informasjonsside
Mission
THORPEX is a 10-year international research and development programme to accelerate improvements in the accuracy of one-day to two-week high impact weather forecasts
for the benefit of society, the economy and the environment.
THORPEX establishes an organizational framework that addresses weather research and forecast problems whose solutions will be accelerated through international collaboration among academic institutions, operational forecast centres and users of forecast products.
THE NORWEGIAN IPY–THORPEX Polar Lows and Arctic Fronts during the 2008 Andøya Campaign by J. E. Kri stj ánsson, I. Barstad, T. Aspelien, I. Føre, Ø. Godøy, Ø. Hov, E. Irvine, T. Iversen,
E. Kolstad, T. E. Nordeng, H. McInnes, R. Randriamampianina, J. Reuder, Ø. Saetra,
M. Shapiro, T. Spengler, and H. Ólafsson A field campaign out of northern Norway in winter 2008, including use of airborne lidar and
targeted observations, provided new insight into the dynamics and predictability of polar lows and Arctic fronts.
E ven today, more than 100 years after the
Norwegian explorer Roald Amundsen first
navigated the Northwest Passage, the Arctic
remains a remote area with little human infrastructure.
At the fringes of the Arctic Basin, where in winter the
frigid Arctic air meets open waters, extreme weather
in the form of poorly understood Arctic fronts and
polar lows can develop. When the Arctic air masses
move over the much warmer ocean, a large heat
exchange between the ocean and the overlying air
takes place, with the potential for the subsequent
formation of convective weather systems. One of the
hotbeds of such marine cold air outbreak (MCAO;
Kolstad et al. 2009) activity is the northern North
Atlantic, the Nordic Seas, and in particular the region
between Svalbard and Norway.
Vilhelm Bjerknes, in a newspaper article in 1904,
described the primary reason for the high frequency
of severe weather over the Nordic Seas as follows:
“The northernmost part of Norway in winter is
one of the stormiest locations on Earth, and the
terrible accidents that occur from time to time,
when large parts of the fishing fleet with crew and
tools are lost, are only too well known. A look at the
climatological conditions shows [that] the reason
for the frequency of the storms [is that] the mean
temperature in January by the outermost Lofoten
Islands is 27 degrees Celsius higher than the mean
for the same latitude around the globe. This is the
effect of the warm waters of the Gulf Stream. At
the same time a Siberian winter cold reigns on the
Finnmark plateau [in northern Norway]. Nature
has, in other words, put an immense steam kettle
side by side with an immense condenser. This steam
engine must always work, and that is what it does,
with great, irregular strokes.”
This study deals with three Arctic weather phenomena
that can give rise to extreme weather that is
often poorly predicted: polar lows (Rasmussen and
Turner 2003), Arctic fronts (e.g., Grønås and Skeie
1999; Drüe and Heinemann 2001), and fierce low-level
winds generated by flow over and around orography
in stably stratified conditions (e.g., Skeie and Grønås
2000; Renfrew et al. 2009). In general, NWP errors
tend to be larger in the Arctic than at midlatitudes
(Nordeng et al. 2007), for the following reasons:
• First, the conventional observational data network
in the Arctic is sparse. Satellites are therefore
crucial in filling the observational gaps, but despite
the excellent temporal coverage by polar-orbiting
satellites, they are not yet able to compensate for
the lack of radiosonde data. Nevertheless, the value
of satellite observations for NWP, for example
through the Advanced Microwave Sounding
Unit (AMSU-A) and the Infrared Atmospheric
Sounding Interferometer (IASI), has been demonstrated
by several investigators (e.g., Cardinali
2009; Hilton et al. 2009).
• Second, the weather phenomena listed in the
previous paragraph are poorly represented in
current numerical weather prediction models.
This deficiency is partly due to the small spatial
scales needed to resolve the processes responsible
for severe weather phenomena (e.g., polar lows are
an order of magnitude smaller than synoptic-scale
cyclones). In addition, moist convection, which
is a major challenge for NWP models (Yano and
Geleyn 2010), is an important component in polar
low development (Rasmussen and Turner 2003).
Furthermore, the NWP models are known to
have large biases in their treatment of the Arctic
planetary boundary layer (Tjernström et al. 2005).
These biases can significantly degrade the ability
to simulate the phenomena in question.
