Quarterly Journal of the Royal Meteorological Society
Q. J. R. Meteorol. Soc. 139: 2134–2147, October 2013 B
An observational study of an arctic front during
the IPY-THORPEX 2008 campaign
Harold Mc Innes,a * Jón Egill Kristjánsson,b Stephan Rahm,c Bjørn Røstinga
and Harald Schyberga
a
The Norwegian Meteorological Institute, Norway
b
University of Oslo, Norway
c Deutsches Zentrum für Luft- und Raumfahrt, Germany
*Correspondence to: H. Mc Innes, The Norwegian Meteorological Institute, Postboks 43, Blindern 0313, Oslo.
E-mail: haroldmi@met.no
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
models. The observations revealed features 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 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 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 features of the frontal zone such as jets,
dry slot and the depth of the front fairly well, although parts of the front were slightly
misplaced. However, the models failed completely in their simulations of one of the
three mesoscale cyclones associated with the front as they located it over the coast of
northern Norway while the correct location was over the Greenland Sea according
to the observations and analysis.
Key Words:
arctic front; observations; wind lidar; mesoscale cylones; NWP models
Received 28 June 2012; Revised 7 November 2012; Accepted 8 November 2012; Published online in Wiley Online
Library 29 January 2013
Citation: Mc Innes H, Kristjánsson JE, Rahm S, Røsting B, Schyberg H. 2013. An observational study
of an arctic front during the IPY-THORPEX 2008 campaign. Q. J. R. Meteorol. Soc. 139: 2134–2147.
DOI:10.1002/qj.2088
et al., 1999). The theory for development of synoptic-scale
cyclones associated with baroclinic instability is described
Due to the available potential energy associated with hori- by Holton (2004), for example, and the role of baroclinic
zontal temperature gradients, a baroclinic zone represents instability in the formation of mesoscale cyclones such as
a potential for the development of extratropical cyclones polar lows was first highlighted by Harrold and Browning
with associated frontal systems. Hence the dynamics and (1969) and later investigated by Van Delden et al. (2003)
structure of fronts and baroclinic zones have been of and Yanase and Niino (2007). Polar lows are often assogreat interest to both forecasters and researchers (Shapiro ciated with arctic fronts (Rasmussen et al., 2003; Shapiro
1.
Introduction
c 2013 Royal Meteorological Society
Observational Study of an Arctic Front
et al., 1987a), which 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 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 the development of a polar low. In their study Grønås and Skeie
simulated an arctic front 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.
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 dropsonde observations combined with
lidar measurements is unique for arctic fronts, and the
present study is based mainly 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 network of meteorological stations in this region.
Compared with 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 jets
based on data from routine soundings and wind observations from a Doppler radar. They investigated six 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 m s−1 at 850–900 hPa. For one of
the cases they also analysed the vertical wind and found a vertical velocity of typically 0.1 m s−1 associated with slantwise
convection above the cold front. In the present study we will
analyse vertical wind speeds measured by the Doppler lidar.
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 crosssections indicated that the frontal zone extended up to
approximately 500–600 hPa. On the warm side of the front
the observations revealed a 34 m s−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
c 2013 Royal Meteorological Society
2135
and 5 January 1989. A cross-section through the cold front
associated with the 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.
Shapiro et al. (1987b) proposed a conceptual model of the
tropopause including an arctic tropopause fold associated
with the arctic front in addition to the tropopause folds
associated 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. 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. (1987b) 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 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 with its 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, for example, Wakimoto and Murphey’s (2008)
observational study.
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
observations captured on 28 February 2008 are of great value
as they both bring new insight 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 although the quality
of the model simulations were sufficient to investigate the
frontal structure, the model simulated temperatures too
low and winds too weak compared with observations. A
study of a polar low that occurred in connection with an
arctic front over the 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.
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
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H. Mc Innes et al.
