ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-15-6981-2015Lifted temperature minimum during the atmospheric evening transitionBlay-CarrerasE.estel.blay@upc.eduPardyjakE. R.PinoD.https://orcid.org/0000-0002-4512-0175HochS. W.CuxartJ.MartínezD.ReuderJ.https://orcid.org/0000-0002-0802-4838Department of Applied Physics, Universitat Politècnica de
Catalunya, BarcelonaTech, Barcelona, SpainDepartment of
Mechanical Engineering, University of Utah, Salt Lake City, UT, USAInstitute for Space Studies of Catalonia (IEEC–UPC), Barcelona,
SpainDepartment of Atmospheric Sciences, University of Utah,
Salt Lake City, UT, USAGrup de Meteorologia, Departament de
Física, Universitat de les Illes Balears, Palma de Mallorca, SpainCenter for Applied Geoscience, University of Tübingen,
Tübingen, GermanyGeophysical Institute, University of
Bergen, Bergen, NorwayE. Blay-Carreras (estel.blay@upc.edu)6981699118July20147November201426February201527February2015This 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://www.atmos-chem-phys.net/15/6981/2015/acp-15-6981-2015.htmlThe full text article is available as a PDF file from https://www.atmos-chem-phys.net/15/6981/2015/acp-15-6981-2015.pdf
Observations of lifted temperature minimum (LTM) profiles in the nocturnal
boundary layer were first reported in 1932. It was defined by the existence
of a temperature minimum some centimetres above the ground. During the
following decades, several research studies analysed this phenomenon
verifying its existence and postulating different hypotheses about its
origin.
The aim of this work is to study the existence and characteristics of LTM
during the evening transition by using observations obtained during the
Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign. Data
obtained from two masts instrumented with thermocouples and wind sensors at
different heights close to the ground and a mast with radiometers are used
to study the role of mechanical turbulence and radiation in LTM development.
The study shows that LTM can be detected under calm conditions
during the day–night transition, several hours earlier than reported in
previous work. These conditions are fulfilled under weak synoptic forcing
when the local flow shifts associated with a mountain–plain circulation in
relatively complex orography. Under these special conditions, turbulence
becomes a crucial parameter in determining the ideal conditions for observing
LTM. Additionally, LTM observed profiles are also related to
a change in the atmospheric radiative characteristics under calm conditions.
Introduction
A lifted temperature minimum (LTM) profile is characterized by
an elevated temperature minimum close to the surface. Depending on the ground
characteristics, LTM is typically located between 10 and 50 cm above
the surface and observed at night. After sunset, if cloudless and calm
conditions exist and ground and air emissivities have similar values, the
air layer just above the ground can cool radiatively faster than the ground
itself and a minimum temperature appears several centimetres above the
surface. LTMs have been studied by means of observations
, numerical simulations
and laboratory experiments .
provided for the first time a detailed description of the
unexpected temperature minimum neglecting advective effects, and suggested
that the LTM might be related with radiation from the ground and the lower
layer of the atmosphere. Several years later, , and
confirmed the results obtained by ,
discarding instrumental errors by using more complex instruments.
took measurements over different terrain types to verify
that LTMs are not produced by advection and defined three
different types of temperature profiles, distinguishing between profiles with
the minimum temperature at the ground and LTM profiles caused
by advection. Additionally, they made measurements at different latitudes to
prove that the phenomenon was not restricted to the tropics. On the contrary,
showed some skepticism about the existence of LTM. For instance, he wondered why LTMs are not
overturned by convective instability. He was also concerned about the
precision of the measurements close to the ground. Later on,
suggested the existence of a haze layer near the ground
to explain the appearance of the LTM. Nevertheless, this approach was
discarded because this layer was never observed and the thermal diffusivity
required for its explanation was not realistic .
More recent studies have shown that LTM observations are common over
different natural, e.g. bare soil, snow and short grass and
artificial surfaces such as concrete or thermofoam
. studied in detail the
importance of surface characteristics for the appearance of LTM.
They demonstrated, by studying LTM formation over different surfaces
(aluminum, thermofoam and concrete), that decreasing surface emissivity
increases the intensity of an LTM and the near-ground temperature gradient.
