Quarterly Journal of the Royal Meteorological Society
An observational study of an arctic front during the IPYTHORPEX 2008 campaign
Journal:
QJRMS
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Manuscript ID:
Wiley - Manuscript type:
Date Submitted by the Author:
n/a
Mc Innes, Harold; The Norwegian Meteorological Institue, Division for
Climate
Kristjansson, Jon; University of Oslo, Geosciences;
Rahm, Stephan; Deutschen Zentrums für Luft- und Raumfahrt (DLR),
Røsting, Bjørn; Norwegian Meteorological Institute,
Schyberg, Harald; Norwegian Meteorological Institute,
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Keywords:
Research Article
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Complete List of Authors:
QJ-11-0169.R3
Arctic front, observations, Wind Lidar, Mesoscale cylones, NWP models
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An observational study of an arctic front during the IPY-THORPEX
2008 campaign
Harold Mc Innes
The Norwegian Meteorological Institute, Norway
Jón Egill Kristjánsson
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University of Oslo, Norway
Stephan Rahm
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Deutsches Zentrum für Luft- und Raumfahrt, Germany
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Bjørn Røsting
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The Norwegian Meteorological Institute, Norway
Harald Schyberg
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Quarterly Journal of the Royal Meteorological Society
The Norwegian Meteorological Institute, Norway
Corresponding author: Harold Mc Innes, The Norwegian Meteorological Institute
Postboks 43 Blindern 0313 OSLO; email: haroldmi@met.no
Quarterly Journal of the Royal Meteorological Society
Abstract
The fact that severe weather associated with polar lows and arctic fronts still comes
unforeseen and puts human life at risk shows that an effort towards increased understanding
of them is required. The observations of an arctic front by dropsondes and Doppler Lidar
carried onboard a research aircraft during the IPY-THORPEX field campaign offered a rare
opportunity to investigate the mesoscale structure of the front and to validate the output from
operational numerical weather prediction (NWP) models. The observations revealed features
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similar to those of polar fronts such as a relatively steep frontal zone, the presence of a strong
low-level jet and an elevated dry slot, making the arctic front appear as a shallow cold front
confined to levels below 700 hPa. The dry slot indicated the presence of a downfolding of the
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tropopause, and together with the observations of an upper-level jet this strongly supports the
inclusion of an arctic tropopause fold connected to the arctic jet stream in a conceptual model
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of the tropopause. A comparison between data from operational numerical weather prediction
models and observations obtained during the flight shows that the models simulated the broad
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features of the frontal zone such as jets, dry slot and the depth of the front fairly well although
parts of the fronts were slightly misplaced. However the models failed completely in their
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simulations of one of the three mesoscale cyclones associated with the front as they located it
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over the coast of Northern Norway while the correct location was over the Greenland Sea
according to the observations and analysis.
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1. Introduction
Due to the available potential energy associated with horizontal temperature gradients, a
baroclinic zone represents a potential for the development of extratropical cyclones with
associated frontal systems. Hence the dynamics and structure of fronts and baroclinic zones
have been of great interest to both forecasters and researchers (Shapiro et al., 1999). The
theory for development of synoptic scale cyclones associated with baroclinic instability is
described by e.g. Holton (2004), and the role of baroclinic instability in the formation of
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mesoscale cyclones such as polar lows was first highlighted by Harrold and Browning (1969)
and later investigated by Van Delden et al. (2003) and Yanase and Niino (2007). Polar lows
are often associated with arctic fronts (Rasmussen et al., 2003; Shapiro et al., 1987 a), which
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are relatively shallow baroclinic zones north of the polar front delimiting the arctic air masses
originating over the sea ice and the warmer maritime polar air. While the link between arctic
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fronts and polar lows is a major motivation for research, a study by Grønås and Skeie (1999)
showed that arctic fronts themselves may be accompanied by severe low level winds without
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the development of a polar low. In their study Grønås and Skeie simulated an arctic front
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caused by an outbreak of cold air between Northern Norway and Spitsbergen with a
numerical weather prediction (NWP) model, mainly focusing on the wind pattern.
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In February and March 2008 a major aircraft-based field campaign (IPY-Thorpex) was
dedicated to the assessment of severe arctic weather systems over the Norwegian and Barents
Seas (Kristjánsson et al., 2011). A total of 12 flights were carried out by the DLR Falcon
research aircraft from the base at Andøya in Northern Norway, and on the 28 February an
Arctic front extending west-east from Greenland to Norway was observed by 15 dropsondes
released from the aircraft as well as by lidar systems carried on board. The large amount of
Quarterly Journal of the Royal Meteorological Society
dropsonde observations combined with lidar measurements is unique for arctic fronts, and the
present study is mainly based on these data. The study’s main objective is to shed light on the
structure of this arctic frontal system through an investigation of temperature, wind and
humidity observed during the flight.
Arctic fronts are believed to occur frequently over the Greenland and Norwegian Seas
(Rasmussen et al., 2003), but the observations of these frontal systems are few due to a sparse
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network of meteorological stations in this region. Compared to arctic fronts, the observations
of polar frontal systems are numerous, as the conventional observational network is denser
further south. Browning and Pardoe (1973) performed a case study of pre frontal low-level
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jets based on data from routine soundings and wind observations from a Doppler radar. They
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investigated 6 different cold fronts over England and in each case the observations revealed
pre frontal low-level jets with maximum wind speeds between 25 and 30 ms-1 at 850 to 900
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hPa. For one of the cases they also analysed the vertical wind and found a vertical velocity of
typically 0.1ms-1 associated with slantwise convection above the cold front. In the present
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study we will analyse vertical wind speeds measured by the Doppler lidar.
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Several field campaigns have addressed cyclonic activity on the polar front, and one of these
was the Fronts and Atlantic Storm-Track Experiment (Fastex: Joly et al., 1997) which took
place in January and February 1997. On 6 February 1997 a cold front connected to a cyclone
located over the Atlantic Ocean was observed by both dropsondes and Doppler radar, and the
data were used by Wakimoto and Murphey (2008) to investigate the mesoscale structure of
the frontal zone. They found a near-surface horizontal temperature gradient of 6 – 7 K per 100
km, and their cross sections indicated that the frontal zone extended up to approximately 500
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– 600 hPa. On the warm side of the front the observations revealed a 34 ms-1 low level jet, and
an elevated slot of dry air 100 – 200 km ahead of the cold front indicated descent in that area.
Neiman et al. (1993) investigated observations of an extratropical cyclone obtained during the
Experiment on Rapidly Intensifying cyclones over the Atlantic (ERICA). The cyclone
developed outside the coast of North America and was observed by dropsondes and airborne
radar on 4 and 5 January 1989. A cross section through the cold front associated with the
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cyclone indicated an almost vertical frontal zone below 800 hPa, gradually sloping westwards
with height. Observations of the cold front as well as the warm front and the bent-back warm
front revealed the presence of a low-level jet and that the frontal zones extended up to levels
of 450 to 550 hPa.