Polar lows and other weather phenomena
associated with MCAOs are treacherous in that they
often arrive from the north during otherwise calm
weather. In the television documentary about the
International Polar Year (IPY)–The Observing System
Research and Predictability Experiment (THORPEX)
flight campaign, a fisherman from northern Norway
explained how he and his brother saw a wall of dark
cal paper by Montgomery and Farrell (1992) and in
a numerical study by Grønås and Kvamstø (1995). It
was not until the 2005 Lofoten Cyclone campaign
(LOFZY; Brümmer et al. 2009) that low-level flights
(less than 50 m above the surface) were carried out
in a polar low environment, thereby allowing direct
measurements of surface fluxes of sensible and latent
heat using high-frequency (100 Hz) sampling of vertical
velocity, temperature, and specific humidity. In
the flight on 7 March 2005, areal averages of sensible
and latent heat of 115 and 190 W m−2, respectively,
were obtained in a shallow and short-lived polar low
over the Barents Sea between Norway and Svalbard.
The highest fluxes of 290 and 520 W m−2, respectively,
were found underneath a cloud band to the north of
the polar low center.
clouds approaching from the north when they were
surprised by a poorly forecasted polar low. Their boat
capsized and his brother drowned. According to the
former director of the weather forecasting office
in Tromsø, Kari Wilhelmsen (in Grønås and Skeie
1999), 56 vessels were lost at sea and 342 people died
in Norwegian waters during the twentieth century.
Undoubtedly a fair share of these deaths were due
to weather associated with MCAOs, because these
phenomena hit more abruptly with less warning in
the form of classical weather signs than synopticscale
cyclones.
Detailed in situ measurements of atmospheric
characteristics in polar lows by research aircraft are
rare. The very first flight of this kind took place on
27 February 1984 during the Norwegian Polar Low
Project, when the National Oceanic and Atmospheric
Administration (NOAA) WP-3D research aircraft
penetrated an intense polar low over the Norwegian
Sea southeast of Jan Mayen (Shapiro et al. 1987).
Surface fluxes of sensible and latent heat were both
estimated at about 500 W m−2 (Shapiro et al. 1987).
However, it is important to note that those estimates
were based on flights at 300 m above the surface
and therefore cannot be regarded as direct measurements.
The observations revealed a warm, moist
inner core all the way up to 580 hPa and maximum
wind speeds up to 35 m s−1 at two locations near the
center of the polar low. Subsequent NWP model
simulations (Grønås et al. 1987) showed that the
model significantly underestimated the strength of
the polar low, even though the initial conditions from
the European Centre for Medium-Range Weather
Forecasts (ECMWF) analysis appeared surprisingly
realistic.
The next two polar low flights were carried
out over the northern Gulf of Alaska during the
1987 Alaska Storms Program (Douglas et al. 1991).
A polar low was penetrated on two consecutive days
(2–3 March 1987) with the NOAA WP-3D, revealing
a gradual deepening of the low and an increase of the
vertical extent of the circulation with time. Similar
to the polar low near Jan Mayen, the Alaskan polar
low developed a warm core, even though it developed
in a baroclinic environment. Returning to the
Norwegian Sea with the NOAA WP-3D, the structure
of a polar low near Greenland was investigated
during the Coordinated Eastern Arctic Experiment
(CEAREX) 1989 campaign (Douglas et al. 1995).
This rather weak system had most of its circulation
below 800 hPa. Nevertheless, an upper-level
vorticity perturbation was also found, suggesting a
possible coupling between low-level and upper-level
In summary, these observational studies indicate
that a complex interplay among low-level baroclinicity,
upper-level forcing, sensible and latent heat fluxes,
and latent heat release contributes to polar low genesis
and intensification. This interplay has been addressed
by theoretical and modeling studies: Some of them
have merely focused on the moist convection associated
with polar lows and have used hurricane theory
to explain polar low evolution. Rasmussen (1979,
1981) suggested conditional instability of the second
kind (CISK) as a crucial mechanism in organizing
the convection, and in support of this (Rasmussen
et al. 2003) seemingly found evidence of a reservoir
of convective available potential energy (CAPE) over
the Norwegian Sea in polar low situations.
Emanuel and Rotunno (1989) coined the term “arctic hurricanes”
for polar lows, and proposed wind-induced
surface heat exchange (WISHE) as the mechanism
for their development. Other studies (e.g., Mansfield
1974; Duncan 1977) have focused on the baroclinic
nature of polar lows. Because of the low tropopause
in Arctic air masses, the weak static stability, and the
large Coriolis parameter, maximum baroclinic wave
growth is expected to occur for much smaller horizontal
scales than at midlatitudes (see, e.g., Holton
2004, 230–238), which is consistent with the observed
size of polar lows (200–500 km in diameter). Recently,
Bracegirdle and Gray (2008), in a climatological study,
found evidence for polar lows over the southern
Norwegian Sea being of “type C” (Plant et al. 2003),
meaning that they are driven by an interplay between
an upper-level potential vorticity (PV) anomaly and
latent heating. Farther north, on the other hand,
Bracegirdle and Gray (2008) typically found stronger
baroclinic and weaker convective contributions.