Lowering surface emissivity with respect the overlying atmosphere can act to
change the temperature profile from a minimum temperature occurring at the
ground to an elevated temperature minimum. Therefore, terrain with an
emissivity close to that of the overlying air favours LTM formation.
summarized the main mechanisms related to
the occurrence of LTM. In his first summary, he introduced
a brief description of a model, which was later described in detail in
. They hypothesized that radiative cooling depends
on ground emissivity and air emissivity gradient. When the air emissivity
gradient is large, the temperature of the air close to the ground decreases
faster than the temperature of the ground and an LTM can be observed. Even
though the model presented a detailed solution for the air temperature
evolution considering surface emissivity, ground cooling and turbulence, it
did not include a detailed discussion of the energy budget near the ground,
which was introduced afterwards by .
Apart from ground thermal characteristics, calm conditions with low
mechanical turbulence are crucial to observe an LTM. For instance, LTM
intensity is weaker for high roughness length surfaces because it increases
both turbulence and emissivity . Moreover, field measurements
and models
show that
advection was weak when an LTM was observed. The LTM has only been reported for
a small number of cases where the friction velocities were above 0.1 ms-1,
and in those cases LTM disappeared relatively quickly
.
were the first to suggest a model which
appears to be in good agreement with observations. They studied the
importance of radiative, conductive and convective fluxes during LTM events.
This model was accepted until and
identified an error in the calculations of and
introduced a new model based on the work by . This model
includes the importance of suspended solid or liquid particles, which can
enhance radiative cooling.
, , and pointed out
the importance of radiation in the formation of LTM.
confirmed that near the surface, radiative cooling can be
orders of magnitude greater than values elsewhere in the boundary layer. With
very light winds, the importance of turbulence is nearly negligible compared
with radiation. Therefore, temperature evolution is mainly governed by
the radiative timescale . Moreover,
showed that a heterogenous distribution of aerosol
concentration can cause hyper-cooling close to the surface, which modifies
the atmospheric radiative cooling.
Another hypothesis explaining the appearance of LTM (or the
temperature maximum at upper levels, around 20–30 cm) during the night in
stable conditions is based on the competition between the radiative warming
of the lower layers (up to 50–70 cm) of the atmosphere, over a rapidly
cooling surface, and the turbulence cooling
. The first
process would drive the heat budget at 20–30 cm, but turbulence cooling
would temporarily be dominant around 10–15 cm.
Finally, daytime LTM measurements have been reported when near-surface temperature
inversions occur under specific conditions over the open Arabian Sea during
the summer monsoon season . These atmospheric conditions,
characterized by strong surface winds and high levels of sea salt particle
concentration in the boundary layer, are far away from the conditions
presented at night or here.
In summary, LTM occurrence varies depending on surface characteristics
(emissivity and thermal inertia), prevailing wind conditions (turbulence) and
atmospheric radiation. In contrast with previous studies, we analyse LTM
occurrences during the evening transition period. It is during this period
when the largest radiative cooling occurs . Our research
objective is to study the relevance of wind characteristics driven by
orography, turbulence, characterized by the Richardson number and radiation on the
appearance of LTM during the evening transition.
The study of the appearance of LTM, besides increasing the
knowledge of the physics of the surface layer, can also be relevant for
agriculture. The lifted temperature minimum can modify the occurrence of frost,
which has adverse effects on crops . Moreover, it can help to
describe the presence of radiation fog because, as it will be shown, the
presence of LTM is related with a variation of the radiation
.
The paper is structured as follows. In Sect. 2 we explain the measurements
used in this study, taken during the Boundary Layer Late Afternoon and Sunset
Turbulence (BLLAST) campaign. In Sect. 3, the temperature profiles are
analysed in detail and LTM characteristics are described. Section 4
investigates and presents the variables influencing LTM: wind characteristics
and friction velocity, turbulence and radiation. Finally, Sect. 5 summarizes
the results.
Measurements
To investigate LTM during the evening transition, we analyse
measurements acquired during the BLLAST field experiment .
This campaign was performed from 14 June to 8 July 2011 in southern France,
near to the Pyrenees. The campaign site extended over an area of
approximately 100 km2 covered with heterogeneous vegetation: mainly
grass, corn, moor and forest.
The most salient BLLAST objective was to obtain a detailed set of
meteorological observations during the evening transition to better
understand the physical processes that control it. For example, to improve the understanding of the effects of entrainment across the boundary layer top,
surface heterogeneity, horizontal advection, clouds, radiation and gravity
waves on the evening transition.