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Shapiro et al. (1987 b) proposed a conceptual model of the tropopause including an arctic
tropopause fold associated with the arctic front in addition to the tropopause folds associated
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with the polar and subtropical fronts. This conceptual model was based on observational
studies of a cold vortex east of Greenland and an outbreak of arctic air over North America.
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Dropsonde and columnar ozone observations from the first case revealed an arctic jet stream
approximately 100 hPa lower than typical polar jet streams, as well as a tropopause fold. The
outbreak over North America reached as far south as Florida, and rawinsonde observations
indicated the presence of an arctic jet stream at 370 hPa north of a polar jet stream at 300 hPa
with associated arctic and polar fronts. In addition to this Shapiro et al. (1987 b) presented a
cross section of potential temperature and wind speed over North America extending from
80° N to 30° N, revealing three jet streams with associated fronts and tropopause folds. The
threefold conceptual model proposed by Shapiro et al. includes an arctic jet at 70° N, a polar
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jet at 45° N and a subtropical jet at 25° N. Except for the fact that the arctic tropopause fold is
considerably deeper and the arctic jet is at a lower altitude compared to it’s polar counterpart,
the structures of polar and arctic frontal systems are similar in this conceptual model. The
observations of the 28 February 2008 frontal system will be discussed in light of the
conceptual model of Shapiro et al., and according to this model we would expect to find a
shallow version of the polar frontal system that was described in e.g. Wakimoto and
Murphey’s (2008) observational study.
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Because of human activity in The Arctic, such as fisheries and an increasing gas and oil
exploration, warnings of weather hazards are of great importance. In this regard the
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observations captured on 28 February 2008 are of great value as they both bring new insight
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into arctic weather systems and they offer a rare opportunity to verify NWP models in this
region. In their numerical study of an arctic front Grønås and Skeie (1999) found that
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although the quality of the model simulations were sufficient to investigate the frontal
structure, the model simulated too low temperatures and too weak winds compared to
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observations. A study of a polar low that occurred in connection with an arctic front over the
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Norwegian Sea on 3 and 4 March 2008 (Kristiansen et al., 2011) indicates that operational
models still have problems with these weather systems. Another polar low that developed
over the Norwegian Sea between 16 and 17 March was also connected to an arctic front, and
in this case model predictions were very poor (Kristjánsson et al. 2011). We will in the
present study use observations to investigate how well two operational NWP models
predicted the structure of the arctic front. Since polar low development to a large degree is
associated with arctic fronts, it is reasonable to believe that a model’s ability to predict polar
lows is at least partly dependent of its ability to simulate arctic fronts.
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2. Synoptic overview
We will here analyse the synoptic scale features that led to the development of the arctic front
by investigating the objective analysis of sea level pressure (SLP), the 925 hPa equivalent
potential temperature (θE) as well as the height of the 500 hPa surface and the 500-1000 hPa
thickness during the period from 27 February 00 UTC to 28 February 12 UTC. The SLP
analysis from 27 February 00 UTC (Figure 1 (a)) shows a rather complicated synoptic
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situation over the Greenland and Norwegian Seas, with a 989 hPa cyclone east of Greenland
at 71.7 º N and 14 º W, a 976 hPa low with its centre outside the Norwegian coast at 66 ° N,
3° E and a developing trough at 72 º N , 2 º E over the Norwegian Sea. An assessment of the
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500 hPa surface and 500-1000 hPa thickness valid at the same time (Figure 1 (b)) reveals that
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the cyclone outside the Greenland coast is a cold core system with a thickness of 4980 m. The
cyclone was associated with an outbreak of cold air that originated over the Greenland Sea
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north of 77° N and was advected southwards during 25 and 26 February by a northerly wind
field. The increasing intensity with height is typical for a cold core cyclone, and follows from
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the thermal wind equation (Van Delden et al., 2003). Both the 925 hPa θE (Figure 1 (a)) and
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500-1000 hPa thickness (Figure 1 (b)) indicate a baroclinic zone over the Norwegian Sea
delimiting the cold air mass towards Greenland from warmer air adjacent to the Norwegian
coast.
Twelve hours later, at 27 February 12 UTC the SLP analysis (Figure 2(a)) shows that the
trough over the Norwegian Sea has moved towards the northwest to 73.3 º N, 4.7 º W and
developed into a 980 hPa cyclone, while the 925 hPa θE analysis (Figure 2 (b)) shows that the
baroclinic zone in the same area has sharpened. The height of the 500 hPa surface and the
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500-1000 hPa thickness (Figure 2 (b)) show that the developing cyclone has a warm core and
we will hereafter refer to it as WC1. The WC1 is located at the northeastern edge of the cold
core low which is slightly further southwest than 12 hours earlier. The low outside the
Norwegian coast now has its centre at 67.1 ° N, 4.5 ° E (Figure 2 (a)), and also this cyclone is
a shallow warm core system (Figure 2 (b)), hereafter termed WC2. The fact that the two warm
core lows are shallow is consistent with the thermal wind equation (Van Delden et al., 2003).
While the SLP distribution over the Greenland Sea gives rise to advection of cold air towards
the southeast, the SLP field off the Norwegian coast gives advection of warm air towards the
northwest.
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The 925 hPa θE analysis from 28 February 00 UTC (Figure 3 (a)) and the 500-1000 hPa
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thickness (Figure 3(b)) show the result of this simultaneous advection of cold air towards the
southeast and warmer air towards the northwest as a reversed frontal zone over the Norwegian
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and Greenland Seas separating relatively warm air to the north and colder air to the south.
WC1 has now moved further towards the northwest and is located over the Greenland Sea
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(Figure 3(a)) at the northern edge of the upper level cyclone, which has approximately the
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same position as 12 hours earlier. At the same time WC2 has moved northwards and is now
located at 70 ° N, 4.6 ° E over the Norwegian Sea.
During the next twelve hours the WC1 moved further towards Greenland, and then
southwards along the eastern coast of Greenland as it filled. At the same time WC2 moved to
the northwest over the Greenland Sea and the SLP analysis from 28 February 12 UTC (Figure
4 (a)) shows this low at 74° N, 12° W while the trough slightly further south shows the
remains of WC1. At 500 hPa (Figure 4 (b)) the upper level cyclone is still pronounced and has
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approximately the same position and strength as 12 hours earlier. The SLP analysis (Figure 4
(a)) shows a 992 hPa closed low over the easternmost part of the Norwegian Sea close to the
Lofoten Islands, and the 500 hPa height and 500-1000 hPa thickness (Figure 4 (b)) show that
also this low has a warm core, and we have termed it WC3. Compared to WC1 and WC2,
WC3 is less intense as it has a relatively broad centre and weak pressure gradients.