(a)
(a)
(b)
(b)
Figure 1. Data from the ECMWF model (+0 h) valid at 27 February
0000 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.
Figure 2. Data from the ECMWF model (+0 h) valid at 27 February
1200 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.
2.
Synoptic overview
Here we will 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 situation over the
Greenland and Norwegian Seas, with a 989 hPa cyclone
east of Greenland at 71.7◦ N and 14◦ W, a 976 hPa low with
its centre outside the Norwegian coast at 66◦ N, 3◦ E and a
developing trough at 72◦ N, 2◦ E over the Norwegian Sea.
An assessment of the 500 hPa surface and 500–1000 hPa
thickness valid at the same time (Figure 1(b)) reveals that 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 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 the thermal wind equation (Van Delden et al.,
2003). Both the 925 hPa θE (Figure 1(a)) and 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◦ N, 4.7◦ W and
developed into a 980 hPa cyclone, while the 925 hPa θE
analysis (Figure 2(b)) shows that the baroclinic zone in the
c 2013 Royal Meteorological Society
same area has sharpened. The height of the 500 hPa surface
and the 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 h earlier. The low outside the Norwegian coast now
has its centre at 67.1◦ N, 4.5◦ E (Figure 2(a)), and this cyclone
also 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.
The 925 hPa θE analysis from 28 February 00 UTC
(Figure 3(a)) and the 500–1000 hPa 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 and Greenland
Seas separating relatively warm air to the north and colder air
to the south. The WC1 has now moved further towards the
northwest and is located over the Greenland Sea (Figure 3(a))
at the northern edge of the upper level cyclone, which has
approximately the same position as 12 h 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 12 h 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
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
Observational Study of an Arctic Front
(a)
(a)
(b)
(b)
2137
Figure 3. Data from the ECMWF model (+0 h) valid at 28 February
0000 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.
Figure 4. Data from the ECMWF model (+0 h) valid at 28 February
1200 UTC. (a) Sea-level pressure (blue every 2 hPa) and equivalent
potential temperature at 925 hPa (dashed dark red, every 2 K). WC2 and
WC3 indicate warm core cyclone 2 and 3 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.
the remains of WC1. At 500 hPa (Figure 4(b)) the upper
level cyclone is still pronounced and has approximately
the same position and strength as 12 h 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 this low also has a warm core, and
we have termed it WC3. Compared with 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 observations obtained
during the flight were not assimilated into the objective
analyses. Nevertheless we found that the European Centre
for Medium-Range Forecasts’ (ECMWF) analysis reflected
the observed locations of the different cyclones and frontal
zone fairly well.
Both the 925 hPa θE analysis in Figure 4(a) and the
500–1000 hPa thickness (Figure 4(b)) 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 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 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 h, 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 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).
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 gives
a suggested wavelength of approximately 700 km, which
is considerably shorter than the wavelength of maximum
baroclinic instability of 4000 km indicated by the twolayer model and the 5500 km for Eady waves (Holton,
2004). However, smaller vertical scale, weak 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 associated
with a shallow baroclinic zone, and showed that the
fastest growing wavelength 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 500 km, corresponding to the observed
wavelength. A study of a two-layer Eady model by Blumen
(1979) showed that baroclinic instability has a maximum at
c 2013 Royal Meteorological Society
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H. Mc Innes et al.
(a)
(b)
(c)
(d)
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.
short wavelengths when the lapse rate of the lowest layer is
close to adiabatic, and this model has been used successfully
to explain the development of mesoscale lows over the North
Sea (Van Delden et al., 2003).
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 five 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 National Oceanic and Atmosphere Administration
(NOAA) infrared satellite image from 28 February 1149 UTC
and Figure 7(b) shows the dropsonde positions together
with the time for each dropsonde overlaid on a satellite
image from 1326 UTC 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 during the flight. In the current section we
will assess the 36 h operational forecasts valid at 28 February
12 UTC from the HIRLAM model (Undén et al., 2002) run
at The Norwegian Meteorological Institute and the ECMWF
model. The objective is to investigate how well the models
simulated this relatively complicated system involving the
arctic front, the three warm-core lows (WC1, 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 parametrization of condensation and cloud processes.