During intensive observational periods (IOPs), the atmosphere was heavily
probed by in situ measurements from masts, towers, tethered balloons,
radiosondes and manned and unmanned airplanes, as well as remote sensing
instruments such as lidar and radar wind profilers.
For the present work, the near surface temperature evolution is analysed
using the measurements taken at two masts (T1 and T2) separated by
approximately 468 m. Figure shows a plan view of the T2 area
and a side view of the T1 and T2 instruments. T1 was located at
43.1275∘ N, 0.36583∘ E and T2 at
43.1238∘ N, 0.36416∘ E. T1 was a 10 m mast
instrumented with four Campbell Scientific CSAT3 sonic anemometer
thermometers and Campbell Scientific E-TYPE model FW05 (12.7 µm
diameter) fine wire (FW) thermocouples at 2.23, 3.23, 5.2 and 8.2 m.
Closer to the ground, there were four additional FW05 12.7 µm FWs
at 0.091, 0.131, 0.191 and 0.569 m, which were only installed during
the IOPs. Temperature data at T1 were recorded at 20 Hz. The
influence of direct or indirect solar radiation has been taken into account
in the measurements. Moreover, showed that as the size of
the thermocouple goes down, the radiative influence is reduced. For
a 25 µm sensor a 0.1 K of error was observed. Our sensor
is half that size; hence, the error of the instrument should be lower than
0.1 K, which is smaller than the values of the LTM
intensity.
T2 was a 2 m mast with eight FW3 (76.2 µm diameter) FWs
located at 0.015, 0.045, 0.075, 0.14, 0.3, 0.515, 1.045 and 1.92 m
recording temperature data at 10 Hz. Additionally, separated
by approximately 2 m from T2, there was also a Campbell Scientific CSAT3
at 1.95 m, recording data at 20 Hz. To unify the measurements
taken by the different instruments, all the recorded data were averaged over
5 min intervals . This information was
complemented with an estimation of the skin temperature provided by
a Campbell Scientific IR120 infrared remote temperature sensor pointing
towards the surface. This infrared sensor measured temperature with a sampling
frequency of 3 Hz before 21 June 2011 and of 1 Hz after this day.
(a) Schematic horizontal view illustrating the location of
the instrumentation around T2, (b) photograph (looking west)
showing the instruments around T2, and (c) photograph (looking south) showing
the instruments around the T1 mast.
Near T2, one Kipp & Zonen CNR1 net radiometer was installed. The CNR1
sensor is able to measure upwelling and downwelling components of both the
shortwave solar (0.305–2.8 µm) and terrestrial radiation
(5–50 µm) separately. The CNR1 was installed at 0.8 m
above the ground.
The ground characteristics below both masts were conducive to observe LTM
. The ground in both cases was covered by long
grass, which has an emissivity of 0.986 . The vegetation
cover has low thermal conductivities which vary from 0.05 to
0.46 Wm-1K-1. However, the surface
surrounding T1 was covered by long grass while the T2 surface had some cut
grass over the terrain, which could cause some heterogeneity in the surface
thermal properties.
pointed out that, over grass-covered surfaces, the minimum
temperature during the night can be found just above the grass instead of
right at the surface. This phenomenon, which is associated with the
vegetative canopy, is sometimes confused with an LTM. observed
an LTM at 0.02 m above the grass. In our case study, the grass height
is short, around 0.03–0.07 m, and the observed LTM height occurred
above 0.1 m from the ground, that is, always above the grass.
For the following analysis, we selected different favourable IOPs with good
data availability from the T1 and T2 areas. The analysis is based on the
observations taken on 24, 25, 27, and 30 June and 1 and 2 July 2011. During these
IOPs, we have measurements from both towers, the infrared surface temperature
sensor and the radiometer. Almost all these IOPs were clear and calm days with
a mountain–plain circulation characterized by weak northerly winds during
the day switching to southerly at night. The synoptic situation did not show
any notable perturbation.
Observed LTM characteristics
During the BLLAST campaign, when LTM occurred it was observed at both masts.
Figure shows the evolution of potential temperature profiles
where an LTM is observed on 24 June 2011 (top panels) and 1 July 2011 (bottom
panels) recorded at T1 (left) and T2 (right). The LTM can be observed on both
days at both masts.
As illustrated in Fig. , three sensors on each tower were used
to detect and characterize LTM. First, the location of the
minimum temperature was identified (θbase). Next, the sensor
closest to the ground was defined as LTM▾.