The research flight took place between 1126 UTC and 1452 UTC on 28 February, but the
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observations obtained during the flight were not assimilated into the objective analyses.
Nevertheless we found that the ECMWF analysis reflected the observed locations of the
different cyclones and frontal zone fairly well.
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Both the 925 hPa θE analysis in Figure 4 (a) and the 500 -1000 hPa thickness (Figure 4(b))
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indicate a reversed frontal zone over the Norwegian and Greenland Seas with an almost zonal
orientation west of 0° W and a horizontal temperature difference across the front of
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approximately 10 K at 925 hPa. The coldest air is on the southern side of this zone with
minimum 500-1000 hPa thickness of approximately 5020 m associated with the cold core just
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east of Greenland, while the thickness north of the frontal zone is approximately 5180 m at
the same longitude. The 925 hPa θE as well as the 500 – 1000 hPa thickness from 28 February
1800 UTC and 29 February 00 UTC (not shown) indicate that this front weakened during the
next 12 hours, and that it was most pronounced during daytime on 28 February.
In Figure 5 we present the NOAA satellite images of the synoptic scale development, with the
early stage of WC1 visible as a cluster of clouds over the Norwegian Sea in Figure 5 (a) from
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27 February 0228 UTC and as a distinct cyclone in Figure 5 (b) from 27 February 1113 UTC.
In figure 5 (c) from 28 February 0359 UTC it is possible to recognize the arctic front as a
cloud band over the Norwegian and Greenland Seas and in the image from 28 February 1149
UTC (Figure 5 (d)) it has become a relatively sharp cloud band extending from the east coast
of Greenland to the coast of Northern Norway, its location being consistent with the frontal
zone indicated by the θE analysis in Figure 4 (a).
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In summary we have seen a complicated development, involving three warm core lows as
waves on the arctic front and an upper level cold cyclone, leading to the almost zonal frontal
zone with the cold air masses to the south. The distance between the warm core cyclones
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gives a suggested wavelength of approximately 700 km, which is considerably shorter than
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the wavelength of maximum baroclinic instability of 4000 km indicated by the two-layer
model and the 5500 km for Eady waves (Holton, 2004). However smaller vertical scale, weak
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static stability and increased Coriolis parameter give maximum baroclinic instability at shorter
wavelengths. Mansfield (1974) applied the Eady theory in a case study of two polar lows
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associated with a shallow baroclinic zone, and showed that the fastest growing wavelength
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was comparable to the observed wavelength of 550 to 750 km. Similarly Moore and Peltier
(1989) applied the primitive equations in a stability analysis of a shallow baroclinic zone in
which a wavetrain of four polar lows developed. They found three different branches of
unstable waves, one of them with a wavelength of maximum growth of 500km, corresponding
to the observed wavelength. A study of a two layer Eady model by Blumen (1979) showed
that baroclinic instability has a maximum at short wavelengths when the lapse rate of the
lowest layer is close to adiabatic, and this model has been successfully used to explain the
development of mesoscale lows over the North Sea (Van Delden et al., 2003).
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3 Validation of the operational NWP models
On 28 February 2008 a flight with the Falcon aircraft was carried out in order to provide
observations of the arctic front, and a total of 15 dropsondes were released in 5 different legs
between 1126 UTC and 1452 UTC. Figure 6 shows the flight path with the position of each
dropsonde, while Figure 7 (a) shows the positions of the dropsondes overlaid on the NOAA
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infrared satellite image from 28 February 1149 UTC and Figure 7 (b) shows the dropsonde
positions together with the time for each dropsonde overlaid a satellite image from 1326 UTC
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the same day. The satellite images of Figure 7 and the ECMWF analyses from 1200 UTC
(Figure 4) and 1500 UTC (not shown) indicate that the frontal zone did not move much
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during the flight. In the current section we will assess the 36 hour operational forecasts valid
at 28 February 12 UTC from the HIRLAM model (Undén et al., 2002) run at The Norwegian
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Meteorological Institute and the European Centre for Medium-Range Weather Forecasts
(ECMWF) model. The objective is to investigate how well the models simulated this
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relatively complicated system involving the arctic front, the three warm core lows (WC1,
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WC2 and WC3), and the upper level cyclone (CC). The HIRLAM model was in 2008 run
with a horizontal grid spacing of 12 km and 40 vertical layers. It applied the Soft TRAnsition
COndensation (STRACO) scheme (Sass and Yang, 2002) for parameterization of
condensation and cloud processes. While surface fluxes of momentum, heat and moisture
over the ocean are expressed by bulk formulas relating these fluxes to the wind and the
thermodynamic state in the lowest model level, where drag coefficients are derived by
formulating expressions for momentum, heat and moisture roughness lengths as described in
Undén et al, 2002.
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The ECMWF model is a global spectral model with a horizontal resolution T799 in 2008,
corresponding to 25 km grid spacing, and it has 91 vertical levels. Cumulus convection is
parameterized by a bulk mass flux scheme involving deep, shallow and mid-level convection.
Data from the ECMWF model were used as initial and boundary data for the HIRLAM
model.
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The dropsonde observations will here be used to verify the SLP fields and the 925 hPa wind
simulated by these models. The dropsondes measured pressure, humidity, temperature and
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horizontal wind with an accuracy of 1 hPa, 0.1 K, 5 % and 0.5 ms-1 respectively and with a
time resolution of one observation every half second (Kristjánsson et al. 2011). The vertical
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resolution of the dropsonde data is 5 – 6 m close to the surface, and we used the last
transmitted pressure to validate the SLP from the models. We estimate that the error in
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observed SLP attributed to this is less than 0.8 hPa.
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Figure 8 (a) shows the SLP field together with the 500 – 1000 hPa thickness from the 36 hour
run of HIRLAM valid at 28 February 12 UTC while Figure 8 (b) shows the same fields from
the ECMWF-model. The figures show that both models simulated a reversed frontal zone
delimiting the cold air to the south from the warmer air to the north. However while the 925
hPa θE analysis from the ECMWF model (Figure 4 (a)) and the 500 – 1000 hPa thickness
analysis (Figure 4 (b)) indicate that the front had a zonal orientation west of 5 º W, both
forecasts gave northwest to southeast orientation, hence placing the western part of the frontal
zone too far north. A comparison of the 500 – 1000 hPa thickness fields from the two
forecasts and the ECMWF analysis reveals that HIRLAM simulated too cold air over the
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Greenland Sea with a minimum 500-1000 hPa thickness of less than 5000 m compared to a
thickness of approximately 5030 m in the analysis. The forecast from the ECMWF model
simulated a thickness of approximately 5020 m in this area, considerably closer to the analysis
than HIRLAM.