While surface fluxes of momentum, heat and moisture over
c 2013 Royal Meteorological Society
75°N
72°N
L3
L2
L5
L4
69°N
L1
66°N
63°N
16°W
8°W
0°
8°E
16°E
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.
the ocean are expressed by bulk formulae 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).
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 parametrized 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.
The dropsonde observations will be used here to verify the
SLP fields and the 925 hPa wind simulated by these models.
The dropsondes measured pressure, humidity, temperature
and horizontal wind with an accuracy of 1 hPa, 0.1 K, 5%
and 0.5 m s−1 respectively, and with a time resolution of
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
Observational Study of an Arctic Front
(a)
2139
(a)
(b)
(b)
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 2008 at 1149 UTC. 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 2008 at 1326 UTC. The green ellipse indicates where the highest
wind speeds associated with the upper-level jet, exceeding 40 m s−1 , were
observed.
one observation every half second (Kristjánsson et al. 2011).
The vertical 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 observed SLP attributed to this is less than 0.8 hPa.
Figure 8(a) shows the SLP field together with the
500–1000 hPa thickness from the 36 h 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,
whereas 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◦ 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 air too cold over
the Greenland Sea, with a minimum 500–1000 hPa thickness
of <5000 m compared with 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,
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, whereas we see that both models simulated a
c 2013 Royal Meteorological Society
Figure 8. The 36 h runs of (a) HIRLAM and (b) the ECMWF model
valid at 28 February 1200 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).
weak ridge in that area. Instead they placed a closed low
further west towards the coast of Greenland. Assessments
of the SLP analysis as well as the 500–1000 hPa thickness
from both models, valid 12 and 6 h earlier (not shown),
indicate that this low corresponds 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 72◦ N and 74◦ N, 4◦ W
and 3◦ E indicate a ridge in this area, considerably stronger
than the 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
its 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 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
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
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H. Mc Innes et al.
from 1149 UTC (Figure 5(d)) did not 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
instead of moving the cyclone northwestwards to Greenland,
they moved WC2 northnortheastwards along the Norwegian
coast. This is indeed a serious failure as the models in
such a case could mislead forecasters to issue warnings on
dangerous weather along the coast.
A minimum SLP of 988 hPa was observed by dropsonde
2 (Figure 7(a)) over the Norwegian Sea at 70.8◦ N, 7.2◦ 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)).
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 the eastsoutheast, with wind speeds
exceeding 20 m s−1 north of the frontal zone between the
east coast of Greenland and the zero meridian. The 25 m s−1
wind from the eastsoutheast 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.
Further south over the Greenland Sea, between 72◦ N and
◦
74 N, the observed wind is 5 to 15 m s−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◦ E and 10◦ E, the wind
directions from HIRLAM and 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 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.
(a)
(b)
Figure 9. Observed wind at 925 hPa (red) and forecast wind (blue) from
36 h runs of (a) HIRLAM and (b) the ECMWF model valid at 28 February
1200 UTC. Only a part of the HIRLAM domain is shown in (a).
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 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 shown in
Figure 10(a), where the horizontal gradient clearly reveals an
4. The mesoscale structure of the front
approximately 150 km wide frontal zone in the northeastern
In the current section we will use the dropsonde data part of the cross-section with a temperature difference of
obtained during the flight to study the mesoscale features about 6 K between the two different air masses at 800 hPa.
of the frontal zone, and we will further investigate how The front has been marked as a dashed line, connecting the
well these features were simulated by the HIRLAM model. leading edge of the baroclinic zone and the frontal inversion.