Finally, the sensor located just above the base sensor
(LTM▴) was identified. An LTM is observed if
θbase-θLTM▾<0andθLTM▴-θbase>0.
During this period, LTM intensity is calculated following :
LTMintensity=θbase-θLTM▾.
The LTM duration was defined as the period when the LTM conditions outlined
above were fulfilled. Table presents a summary of the following LTM
characteristics for the different IOPs: height, intensity absolute values
and duration of the phenomenon.
Characteristics of the LTM at T1 and T2 for all the studied IOPs.
IOPLTMLTM heightLTM heightLTM intensityLTM intensityLTM durationLTM durationT1 (m)T2 (m)T1 (K)T2 (K)T1 (min)T2 (min)24 June 2011Yes0.1310.07–0.140.350.718:15–18:2517:50–18:5025 June 2011Yes0.1310.3–0.5–17:50–18:2027 June 2011No––––––30 June 2011Yes0.1310.07–0.140.30.517:55–18:1517:55–18:151 July 2011Yes0.1310.07–0.140.350.717:35–17:5517:30–18:202 July 2011Yes0.1310.07–0.140.30.517:35–18:0517:10–18:10
Temporal evolution of vertical potential temperature
profiles with an observed LTM on 24 June 2011 (top) and 1 July 2011 (bottom)
measured at T1 (left) and T2 (right).
An LTM was observed during the evening transition for all IOP days except on
27 June 2011. An LTM forms at similar heights on both towers. For example, at
T1 a height of around 0.131 m was typical, while LTM heights were
between 0.075 and 0.14 m (except on 25 June 2011) at T2.
Unfortunately, limitations in the vertical resolution of the measurements
prevent a more precise determination of the LTM heights. In spite of this
consistency, there are clear differences between the detailed LTM
characteristics on different IOPs and at the different towers. On 24 June
2011, an LTM was observed during 10 min at T1 and for 40 min
at T2. Greater LTM intensity (0.7 K) was observed at T2
compared to T1 (0.35 K). On 25 June 2011, an LTM was detected at T2 at
a slightly higher level, around 0.3 m with an intensity of
0.5 K. This height is in the range of LTM heights reported by
. On 25 June 2011, FWs were installed at T1 after 19:30 UTC (universal time coordinated); therefore, LTM comparisons cannot be made.
A completely different situation was observed on 27 June 2011; with no clear
LTM development. T2 measurements showed indications of an LTM formation which
did not progress (not shown).
Illustration of the methodology used to identify LTM and quantify
its intensity.
On 30 June 2011, T1 showed a slightly lower-intensity (0.3 K) LTM
starting around 18:00 UTC and lasting less than 20 min. A slightly
lower-intensity LTM was also observed at T2 with an intensity of
0.5 K. On 1 July 2011 a clearly marked (0.7 K) LTM was
observed at T2 during 1 h. On the other hand, T1 showed
a less pronounced LTM (0.35 K), which persisted only 20 min.
Finally, on 2 July 2011 T2 showed an LTM intensity of around 0.5 K
with a duration of more than 1 h. However, T1 showed an intensity of 0.35 K with a duration of 40 min.
Due to the variations in sensor heights at the two locations, the LTM
intensity can vary from one tower to the other. Day to day variations at
a single location, however, can be compared. Specifically, our definition of
LTM intensity is based on the temperature measured closest to the ground
which, in order to detect an LTM, needs to be warmer than the LTM. The elevation
of the sensor closest to the ground differs for T1
and T2 (about 9 and 1.5 cm, respectively); thus, the two locations'
intensities are not strictly comparable. As shown in Table 1, the LTM
intensity at T2 is always roughly twice the value observed at T1, which is
most likely due to the fact that the lowest thermocouple at T1 is still
influenced by the cold air associated with the LTM and an additional increase
in temperature towards the surface is not resolved.
The analysis of wind conditions is crucial for understanding the influence of
mechanical turbulence on the formation of LTM. Since during all of the
IOPs presented in the analysis weak synoptic forcing occurred,
orography will be the main driving mechanism of surface winds during the evening
transition .