A SLP of 988.4 hPa measured by dropsonde 6 and a SLP of 987.1 hPa measured by
dropsonde 8 (Figure 7 (a)) indicate the presence of two lows over the Greenland Sea,
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corresponding to WC2 and the remainder of WC1, shown as a trough over the Greenland Sea
in the SLP analysis from 28 February 12 UTC (Figure 4 (a)). The locations of these two lows
based on the dropsonde data have been marked in Figure 8, while we see that both models
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simulated a weak ridge in that area. Instead they placed a closed low further west towards the
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coast of Greenland. Assessments of the SLP analysis as well as the 500-1000 hPa thickness
from both models, valid 12 and 6 hours earlier (not shown), indicate that this low corresponds
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to WC1, while neither of the models produced a cyclone corresponding to WC2. SLP
observations ranging from 993.7 to 995.4 hPa over the Norwegian Sea (Figure 7 (a)) between
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72° N and 74° N, 4° W and 3° E indicate a ridge in this area, considerably stronger than the
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ridge that the models simulated over the Greenland Sea, and we may hence conclude that both
the models placed the ridge more than 5 ° too far southwest and underestimated its strength.
Also in the SLP analysis (Figure 4 (a)) the ridge was too weak as the SLP was between 990
and 992 hPa, but it’s position was consistent with the observations.
Further towards the Norwegian coast both HIRLAM and the ECMWF model predicted
decreasing SLP and a closed warm core cyclone over the Lofoten Islands with a SLP of 984
and 982 hPa respectively and a 500 – 1000 hPa thickness of 5220 - 5260 m (Figure 8). The
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southeasternmost dropsonde, which was released close to the Lofoten Islands, measured a
SLP of 991 hPa, which is several hPa higher than predicted by both models. Furthermore the
satellite image from 1149 UTC (Figure 5 (d)) didn’t show any circulation or cloud
enhancement in that area, which it would if a low was present. This strongly indicates that the
models misplaced the cyclone in this area, and an investigation of the HIRLAM and ECMWF
forecasts valid at 27 February 12 UTC (+12 h) and 28 February 00 UTC (+ 24 h) (not shown)
indicates that this cyclone corresponds to the WC2. Both models simulated the formation of
WC2 over the Norwegian Sea in good agreement with the analysis shown in Figure 2 (a), but
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instead of moving the cyclone northwestwards to Greenland, they moved WC2 towards NNE
along the Norwegian coast. This is indeed a serious failure as the models in such a case could
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mislead forecasters to issue warnings on dangerous weather along the coast.
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A minimum SLP of 988 hPa was observed by dropsonde 2 (Figure 7 (a)) over the Norwegian
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Sea at 70.8 º N, 7.2 º E, and in this area HIRLAM simulated a local minimum in SLP of 986
hPa, whereas the ECMWF model simulated a 988 hPa trough, which we believe corresponds
to WC3 (see e.g. Figure 7 (a)).
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We have further assessed the 925 hPa winds from the HIRLAM (Figure 9 (a)) and the
ECMWF models (Figure 9 (b)) together with the winds observed from the dropsondes at the
same level. Figure 9 shows that both models simulate winds from ESE with wind speeds
exceeding 20 ms-1 north of the frontal zone between the east coast of Greenland and the zero
meridian. The 25 ms-1 wind from ESE observed by the northernmost dropsonde indicates the
presence of strong low-level winds in this area, and we will discuss this further in section 4.3.
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Further south over the Greenland Sea, between 72 ° N and 74 ° N, the observed wind is 5 to
15 ms-1 from the south (Figure 9) due to the pressure gradient with increasing SLP towards
the east (Figure 7(a)) and the decaying WC1 to the west. The weak circulation produced by
HIRLAM near 73 ° N and 8 ° W (Figure 9 (a)) is not seen in the ECMWF simulation (Figure
9 (b)) and does not correspond to any of the warm core cyclones. As previously described,
both models failed to simulate the ridge over the Norwegian Sea, and therefore the SLP
distribution and the wind direction disagree with the observations in this area. Further east
over the Norwegian Sea, between 0 º E and 10 º E, the wind directions from HIRLAM and
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ECMWF were in accordance with the observations but the wind speeds from the models were
too high (Figure 9). The deviation between the observed and predicted wind directions for the
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southeasternmost dropsonde is a consequence of the cyclone centre being mistakenly placed
over the Lofoten Islands by both models, as mentioned earlier in this section.
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4. The mesoscale structure of the front
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In the current section we will use the dropsonde data obtained during the flight to study the
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mesoscale features of the frontal zone, and we will further investigate how well these features
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were simulated by the HIRLAM model. This will be done by assessing the cross sections of
potential temperature, wind speed and relative humidity from the different dropsonde legs,
and we will pay most attention to Leg 4 (Figure 6) as the data coverage from the four
dropsondes released in that flight leg was relatively good for all parameters and the
temperature gradients were strong at this location. The first sonde of this leg was released at
1306 UTC at 72.4 ° N and 9.8 ° W while the last sonde was released at 1335 UTC at 73.5 ° N,
0.1 ° E. The alignment of the frontal cloud band (Figure 7 (b)) suggests that this dropsonde
leg (sondes 9 – 12) was almost perpendicular to the front and that the first and second of the
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sondes were dropped in the cold air on the southern side of the front, while the third and
fourth sondes were dropped within the frontal zone.
4.1 Potential temperature
The cross section based on dropsonde observations of potential temperature from Leg 4 is
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shown in Figure 10 (a), where the horizontal gradient clearly reveals an approximately 150
km wide frontal zone in the northeastern part of the cross section with a temperature
difference of about 6 K between the two different air masses at 800 hPa. The front has been
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marked as a dashed line, connecting the leading edge of the baroclinic zone and the frontal
inversion. Near the surface, the gradient is much weaker, probably due to vertical mixing in
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the conditionally unstable air on the cold side of the front, evidenced by convective cloud
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structures in Figure 7. The relatively shallow baroclinic zone has a slope of approximately 1
to 70 towards the southwest and delimits a low level pool of cold air to the southwest from the
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warmer air to the northeast. Southwest (left) of dropsonde 11 the frontal zone extends as a
frontal inversion, capping the 268 – 270 K low level cold air. The temperature profile from
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dropsonde 11 (not shown) indicates that this frontal inversion is between 820 and 720 hPa,
and further southwest dropsonde 10 (not shown) indicates an inversion between 720 and 600
hPa. The cross section of potential temperature through the same leg based on a 36 hour
prediction from HIRLAM valid at 28 February 12 UTC, shown in Figure 10 (b), indicates that
the model simulated the location of the frontal zone in the northeasternmost part of the
dropsonde leg fairly well. Also the pool of cold air in the southwestern part of the leg is
mainly in accordance with the observations, although an assessment of the 270 K isopleth
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shows that the simulated cold pool is too deep and the isopleth too steep compared to the
observations.