This will be done by assessing the cross-sections of potential Near the surface, the gradient is much weaker, probably
temperature, wind speed and relative humidity from the due to vertical mixing in the conditionally unstable air on
different dropsonde legs, and we will pay most attention the cold side of the front, evidenced by convective cloud
to leg 4 (Figure 6) as the data coverage from the four structures in Figure 7. The relatively shallow baroclinic zone
dropsondes released in that flight leg was relatively good for has a slope of approximately 1 to 70 towards the southwest
all parameters and the temperature gradients were strong and delimits a low-level pool of cold air to the southwest
at this location. The first sonde of this leg was released from the warmer air to the northeast. Southwest (left) of
at 1306 UTC at 72.4◦ N and 9.8◦ W while the last sonde dropsonde 11 the frontal zone extends as a frontal inversion,
c 2013 Royal Meteorological Society
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
Observational Study of an Arctic Front
(a)
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pressure (mb)
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650 27
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850 270
900
950
500
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distance (km)
(b)
Figure 11. Cross-section of potential temperature (every 2 K) through leg
1 based on dropsonde data with the westernmost dropsonde to the left.
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
dropsondes through leg 2 (not shown) revealed the same
features as the data from legs 4 and 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.
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 1200 UTC.
capping the 268–270 K low-level cold air. The temperature
profile from 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 h
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 shows that the simulated cold pool is too deep and
the isopleth too steep compared with 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◦ . The cross-section of potential
temperature (Figure 11) shows a pool of relatively cold air
underneath an inversion in the western part of the crosssection, while there is 5–6 K warmer air in the eastern part.
A shallow low-level frontal zone separates these air masses,
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-section was well
reproduced by the 36 h HIRLAM forecast (not shown), while
the 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
c 2013 Royal Meteorological Society
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 emitting laser pulses at 2.02 µm wavelength with
1–1.5 mJ energy and 500 Hz repetition rate. The advantage
of such a coherent Doppler lidar is the high accuracy of the
Doppler shift obtained, 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−1 .
Any degradation/dealignment of the lidar results in a lower
SNR, which has only a minor impact on the accuracy
achieved. Furthermore, this lidar is equipped with a double
wedge scanner made by Deutsches Zentrum für Luft und
Raumfahrt (DLR). The scan pattern is a 20-point step-andstare conical scan looking downward with a 20◦ half-cone
angle, the duration of one scan (= one revolution) being
roughly 30 s. From all 20 stare positions the Doppler shift is
estimated, the influence of the platform motion subtracted,
and then the three-dimensional wind vector is calculated by
an inversion of the line of sight (LOS) wind speed obtained.
Thus a height-resolved wind profile is obtained every half
minute. The vertical resolution of 100 m is determined
mainly by the pulse length of the emitted laser pulse, which
is approximately 600 ns, and the horizontal resolution is
given mainly by the opening angle of the cone and the
aircraft movement during one scan. For example, at 10 km
altitude of the aircraft and a velocity of 200 m s−1 the
footprint of the scan is approximately 7 km wide and has a
length of 13 km.
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, the higher the backscattered
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
2142
H. Mc Innes et al.
(a)
(b)
Figure 12. Observed wind in m s−1 through leg 4. (a) Cross-section based
on dropsonde data (every 2 m s−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-section measured by the Doppler lidar onboard the aircraft.
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.
For example, 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.
4.3.
The horizontal wind pattern
Here we will 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 m s−1 jet
covering major parts of the troposphere above the frontal
inversion, which was discussed in 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, while
wind speeds exceeding 15 m s−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 m s−1 .
We have also investigated the wind data measured by
the Doppler lidar, and we show the 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 than the
80–90 km resolution of the dropsonde data from leg 4. As
c 2013 Royal Meteorological Society
previously mentioned the presence of aerosols is necessary
to give backscatter and hence wind measurments, while
deep clouds will attenuate the laser beam. This explains why
in the northeastern part of the leg we obtain 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 m s−1 between 6 and 7 km altitude. This shows that
while the dropsonde data provide 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.