Figure shows the temporal evolution of the averaged 2 m
wind speed and direction every 5 min observed at T1 and T2. The
observed wind directions shown in Fig. a and b clearly indicate for
most of the days a typical mountain–plain circulation :
daytime plain–mountain wind (northerly over the Lannemezan Plateau toward
the Pyrenees), early evening calm conditions and nighttime mountain–plain
wind (southerly). The wind speed observations (see Fig. c, d)
indicate slightly weaker winds at T2, most likely due to the presence of
trees near to T2 and to the difference in the surface cover. Before 17:30 UTC,
2.5 and 2 ms-1 wind speeds were observed at T1 and T2,
respectively. At 17:30 UTC, the wind speed started to decrease except on
27 June 2011, indicating the beginning of the evening calm period. However,
the decrease rate was not the same for all the IOPs, being faster on 24 June
and 1 and 2 July 2011. The wind speed continued decreasing until 18:30–19:00 UTC
when the wind was around 0.5 ms-1 at both masts. During this
period, the wind direction turned from northerly to southerly progressively
(see Fig. a, b). After 19:00 UTC, surface flows from the mountains
dominated, with increasing wind speed (see Fig. c, d).
In order to analyse why the wind-speed decay during the evening was
different for the analysed days, a WRF-mesoscale simulation
was performed with 3 km horizontal resolution from
29 June at 00:00 UTC until 3 July 2011 at 00:00 UTC. When analysing the atmospheric
conditions at low levels during the evening, a surface northerly wind is
simulated at Lannemezan (43∘12′ N, 0.39∘ E)
during the 3 days. However, on 30 June 2011 this northerly wind is
simulated until a later hour than on 1 and 2 July 2011. This is due to the
lower temperatures simulated in the Pyrenees mountain range on 30 June 2011
(not shown). A similar reason could explain the lowest wind decrease observed
on 25 June 2011.
In stable conditions, postulated that the wind speed at
0.25 m must be less than 0.4 ms-1 to observe an LTM over
short grass. In our study case, sensors measuring wind speed were located at
2 m. Therefore, we need to extrapolate this value to 0.25 m
to be able to compare with previous results. To do this a log-law
approximation for neutral stability conditions was utilized, namely
v≈vrefln(z/z0)ln(zref/z0),
where v is the wind speed at height z, vref is the
wind speed at height zref=2m, and z0 is the roughness
length (0.03 m in our case). The results from this approximation show
that for all the analysed days except 27 June 2011, the wind speed at
0.25 m is below 0.4 ms-1.
Temporal evolution, from 17:30 to 20:00 UTC, on all the studied days
of the observed 2 m wind direction (top) and speed (bottom) averaged
every 5 min at T1 (left) at 2.3 m and T2 (right) at
2 m.
Turbulence
The gradient Richardson number (Rig) is a crucial parameter in
the study of the LTM during stable night conditions. observed
that Rig>0.1 is needed to observe an LTM over different terrain
in stable conditions. The gradient for the Richardson number is defined as
Rig=gθv‾∂θv‾/∂z∂U/∂z2+∂V/∂z2,
where g is the gravity acceleration, θv‾ is
the virtual potential temperature, and U, V the horizontal wind
components.
To estimate Rig, the potential temperature vertical gradient was
computed using the θLTM▴ and θbase
as, by definition, it is not possible to observe an LTM unless the
∂θv‾/∂z is positive directly
above the height where the LTM is observed. Moreover, as we
do not have measurements of the wind speed at the LTM height or at
LTM▴, we approximate U and V using Eq. ().
Figure shows the temporal evolution of Rig
during the evening transition obtained by using the data measured at T1 on
all the studied days. As expected, as the stable surface layer develops,
Rig significantly increased for all the days studied except
for 27 June 2011, when Rig remains nearly constant and close to
zero. During this day, an LTM was not observed because large mechanical
turbulence in the lower part of the boundary layer existed.
Temporal evolution of the Richardson number from 17:30 to
19:00 UTC on all the studied days at T1.
Temporal evolution of u* from 16:00 to 24:00 UTC on all the
studied days at (a) T1 and (b) T2.
An opposite situation occurred on 24 June and 1 and 2 July 2011. On these
days a large increase of the Rig values is observed when
Rig becomes positive and the LTM appeared. The large increase in the
Rig is related to a fast decrease of mechanical
turbulence. Therefore, on these 3 days, LTMs were clearly
observed with a large LTM intensity. On 25 and 30 June 2011 there was a less-pronounced increase of the Rig values. These days have
a smoother decrease of turbulence as well as a lower intensity of LTM.