We have also investigated potential temperature based on dropsondes 1 - 4 of Leg 1, which
was flown between 1126 and 1220 UTC (Figure 7 (b)), cutting the frontal zone with an angle
of approximately 30 º. The cross section of potential temperature (Figure 11) shows a pool of
relatively cold air underneath an inversion in the western part of the cross section, while there
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is 5 – 6 K warmer air in the eastern part. These air masses are separated by a shallow low
level frontal zone, and the frontal structure found from the dropsondes of Leg 1 is hence
consistent with the observations of Leg 4. The pool of cold air in the western part of the cross
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section was well reproduced by the 36 hour HIRLAM forecast (not shown) , while the
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temperatures in the eastern part were approximately 4 K higher than observed. This is
consistent with the comparison between predicted 500 – 1000 hPa thickness and the analysis
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performed in Section 2, which indicated that the model predictions were too warm in this
area. An assessment of the cross section of potential temperature based on the three
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dropsondes through Leg 2 (not shown) revealed the same features as the data from Leg 4 and
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Leg 1, such as the low-level pool of cold air south of the front and the warmer air towards the
north. This is also consistent with the findings of Kristjánsson et al. (2011), who investigated
observations of potential temperature through Leg 3.
4.2 The Doppler Lidar
When studying the wind distribution we benefited from lidar measurements in addition to the
dropsonde data. The lidar used here is a coherent Doppler lidar from Lockheed Martin, former
CTI, emitting laser pulses at 2.02 µm wavelength with 1 to 1.5 mJ energy and 500 Hz
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repetition rate. The advantage of such a coherent Doppler lidar is the high accuracy of the
obtained Doppler shift, and furthermore it is straightforward to apply quality criteria. As long
as the resulting spectra have a sufficient signal to noise ratio (SNR) the processed Doppler
shift has a high accuracy, here much better than 1 m/s. Any degradation/dealignment of the
lidar results in a lower SNR that has only a minor impact on the achieved accuracy.
Furthermore this lidar is equipped with a DLR made double wedge scanner. The scan pattern
is a 20 point step and stare conical scan looking downward with 20° half cone angle, the
duration of one scan (=one revolution) being roughly 30 s. From all 20 stare positions the
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Doppler shift is estimated, the influence of the platform motion subtracted, and then the 3
dimensional wind vector is calculated by an inversion of the obtained LOS wind speed. Thus
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a height resolved wind profile is obtained every half minute. The vertical resolution of 100 m
is mainly determined by the pulse length of the emitted laser pulse which is approximately
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600 ns, and the horizontal resolution is given mainly by the opening angle of the cone and the
aircraft movement during one scan. E.g. at 10 km altitude of the aircraft and a velocity of 200
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m/s the footprint of the scan is approximately 7 km wide and has a length of 13 km.
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The range of a lidar based on Mie-scattering depends on the density of aerosols in the
measurement volume, as the higher the aerosol density is the higher is the backscattered laser
power. On the other hand the laser beam and the backscattered light suffer from losses due to
attenuation on the way from the lidar to the measurement volume and back which are also
proportional to the backscatter coefficient. As a result there is an optimal aerosol density
(backscatter coefficient) for a maximum range of the lidar measurement. E.g. thick clouds
give a huge signal from their surface, but attenuate the laser beam quite rapidly so that no
measurement from inside the cloud is possible.
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4.3 The horizontal wind pattern
We will here investigate the wind pattern associated with the front, and start by assessing the
cross section of wind speed based on the four dropsondes of Leg 4 (Figure 12 (a)). The cross
section reveals a southeasterly 25 - 30 ms-1 jet covering major parts of the troposphere above
the frontal inversion, which was discussed in the subsection 4.1. Around dropsonde 11 the jet
extends down to approximately 3 km and has local maxima close to 3.5 km and at 6 - 7 km,
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while wind speeds exceeding 15 ms-1 extend down to 1.5 km in the northeasternmost part of
the leg. In the cold air below the frontal inversion, the wind is much weaker from a southerly
direction with wind speeds mainly between 4 and 8 ms-1.
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We have also investigated the wind data measured by the Doppler lidar, and we show the
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cross section of wind speed based on the lidar data from Leg 4 in Figure 12 (b). The data from
the lidar have a horizontal resolution of 6 – 7 km in this case, which is considerably higher
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than the 80 – 90 km resolution of the dropsonde data from Leg 4. As previously mentioned
the presence of aerosols is necessary to give backscatter and hence wind measurments, while
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deep clouds will attenuate the laser beam. This explains why we in the northeastern part of the
leg get data down to approximately 4 km, as the frontal cloud band gives a strong backscatter.
Southwest of the cloud band there are data down to 6 – 7 km and then again from 1.5 km and
down to the surface, which is probably connected to a low density of aerosols down to 1.5
km while sea salt aerosols and possibly thin low-level clouds produce backscatter below this
level. When comparing the lidar-based cross section with the corresponding cross section
from the dropsondes (Figure 12 (a)), we see that the lidar revealed stronger winds associated
with the jet, measuring wind speeds exceeding 40 ms-1 between 6 – 7 km altitude. This shows
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that while the dropsonde data give a good picture of the wind pattern, their horizontal
resolution is too coarse to reveal the more detailed variations and hence the core of the jet.
A comparison between the wind speeds measured by the dropsondes and the lidar (not shown)
indicates that they are consistent. We further investigated the wind direction from the lidar
(not shown), and as for the dropsondes we found southerly low-level wind in the southwestern
part of the leg while at upper levels the wind was from the southeast through the entire leg.
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In the 36 hour HIRLAM run (not shown) the broad features are mainly consistent with the
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corresponding cross section based on the dropsonde data (Figure 12 (a)), with strong winds
exceeding 30 ms-1 above the sloping frontal inversion and much weaker winds closer to the
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surface. However the model failed to simulate the strongest winds associated with the upperlevel jet (Figure 12 (b)), attaining a maximum wind speed of 34 ms-1 as compared to more
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than 40 ms-1 measured by the lidar. HIRLAM simulated surface winds of approximately 20
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ms-1 in the northeastern part of Leg 4, indicating the presence of a low-level jet. The SLP
analysis from ECMWF from 28 February 1200 UTC (Figure 4 (a)) shows a strong horizontal
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gradient north of the frontal zone, and we would expect to observe high surface wind speeds
in this area. An investigation of winds observed from QuikScat between 04 and 06 UTC
(Figure 13 (a)) and between 17 and 19 UTC (Figure 13 (b)) revealed easterly wind between
15 and 18 ms-1 north of the frontal zone. This shows that there was a low-level jet associated
with the front, but almost all the dropsondes were released too far south to observe it.
