near-surface level within few days as already suggested by
The vortex becomes more and more unstable during the Antarctic spring,
and one may assume that this mixing event is already a first sign
of the vortex break-up.
Finally, the reason for the large downward transport is a mixture of strong Rossby wave activity, the presence of a permeable thermal tropopause, RWB events and the additional radiative cooling with corresponding subsidence of the air masses.
Detailed observations of dehydration and STE in the Antarctic UT/LS are very rare. In this study, we used high-resolution in situ and remote sensing measurements taken from aboard the HALO aircraft in combination with an extensive trajectory case study to answer the following questions:
Can we confirm dehydration in the lowermost stratosphere and at the bottom
of the Antarctic vortex? We observed strongly dehydrated air in the lower
vortex at potential temperatures of 350 to 360 How deep do dehydrated air masses from the vortex descend into the troposphere?
Dehydrated air masses are also observed with the high-resolution remote
sensing instrument GLORIA down to the thermal tropopause, as well as below
this point, down to 7 Where and how have Antarctic dehydrated stratospheric air masses been transported?
With an extensive trajectory case study, dry air masses below the thermal
tropopause in the southern hemispheric polar and midlatitude region are
determined to have stratospheric origin. The enhanced permeability of the
Antarctic thermal tropopause in winter and spring allowing isentropic
downward transport is confirmed
Sincere thanks go to the whole CLaMS team and in particular to Jens-Uwe Grooß for supporting the trajectory calculations. We acknowledge the NASA Langley Research Atmospheric Science Data Center for providing the CALIPSO data. In addition, we acknowledge the European Centre for Medium-Range Weather Forecasts for meteorological reanalysis data support. The development and application of the HAI instrument was financially supported by the German Science Foundation (DFG). We thank all members of the GLORIA instrument team for their great efforts in developing the first IR limb imager. The GLORIA hardware was mainly funded by the Helmholtz Association of German Research Centres through several large investment funds. Great thanks go to the coordinators of the TACTS campaign, Andreas Engel and Harald Bönisch, and of course to the whole HALO community for the great work during both campaigns. We thank Jens-Uwe Grooß for assisting the flight planning by CLaMS model forecasts supported by the German Research Foundation (DFG) through the project LASSO (HALO-SPP 1294/GR 3786). Special thanks go to Anna Luebke for language revision. Finally, we greatly acknowledge the two reviewers, Heini Wernli and Howard Roscoe, as well as Adrian Tuck, who provided constructive and fruitful comments leading to an improvement of the study. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: F. Khosrawi