As mentioned, suggested a minimum Rig threshold
for LTM formation of Rig≳ 0.1. During nighttime, when the
main destabilizing force is mechanical turbulence, Rig can be
used to define the conditions for observing LTM. However, this
Rig threshold cannot be compared with our results because we
observe an LTM when ∂θv‾/∂z is
changing at the surface. Therefore, we cannot define an exact threshold for
LTM formation and we focus our analysis in the change of the increase rate of
the Rig values.
Decrease of mechanical turbulence during the afternoon transition can be also
studied by using friction velocity (u*). Figure shows the
temporal evolution of u* at 2 m during the evening transition for
all the studied days with a 5 min average. Due to the orography,
during the afternoon, u* decreased from around 0.25 ms-1 to
values below 0.1 ms-1 (around 18:30 UTC at T1 and 18:00 UTC at
T2). Afterwards it slightly increases but remains at lower values.
pointed out that an LTM can occur with friction
velocities greater than about 0.1 ms-1, but the layer slowly
fades away. In our study case, during most of the IOPs u* was reduced to
values lower than 0.1 ms-1 shortly after the LTM occurrence
except on 27 June 2011, when friction velocity clearly presented values
higher than 0.1 ms-1 during the evening transition at both
masts. Therefore, during this day turbulence prevented the appearance of
an LTM. Moreover, on 30 June 2011 u* had low values but only during a short
period during which an LTM occurred (see Fig. a, b).
used wind speed fluctuations to analyse turbulence and its
influence on LTM occurrence. Figure shows the horizontal wind speed
measured at 20 Hz and its mean value (a 500 s moving average)
for two different IOPs, 24 June and 27 June 2011, which represent the most
extreme cases. The LTM occurrence on 24 June (see Table 1) is associated with
a clear decrease not only of mean wind speed but also of wind speed
fluctuations (see Fig. a). On the contrary, on 27 June, when an LTM is
not observed, Fig. b shows that neither mean wind speed nor
turbulence intensity decrease during the evening transition. By comparing
these facts with the parameters described in Table 1, we can directly relate
turbulence and mean wind velocity with the intensity of the LTM. IOPs with
a clear decrease on turbulence during the afternoon transition, such as
24 June and 1 or 2 July 2011, present larger LTM intensity. Those days with a lower
or non-existing decrease of wind speed fluctuations have a less-pronounced
LTM or no LTM present.
Temporal evolution of mean wind speed and deviation from mean wind
speed on 24 (top) and 27 June 2011 (bottom).
Radiation
, and
pointed out the radiative origin of LTM. For this reason, we also analyse the
radiation measurements taken by the radiometers located near T2.
Unfortunately, during all the days of the campaign, a shadow produced by the
60 m tower located 160 m to the northwest of T2 affected
the shortwave and net radiation measurements. Consequently, here we can only
analyse the upwelling longwave radiation recorded by the Kipp & Zonen
CNR1 radiometer located at 0.8 m. Additionally, we estimate longwave
radiation at the LTM height by using the conservation of heat equation
:
∂θ‾∂t+Uj‾∂θ‾∂xj=νθ∂2θ‾∂xj2-1ρ‾Cp∂Q*∂xj-LvEρ‾Cp-∂(uj′θ′‾)∂xj,
where xj represents (x,y,z) for j=(1,2,3), θ‾ and θ′ are the mean and fluctuating components of the potential temperature, νθ
is the kinematic molecular diffusivity for heat in air, Q* is the net
radiation, Lv is the latent heat of vaporization of water, E is the
phase change rate, ρ is density of the air, Cp is the specific
heat at constant pressure for moist air and Uj and uj′ are the mean and fluctuations of the wind components
(u,v,w) for j=(1,2,3).
The first term represents the tendency of the potential temperature. The second term
describes the advection of heat by the mean wind. The third term is the mean
molecular conduction of heat. The fourth term represents the net radiation flux
divergence. The fifth term describes the latent heat release and the sixth
term is the divergence of the turbulent heat flux. Despite that large values of
latent heat were measured at noon during the BLLAST campaign, the fifth term of
Eq. (5) is smaller when compared with the other terms.