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We have also investigated the data from the dropsondes of Leg 2 (Figure 6), which extends
northwards to 74.5 ° N, 8.9 ° W. The cross section of horizontal wind from Leg 2 (Figure 14
(a)) shows a southeasterly jet above the cold air in the southeastern part (right) of the cross
section, while in the northwest (left) there are strong winds extending almost all the way
down to the surface with a maximum wind speed of 40 ms-1 at approximately1.6 km altitude.
Since most of the wind data below 1 km were missing for dropsonde 6, we were not able
extend the cross section below this level. However, the sparse wind data below this level
showed wind speeds exceeding 25 ms-1, indicating that the low-level jet did extend at least as
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far south as dropsonde 6.
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A corresponding cross section from the lidar is shown in Figure 14 (b). Also here the range of
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the lidar varied due to variations in the cloud cover, but the wind speed was observed down to
5 km above the surface through most of the leg and down to 3 - 4 km between 100 and 200
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km. As for Leg 4, the lidar was able to measure low-level winds south of the frontal cloud
band. Between 80 and 120 km wind speeds up to 40 ms-1 were observed by the lidar at
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approximately 5 to 7 km altitude, while the dropsondes indicated that the wind speed was
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approximately 32 ms-1 in this area (Figure 14 (a)). Clearly the spatial resolution provided by
the 3 dropsondes released in Leg 2 was too coarse to capture the detailed distribution of the
wind, thereby missing the 40 ms-1 core of the upper level jet which was located between
dropsondes 4 and 5.
We have also investigated wind data from the dropsondes and the lidar from Leg 3 (Figure 6),
and also here the lidar-based cross section (not shown) picked up the core of the upper level
jet with wind speeds up to 40 ms-1, while the dropsonde data were too coarse to reveal these
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details. Further east, the wind measured by the lidar during Leg 1 (Figure 6) indicated a 10 –
15 ms-1 weaker upper-level jet, confined to areas west of 4 - 5° E. Based on the wind lidar
data from Leg 4 (Figure 12 (b)), Leg 2 (Figure 14 (b)) and Leg 3 (not shown) we would argue
that the strongest part of the upper level jet was between 4º and 10º W over the Greenland Sea
(Figure 7 (b)). As for the low-level jet the dropsonde-based cross sections from Leg 2 (Figure
14 (a)) and Leg 3 (not shown) indicated that it was only observed by sonde 6, which was the
northernmost sonde of the flight. Most of the low-level jet was missed, and it would have
been desirable to deploy dropsondes further north into the warmer air, but poor guidance from
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NWP models made the planning of this flight difficult (Kristjánsson et al., 2011).
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4.4 Vertical wind measured by the lidar
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Cross sections of vertical wind speed measured by the Doppler Lidar through Leg 4 and Leg 2
are shown in Figures 15 (a) and 15 (b) respectively. Despite the gaps in the lidar data due to
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deep clouds, the figures indicate ascending air associated with the frontal zone. Figure 15 (a)
shows vertical updrafts of 0.1 to 1 ms-1 above the front between 200 and 300 km, and vertical
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motion of the same magnitude is also seen above the front in Figure 15 (b), where there are
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signals down to 2 km above the surface. The location of this ascent is consistent with the
slantwise ascent found by Browning and Pardoe (1973) above a cold front, but they suggested
a typical vertical velocity of 0.1 ms-1. The figures indicate both positive and negative vertical
motion in the low level cold air between 0 and 150 km in Figures 15 (a) and (b), associated
with shallow convection, evidenced by the shallow clouds south of the frontal cloud band in
the satellite images in Figure 7. In the upper parts of the troposphere both cross sections
indicate descending air. This could be related to a downfolding of the tropopause which will
be discussed in the next sub section.
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4.5 Frontal cloud band and dry slot
We will here study observations of relative humidity with respect to water (RHw) in order to
gain further insight into the distribution of dry and moist air. The cross section of RHw
through Leg 4 (Figure 16 (a)) shows the frontal cloud band as a deep layer of moist air in the
northeastern (right) part of the leg, with a RHw of more than 80 % up to approximately 650
hPa and more than 60 % up to 400 hPa. An investigation of the cross section of RHi (relative
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humidity with respect to ice) indicated that the cloud band was fairly deep as values of RHi
exceeded 90 % up to approximately 380 hPa in the northeastern part of the leg (not shown).
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In the southwestern part of the leg, between 0 and 150 km, we recognize the pool of cold air
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beneath the frontal inversion as humid air with RHw around 80 %, while above the top of the
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frontal inversion (dashed line in Figure 16 (a)) the air is extremely dry with RHw less than 20
%. This slot of dry air could be a sign of descending air associated with a downfolding of the
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tropopause above the front (Wallace and Hobbs, 2006). The 36 hour HIRLAM prediction
(Figure 16 (b)) is consistent with the observations in that it predicts a slice of dry air in the
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southwestern (left) part of the cross section, but this slice is narrower than in the observations
and extending further towards the northeast, where it undercuts the moist air associated with
the frontal cloud band. Although HIRLAM seems to simulate a filament of dry air extending
too deep into the troposphere and too far towards the northeast, a pocket of dry air was
detected from dropsonde 11 at 870 hPa (Figure 16 (a)), and relatively dry air was observed
down towards the surface in this area. We performed a manual analysis of the RH data from
the dropsondes (not shown) and this indicated that the pocket of dry air observed from
dropsonde 11 is an extension of the dry slot. Based on this we would argue that the narrow
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tongue of dry air down to 850 Pa simulated by HIRLAM is realistic, and that the lack of this
in figure 16 (a) is a result of interpolating data with coarse horizontal resolution. Between 0
and 100 km the shape of the dry slot in Figure 16 (a) is mainly consistent with the manual
analysis, and the RHw of 80 % predicted by HIRLAM at approximately 450 hPa and 100 km
is inconsistent with the observations.
We also studied the cross section of RHw based on the dropsondes from Leg 1 (Figure 17),
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and also here the dry slot on the cold side of the front (left) revealed itself as extremely dry air
extending down to 850 hPa and undercutting moist air that was associated with the frontal
cloud band. Likewise, the cross sections of RHw based on the dropsondes of Leg 2 and Leg 3
strongly indicated a dry slot.