In the 36 h HIRLAM run (not shown) the broad features
are mainly consistent with the corresponding cross-section
based on the dropsonde data (Figure 12(a)), with strong
winds exceeding 30 m s−1 above the sloping frontal inversion
and much weaker winds closer to the surface. However, the
model failed to simulate the strongest winds associated with
the upper-level jet (Figure 12(b)), attaining a maximum
wind speed of 34 m s−1 as compared with more than 40 m s−1
measured by the lidar. HIRLAM simulated surface winds of
approximately 20 m s−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 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
(winds.jpl.nasa.gov/) between 04 and 06 UTC (Figure 13(a))
and between 17 and 19 UTC (Figure 13(b)) revealed easterly
wind between 15 and 18 m s−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.
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 m s−1 at approximately1.6 km altitude. As 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 m s−1 , indicating that the low-level jet
did extend at least as far south as dropsonde 6.
A corresponding cross-section from the lidar is shown
in Figure 14(b). Also here the range of 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 km.
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
Observational Study of an Arctic Front
(a)
(a)
(b)
(b)
Figure 13. Sea -evel winds observed from QuikScat (winds.jpl.nasa.gov/)
28 February 2008 between 0400 and 0600 UTC (a) and between 1700
and 1900 UTC (b). The positions of dropsonde 6 and dropsonde 12 are
indicated by S6 and S12 respectively.
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 m s−1 were observed by the lidar
at approximately 5–7 km altitude, while the dropsondes
indicated that the wind speed was approximately 32 m s−1
in this area (Figure 14(a)). Clearly the spatial resolution
provided by the three dropsondes released in leg 2 was
too coarse to capture the detailed distribution of the wind,
thereby missing the 40 m s−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 here also the lidarbased cross-section (not shown) picked up the core of the
upper level jet with wind speeds up to 40 m s−1 , while
the dropsonde data were too coarse to reveal these details.
Further east, the wind measured by the lidar during leg 1
(Figure 6) indicated a 10–15 m s−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 4◦ and 10◦ 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 NWP models made the
planning of this flight difficult (Kristjánsson et al., 2011).
4.4. Vertical wind measured by the lidar
Cross-sections of vertical wind speed measured by the
Doppler lidar through leg 4 and leg 2 are shown in
c 2013 Royal Meteorological Society
2143
Figure 14. Observed wind speed (m s−1 ) through leg 2. (a) Cross-section
based on dropsonde data (every 2 m s−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.
Figures 15(a) and 15(b) respectively. Despite the gaps in the
lidar data due to deep clouds, the figures indicate ascending
air associated with the frontal zone. Figure 15(a) shows
vertical updrafts of 0.1 to 1 m s−1 above the front between
200 and 300 km, and vertical motion of the same magnitude
is also seen above the front in Figure 15(b), where there
are 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 m s−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 15(b), associated with shallow convection, as 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 subsection.
4.5. Frontal cloud band and dry slot
Here we will study observations of relative humidity with
respect to water (RH w ) in order to gain further insight
into the distribution of dry and moist air. The cross-section
of RH w 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 RH w of more than 80% up to
approximately 650 hPa and more than 60% up to 400 hPa. An
investigation of the cross-section of RH i (relative humidity
with respect to ice) indicated that the cloud band was fairly
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
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H. Mc Innes et al.
(a)
(a)
(b)
(b)
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.
deep as values of RH i exceeded 90% up to approximately
380 hPa in the northeastern part of the leg (not shown).