This term on 1 July 2011, for instance, was approximately
0.15 Kms-1 during daytime but decreased to values close to
0.01 Kms-1 during the evening transition.
Temporal evolution of upwards longwave radiation (Lu)
(a) measured at 0.8 m on 24, 25, 27 and 30 June 2011 and 1
and 2 July 2011 and (b) estimated, by using Eq. (), at LTM
height on 24 and 25 June 2011 and 1 and 2 July 2011 using Eq. ().
If we consider very light winds, horizontal homogeneity and neglect
subsidence, the heat equation can be written as
∂θ‾∂t=νθ∂2θ‾∂z2-1ρ‾Cp∂Q*∂z-∂(w′θ′‾)∂z.
We integrate this equation from the ground to LTM height and average it
every 5 min. We obtain an approximation for the radiation at LTM height,
which reads
Q*ρ‾Cpz=LTM=-νθ∂θ‾∂zz=0m+Q*ρ‾Cpz=0m-w′θ′‾z=2m.
It is important to note that the tendency of potential temperature vertically
integrated from the surface to the LTM height is much smaller than the other
terms and for this reason is neglected.
The second term of Eq. () is computed by using the temperature
measured by the IR120 infrared surface temperature sensor and the lowest
thermocouple located at 0.015 m, and we approximate νθ to
the ground molecular diffusion value. Moreover, to estimate the heat flux we
use the measurements at the lowest SAT, located at 2 m, even though it is
outside the integration domain. During evening transition, most of Q*ρ‾Cpz=0m and Q*ρ‾Cpz=LTM correspond to longwave
radiation. Therefore, considering that the main contributor of the upwelling
longwave radiation (Lu) is the ground, we compute the longwave radiation emitted
at the ground using the ground temperature (Tg) measured by the IR120
infrared surface temperature sensor as
Q*ρ‾Cpz=0m≃Luz=0=εσbTg4,
where ε is the emissivity of the ground (0.986) and σb is the Stefan–Boltzmann constant.
To discard LTM produced by variations of the ground characteristics during
the LTM period, we analysed the evolution of ground emissivity by using the
measurements of longwave radiation at 0.8 m and temperature at
0.015 m. The results do not shown any particular modification during
the occurrence of LTM. Moreover, a sensitivity study changing the value prescribed of
the surface emissivity has been also performed
without qualitatively modifying the results presented below.
Figure a shows the temporal evolution of the upwelling longwave
radiation measured by the Kipp & Zonen CNR1 net radiometer at
0.8 m. During the afternoon transition, we observe a nearly constant
decay rate for the upwelling longwave radiation at 0.8 m. Longwave
radiation at the ground calculated by using Eq. () presents
a similar evolution (not shown). However, we cannot correlate these two
upwelling longwave radiations to analyse if there is any difference to
explain the appearance of the LTM because the IR120 infrared surface
temperature sensor and the longwave net radiation sensor have different
response times (<1s for the IR120 infrared camera and 18 s
for the Kipp & Zonen CNR1 net radiometer). Moreover, both sensors were
not sampling using the same data logger. Consequently, we focus on analysing
the differences in the decay rate of upwelling longwave radiation at
0.8 m and the longwave radiation at LTM height calculated by using
Eq. ().
Figure b shows the temporal evolution of the longwave radiation at
the LTM height estimated by using Eq. (). This figure does not
include the longwave radiation at the LTM height for 27 and 30 June 2011
because of some problems occurred with the IR surface temperature sensor
measurements during these IOPs. In contrast to Fig. a, the longwave
radiation decay rate is not constant and increases around 17:30–18:30 UTC,
when the LTM appears for some IOPs. This increase in the longwave radiation
decay rate can lead to a more rapid local decrease in air temperature and the
formation of an LTM.
It is important to note that with the deployed instruments during the
campaign, we are not able to study the vertical profile of the air
emissivity. We use longwave radiation measured at 0.8 m and the
closest measurements of temperature (2 m) to estimate air emissivity,
and no variation of the air emissivity occurred around the time of the LTM
for any of the analysed days (not shown).
reported that LTM intensity decreases when clouds were
present, also suggesting the importance of radiation in the phenomenon. By
analysing the ceilometer measurements obtained during BLLAST (not shown),
a completely clear sky is reported for all the IOP evening transitions except on
30 June 2011. From the previous section, we know that during this day even
though the conditions of turbulence were acceptable to observe LTM and LTM
presented similar values to other IOPs, there was a combination of low
intensity and short duration not present in other IOPs. These
LTM characteristics can be also caused by the presence of
clouds.