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5. Short summary of the observed frontal structure
Based on the analysis of the observations described in section 4 (mainly data from Leg 1, Leg
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2 and Leg 4) we have summarized the main features associated with this frontal zone in
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Figure 18. Combining the dropsondes released between 1126 UTC and 1335 UTC with the
ECMWF analyses from 28 February 1200 UTC (Figure 4) and 1500 UTC (not shown), as
well as the NOAA satellite images from 1149 UTC (Figure 7 (a)) and 1326 UTC (figure 7
(b)), we find that the frontal zone was quasi stationary at the time, although cyclone WC2 was
moving westwards. In Figure 18 (a) the front’s position at 925 hPa has been marked on the
NOAA satellite image from 28 February 1149 UTC together with the edge of the dry slot
which is parallel to the front at 750 hPa. The front was marked in accordance with the
observed wind directions (Figure 9) as well as the cross sections of potential temperature
(Figures 10 (a) and 11), while we marked the edge of the dry slot based on the cross sections
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of RH (Figures 16 (a) and 17). A summary of the vertical cross section of the frontal zone is
presented in Figure 18 (b), where we have depicted the front as a line representing the leading
edge of the cold air with a slope that is gradually reduced with altitude, extending
southwestwards as a frontal inversion layer capping a pool of cold air. While it is difficult to
clearly distinguish the front from its extension, we have marked the transition from front to
inversion layer where the front gradually loses its slope, which is consistent with the analyses
of Grønås and Skeie (1999) and Shapiro et al. (1989). The red arrows in Figure 18 (a) indicate
the upper-level jet which extended over most of the troposphere above the frontal inversion,
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while the low level jet on the warm side of the front is depicted by green arrows. Data from
QuikScat indicated that the low level jet extended down to the surface, but the dropsondes
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were released too far south observe this jet. The maximum wind speeds associated with the
upper level jet were found between 5.5 and 8 km, corresponding to approximately 470 - 300
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hPa, and in the vertical cross section (Figure 18 (b)) the cores of the upper-level jet and the
low-level jet are shown as arrows directed into the picture .
6. Discussion and conclusion
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6.1 Discussion
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During 27 February 2008 a relatively complex synoptic situation involving a stationary upperlevel cold cyclone off the east coast of Greenland and two intense northwestwards moving
warm core lows over the Norwegian Sea gave rise to advection of cold air towards the
southeast and warm air towards northwest, creating a frontal zone which delimited relatively
warm air to the north and colder air to the south. On 28 February the mesoscale structure of
this arctic front was observed by dropsondes and a remotely sensing wind lidar carried on
board the DLR Falcon research aircraft. In the cross sections based on the dropsonde data the
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front appeared as a 150 km broad baroclinic zone and further southwest as an inversion layer
capping the pool of cold air, which was confined to levels below 700 – 800 hPa. The frontal
zone had a near-surface temperature gradient of approximately 5 K per 100 km, which is of
the same magnitude as found in an observational study of a cold front by Wakimoto and
Murphy (2008). Whereas the cold front analysed by Wakimoto and Murphy and the cold front
observed during the ERICA field campaign (Neiman et al., 1993) extended up to
approximately 500 hPa, the present front was mainly confined to levels below 700 hPa.
Hence the front was shallow compared to observed polar cold fronts, but it was deeper than
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the arctic fronts investigated by Grønås and Skeie (1999) and Shapiro et al. (1989), both of
which were confined to levels below 850 hPa.
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As in the case of the arctic front investigated by Grønås and Skeie, both observations from
QuikScat and the northernmost dropsonde indicate that the front in the present study was
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accompanied by severe low level winds which could be a danger to human activity in the
area. These strong winds were a manifestation of a low-level jet on the warm side of the
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front, but most of this jet was missed by the dropsondes, as they were released too far south.
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In their case study of low-level jets ahead of cold fronts, Browning and Pardoe (1973) found
that the strongest winds were at 900 to 850 hPa with wind speeds between 25 and 30 ms-1.
The 40 ms-1 wind observed at 1600 m (800 hPa) indicates that the present jet was very strong,
and more observations would have been desirable.
The presence of a southeasterly upper-level jet was revealed by the dropsonde data (Figure 7
(b)), but they were too coarse to reveal its detailed structure. The Doppler lidar provided
detailed observations of the upper-level winds, which exceeded 40 ms-1 in the core of the jet.
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These high wind speeds were associated with the steep pressure gradients north and east of
the upper level cyclone (Figure 4 (b)), and were located near the edge of the cloud band. The
core of the upper-level jet was observed at approximately 400 hPa (6.5 km) above sea level,
and these observations support Shapiro et al. (1987 b) who included an arctic jet stream in
their conceptual model of the tropopause. In the present study the dropsonde data revealed a
slot of dry air (Figure 16 (a) and Figure 17) which is likely to be an indication of subsidence.
Mc Innes et al. (2009) found that a similar dry slot observed over a lee cyclone southeast of
Greenland was associated with subsidence of air from a downfolding in the tropopause,
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evidenced by high ozone concentrations measured from an aircraft. We investigated the cross
section of PV based on a 6 hour simulation of HIRLAM valid at 28 February 12 UTC for the
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four legs and found downfoldings in the PV isopleths in the same area as the dry slot (not
shown), which indicate that the dry air also in this case is associated with a tropopause fold.
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Both the indications of a tropopause fold and the upper level jet found in the present study are
consistent with the threefold model of the tropopause argued by Shapiro et al. (1987 b).
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Severe low-level winds clearly show the importance of paying attention to arctic fronts in
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order to provide warnings of hazardous weather, and adequate NWP simulations are essential
in this regard. Although both NWP models assessed in the present study simulated the frontal
zone and strong low-level winds, it is disappointing that they located the westernmost part of
the system too far north and placed a warm core low over the coast of Northern Norway
instead of over the Greenland Sea. These problems are not unexpected as previous studies
such as Kristiansen et al. (2011) and Kristjánsson et al. (2011) have shown that NWP models
have problems in simulating cyclonic development associated with arctic fronts in this area.
A study of Mc Innes et al (2011) showed that increasing the spatial resolution of a NWP
model could increase the skill of polar low simulations, and it is reasonable to believe that
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increasing the resolution of operational model runs would improve the forecasts of mesoscale
systems associated with arctic fronts as well.
6.2 Concluding remarks
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The observational data obtained on 28 February 2008 revealed mesoscale structures of an
arctic front that was quite similar to previous observational studies of polar cold fronts with
respect to low-level jet, tropopause fold and horizontal temperature gradient. While the
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dropsonde data revealed the main features of the frontal system, the use of Doppler Lidar
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turned out to be essential for exploring the detailed structure of the upper level jet. It also
provided valuable information on the vertical motions, both above the cold front and in the
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pool of cold air below the frontal inversion. Based on these findings we would recommend
the use of such lidars in future field campaigns despite limitations in the presence of thick
clouds or too low concentrations of aerosols.