In the southwestern part of the leg, between 0 and
150 km, we recognize the pool of cold air beneath the
frontal inversion as humid air with RH w around 80%,
while above the top of the frontal inversion (dashed line in
Figure 16(a)) the air is extremely dry, with RH w less than
20%. This slot of dry air could be a sign of descending
air associated with a downfolding of the tropopause above
the front (Wallace and Hobbs, 2006). The 36 h HIRLAM
prediction (Figure 16(b)) is consistent with the observations
in that it predicts a slice of dry air in the southwestern
(left) part of the cross-section, but this slice is narrower
than in the observations and extends 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 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
c 2013 Royal Meteorological Society
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 1200 UTC.
manual analysis, and the RH w of 80% predicted by HIRLAM
at approximately 450 hPa and 100 km is inconsistent with
the observations.
We also studied the cross-section of RH w based on the
dropsondes from leg 1 (Figure 17), and here also 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 RH w based on the dropsondes of legs 2
and 3 strongly indicated a dry slot.
5.
Short summary of the observed frontal structure
Based on the analysis of the observations described in section
4 (mainly data from legs 1, 2 and 4) we have summarized the
main features associated with this frontal zone in 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
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
Observational Study of an Arctic Front
2145
(a)
Figure 17. Cross-section of relative humidity with respect to water (%)
through leg 1. The westernmost dropsonde is to the left.
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 of RH
(Figures 16(a) and 17). A summary of the vertical crosssection 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, 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 were released too
far south to 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 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.
(b)
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. Arrows directed into the picture
mark the upper-level and low-level jets and a dashed blue line depicts the
dry slot.
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 and
Fedor, 1993) extended up to approximately 500 hPa, the
present front was confined mainly to levels below 700 hPa.
6. Discussion
Hence the front was shallow compared with observed polar
cold fronts, but it was deeper than the arctic fronts investiDuring 27 February 2008 a relatively complex synoptic gated by Grønås and Skeie (1999) and Shapiro et al. (1989),
situation involving a stationary upper-level cold cyclone off both of which were confined to levels below 850 hPa.
As in the case of the arctic front investigated by Grønås
the east coast of Greenland and two intense northwestwards
moving warm-core lows over the Norwegian Sea gave rise and Skeie, observations from both QuikScat and the
to advection of cold air towards the southeast and warm northernmost dropsonde indicate that the front in the
air towards the northwest, creating a frontal zone that present study was accompanied by severe low-level winds,
delimited relatively warm air to the north and colder air to which could be a danger to human activity in the area. These
the south. On 28 February the mesoscale structure of this strong winds were a manifestation of a low-level jet on the
arctic front was observed by dropsondes and a remotely warm side of the front, but most of this jet was missed by
sensing wind lidar carried on board the DLR Falcon research the dropsondes, as they were released too far south. In their
aircraft. In the cross-sections based on the dropsonde data case study of low-level jets ahead of cold fronts, Browning
the front appeared as a 150 km broad baroclinic zone and and Pardoe (1973) found that the strongest winds were at
further southwest as an inversion layer capping the pool of 900 to 850 hPa with wind speeds between 25 and 30 m s−1 .
c 2013 Royal Meteorological Society
Q. J. R. Meteorol. Soc. 139: 2134–2147 (2013)
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H. Mc Innes et al.
The 40 m s−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 m s−1 in the core of the jet. 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. (1987b), 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
(Figures 16(a) and 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, as evidenced by high ozone concentrations
measured from an aircraft. We investigated the cross-section
of a potential vorticity (PV) based on a 6 h simulation of
HIRLAM valid at 28 February 12 UTC for the 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
in this case also is associated with a tropopause fold. 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. (1987b).
Severe low-level winds clearly show the importance of
paying attention to arctic fronts in 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
increasing the resolution of operational model runs would
improve the forecasts of mesoscale systems associated with
arctic fronts as well.
7.
Concluding remarks
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 dropsonde data
revealed the main features of the frontal system, the use of
Doppler lidar 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 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
c 2013 Royal Meteorological Society
limitations in the presence of thick clouds or concentrations
of aerosols that are too low.
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.
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 that have been of great help in our work with the
manuscript.
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