Conclusions
The presence of a lifted temperature minimum during the evening
transition is studied by means of observations taken during the BLLAST
campaign. The campaign site presented ground characteristics suitable for
observing LTM with large ground emissivity and thermal inertia.
During this period of the day, LTMs were observed at different
heights and with different intensity and duration during all IOPs except on
27 June 2011.
With the instrumentation deployed during the campaign we were not able to
verify all the previous hypotheses to explain the appearance of LTM. For
instance, the presence of aerosols at lower height were not monitored during
the campaign.
Additionally, it would be difficult to analyse, by using observations, the
budget between radiation warming and turbulence cooling during the evening
transition. While small Kaijo–Denki sonics could be used at 15 and 30 cm
to measure cooling via sensible heat flux divergence, radiation measurements
would be much more difficult at those heights close to the surface, and not
possible with commercial pyranometers.
Moreover, it is important to note that the research study focusses on the
afternoon transition. To our knowledge, the heat budget (the competition
between turbulent fluxes and radiation divergence) at the different levels
close to the surface has not been studied during this period of the day. In
fact, the current MATERHORN observational campaign was
partially designed to study the evolution of the heat budget during the
afternoon/evening transition.
By studying the wind conditions characterized by a mountain–plain flow, we
conclude that the days with a more marked decrease of mean wind speed and
wind speed fluctuations (24 June or 1 July 2011) have a more intense LTM. On
the other hand, on the days without a reduction of wind speed, such as
27 June 2011, LTM cannot be observed during the evening
transition.
Analysing Rig during the evening transition, we observe that
the LTM is detected on days with a faster increase of Rig, i.e.,
a faster decrease of mechanical turbulence. However, due to the fact that
∂θv‾/∂z is changing sign during
the evening transition, no threshold of Rig can
be defined.
Finally, the longwave-radiative conditions are analysed. We study the
differences in the decay rate of the upwelling longwave radiation at
0.8 m and the longwave radiation at LTM height. Longwave radiation at
LTM height decays at two different rates in contrast to the upwelling longwave
radiation decay at 0.8 m which is constant in time. This change in
the radiative conditions can modify the temporal evolution of the potential
temperature creating the LTM.
To conclude, during evening transition it is possible to observe the lifted
temperature minimum over a terrain with moderate/large emissivity and thermal
inertia. In this study case, really calm conditions were observed during
evening transition due to the presence of the Pyrenees which
produces an early evening calm period easily defined through a change in the
wind velocity and turbulence. Moreover, a change in the radiative conditions
was observed during an LTM period which confirms its radiative origin.
Acknowledgements
This project was performed under the Spanish MINECO projects CGL2009-08609,
and CGL2012-37416-C04-03. The MODEM radio sounding station and the UHF
wind profiler have been supported by CNRS, Université Paul Sabatier and
the FEDER program (contract no. #34172 Development of the instrumentation of
Observatoire Midi-Pirénées-PIRENEA-ESPOIR). The 60 m tower equipment
has been supported by CNRS, Université Paul Sabatier and the European POCTEFA
720 FluxPyr program. One EC station was supported by Wageningen University
and two EC stations were supported by the University of Bonn and DFG project
SCHU2350/21.
The BLLAST field experiment was made possible thanks to the contribution of
several institutions and support: INSU-CNRS (Institut National des Sciences
de l'Univers, Centre National de la Recherche Scientifique, LEFE-IDAO
program), Météo-France, Observatoire Midi-Pirénées (University of
Toulouse), EUFAR (EUropean Facility for Airborne Research) and COST ES0802
(European Cooperation in Science and Technology). The field
experiment would not have occurred without the contribution of all
participating European and American research groups, which all have
contributed in a significant amount. The BLLAST field experiment was hosted by
the instrumented site of Centre de Recherches Atmosphériques, Lannemezan,
France (Observatoire Midi-Pirénées, Laboratoire d'Aérologie). The
BLLAST data are managed by SEDOO, from Observatoire Midi-Pirénées.
This work was partially supported through funding from the U.S. Office of Naval Research award #N00014-11-1-0709, Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program.
Finally, we would like to thank K. R. Sreenivas from the Jawaharlal
Nehru Centre for Advanced Scientific Research and S. Wacker from the
Physikalisch-Meteorologisches Observaturium Davos for fruitful discussions
about atmospheric radiation.
Edited by: S. Galmarini
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