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Although the structure of the frontal zone was fairly well predicted by the operational NWP
models, they completely misplaced one of the mesoscale cyclones, providing misleading
guidance to forecasters. As human activity (and hence the importance of reliable forecasts) is
increasing in the Arctic, an effort towards improved predictions of weather systems associated
with arctic fronts seems to be imperative.
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Acknowledgements
This study has received support from the Norwegian Research Council through the project
“THORPEX-IPY: Improved forecasting of adverse weather in the Arctic – present and
future” (grant no. 175992). During this study we had useful discussions with Øyvind Sætra
and Pål Sannes at the Norwegian Meteorological Institute. We also acknowledge the two
reviewers for constructive and useful comments which have been of great help in our work
with the manuscript.
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Figure 1: Data from the ECMWF model (+ 0h) valid at 27 February 00 UTC. (a) Sea level pressure (Blue every
2 hPa) and equivalent potential temperature at 925 hPa (dashed dark red, every 2 K). (b) Height of 500 hPa
surface (green, every 20 m) and 500-1000 hPa thickness (dashed red, every 20 m). CC indicates cold core.
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Figure 2: Data from the ECMWF model (+ 0h) valid at 27 February 12 UTC. (a) Sea level pressure (Blue every
2 hPa) and equivalent potential temperature at 925 hPa (dashed dark red, every 2 K). WC1 and WC2 indicate
warm core cyclone 1 and 2 respectively. (b) Height of 500 hPa surface (green, every 20 m) and 500-1000 hPa
thickness (dashed red, every 20 m). CC indicates cold core.
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Figure 3: Data from the ECMWF model (+ 0h) valid at 28 February 00 UTC. (a) Sea level pressure (Blue every
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warm core cyclone 1 and 2 respectively. (b) Height of 500 hPa surface (green, every 20 m) and 500-1000 hPa
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Figure 4: Data from the ECMWF model (+ 0h) valid at 28 February 12 UTC. (a) Sea level pressure (Blue every
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Figure 5: NOAA infrared images from (a) 27 February 0028 UTC, (b) 27 February 1113 UTC, (c) 28 February
0359 UTC and (d) 28 February 1149 UTC.
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Figure 6: Flight track with dropsonde positions indicated by black dots. Star indicates first dropsonde and square
last. L1 indicates dropsonde leg 1, L2 indicates dropsonde leg 2 etc.
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Figure 7: a) Dropsonde positions indicated by filled circles together with observed sea level pressure (green)
overlaid on a NOAA infrared image from 28 February 1149 UTC 2008. The positions of the cyclones are
depicted by WC1, WC2, and WC3 while H indicates a ridge. (b) Dropsonde positions and time (yellow) overlaid
on a NOAA infrared image from 28 February 1326 UTC 2008. The green ellipse indicates where the highest
wind speeds associated with the upper-level jet, exceeding 40 ms-1, were observed.
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Figure 8: 36 hour runs of HIRLAM (a) and the ECMWF model (b) valid at 28 February 12 UTC for sea level
pressure (SLP) (blue, every 2 hPa)) and 500 – 1000 hPa thickness (dashed red, every 20 m). Filled circles
indicate dropsonde positions. The locations of the three warm core lows based on dropsonde data are marked
WC1, WC2, and WC3. Only a part of the HIRLAM domain is shown in (a).
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Figure 9: Observed wind at 925 hPa (red) and forecasted wind (blue) from 36 hour runs of HIRLAM (a) and the
ECMWF model (b) valid at 28 February 12 UTC. Only a part of the HIRLAM domain is shown in (a).
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Figure 10: Cross section of potential temperature (every 2 K) through Leg 4. (a) Based on dropsonde data with
the southwesternmost dropsonde to the left. The dashed line indicates the position of the front. (b) The same
cross section from the operational HIRLAM run (+ 36 h) valid at 28 February 12 UTC.
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Figure 11: Cross section of potential temperature (every 2 K) through Leg 1 based on dropsonde data with the
westernmost dropsonde to the left.
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Figure 12: Observed wind in ms-1 through Leg 4. (a) Cross section based on dropsonde data (every 2 ms-1) with
the southwesternmost dropsonde to the left. The arrows show the direction of the horizontal wind at different
levels, with the length of the arrow being proportional to the wind speed. The dashed line indicates the position
of the front. (b) The corresponding cross measured by the Doppler Lidar onboard the aircraft
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Figure 13: Sea level winds observed from QuikScat (winds.jpl.nasa.gov/) 28 February 2008 between 04 and 06
UTC (a) and between17 and 19 UTC (b). The positions of dropsonde 6 and dropsonde 12 are indicated by S6
and S12 respectively.
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Figure 14: Observed wind speed (ms-1) through Leg 2. (a) Cross section based on dropsonde data (every 2 ms-1)
with the northwesternmost dropsonde to the left. The arrows show the direction of the horizontal wind at
different levels, with the length of the arrow being proportional to the wind speed. The black dashed line
indicates the front. (b) The corresponding cross section measured by the Doppler Lidar onboard the aircraft.
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Figure 15: Cross section of vertical wind through Leg 4 (a) and Leg 2 (b) measured by the Doppler Lidar
onboard the aircraft. The dashed black line indicates the position of the front.
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Figure 16: (a) Cross section of relative humidity with respect to water (%) through Leg 4 with the
southwesternmost dropsonde to the left. The dashed black line indicates the front. (b) The same cross section
from the operational HIRLAM run (+ 36 h) valid at 28 February 12 UTC.
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Figure 17: Cross section of relative humidity with respect to water (%) through leg 1. The westernmost
dropsonde is to the left.
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Figure 18: (a) The reversed Arctic front indicated by a violet solid line overlaid on the NOAA infrared satellite
image from 28 February 1149 UTC. W and C indicate the warm and cold air masses respectively. The arrows
depict the observed jet and the airflow direction. Red arrows indicate the upper level jet, and green arrows the
low-level jet. The dashed blue line indicates the northeastern edge of the dry slot. (b) A vertical cross section
showing the front as a solid violet curve and dashed violet curves indicating the inversion in the extension of the
front. The upper level jet and the low-level jet are marked by arrows directed into the picture. The dry slot is
depicted by a dashed blue line.
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Quarterly Journal of the Royal Meteorological Society
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Quarterly Journal of the Royal Meteorological Society
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Quarterly Journal of the Royal Meteorological Society
Quarterly Journal of the Royal Meteorological Society
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