The model-based polar low studies of Mailhot et al.
(1996) and Føre et al. (2011a, manuscript submitted to
Quart. J. Roy. Meteor. Soc.) obtained extremely high
sensible heat fluxes (1,200–1,400 W m−2) near the ice
edge, and the latter study found sensible heating to
be a crucial factor in explaining the polar low deepening.
The field campaign described below seeks to
provide observational data that will enable scientists
to validate these diverse conceptual models, while
at the same time providing a test bed for the NWP
models, thereby contributing to improved forecasting
of polar lows.
Shallow Arctic fronts define the southern extension
of the cold Arctic boundary layer originating
over sea-ice/land and may extend several hundreds
of kilometers southward during major cold air outbreaks.
The stable air of the frontal zone is eroded
by a convective boundary layer building up over
the sea, and model simulations indicate that the
corresponding large surface fluxes and release of
latent heat are important for the frontal circulation
(Grønås and Skeie 1999). Although their small spatial
scale precludes routine detection, shallow Arctic
fronts generating strong winds are believed to occur
frequently in situations with off-shelf flow in winter.
Only two detailed observations of Arctic fronts exist
(Shapiro et al. 1989; Drüe and Heinemann 2001),
and new observational data are essential for a better
understanding of the phenomenon.
When the stable Arctic air impinges on the mountains
of Spitsbergen, strong jets and downslope winds
can occur, aided by drainage and katabatic flow from
the ice sheets over the interior of Svalbard, and the
flow becomes highly nonlinear. For instance, Skeie
and Grønås (2000) found speed-up factors larger
than 2 relative to the upstream wind speed in easterly
flow impinging on Spitsbergen. Previous studies also
indicate that in order to simulate such wind storms,
very high spatial resolution is required (e.g., Sandvik
and Furevik 2002). These previous studies are almost
exclusively model based, and there is an urgent need for
observational data to validate the model simulations.
The purpose of this paper is to describe a field
campaign, operated out of Andøya in northern
Norway in 2008, gathering unique atmospheric
data on the weather phenomena described above.
The campaign was jointly funded by the Norwegian
Research Council, the German Aerospace Center
[Deutsches Zentrum für Luft- und Raumfahrt
(DLR)], and the European Facility for Airborne
Research (EUFAR). One of the international clusters
within the International Polar Year effort was IPY–
THORPEX, consisting of, e.g., the Greenland Flow
Distortion experiment (GFDex: Renfrew et al. 2008),
Storm Studies in the Arctic (STAR; Hanesiak et al.
2010) and the Norwegian IPY–THORPEX, which
we here report on. IPY–THORPEX is linked to the
THORPEX program of the World Meteorological
Organization (WMO), and the overall objective of
the Norwegian IPY–THORPEX project is to improve
the accuracy of high-impact weather forecasts in the
Arctic region. In order to achieve this objective, the
following research activities were initiated: (i) a comprehensive
field campaign in the late winter of 2008
described herein, (ii) investigations of the synopticscale
conditions preceding the development of the
phenomena responsible for Arctic weather extremes,
(iii) the development of a probabilistic forecasting
system [the Limited-Area Model Ensemble Prediction
System (LAMEPS)] for the Norwegian Arctic, and
(iv) enhanced use of new satellite data, in particular
from the IASI instrument on the European polar
satellite MetOp.
The field campaign sought to address the following
research questions:
• What is the relative role of low-level baroclinicity,
upper-level forcing, surface fluxes and latent
heat release from deep convection for polar low
development?
• To what extent can conceptual models from hurricane
theory (CISK, WISHE) be applied to polar
lows?
• Is there a potential for improving weather forecasts
(deterministic as well as ensemble) in the Arctic
through targeted observations by aircraft?
It also was designed to meet the following needs:
• Documenting the structure of Arctic fronts
• Investigating the capability of airborne lidar
systems to capture mesoscale wind and humidity
structures in the Arctic
• Leaving a legacy for future research by providing
unique high-quality observational data to the
international scientific community
• Leaving a legacy for weather forecasting by providing
and exploiting new observational platforms
[e.g., unmanned aerial systems (UAS)] in the datasparse
Arctic region
In the following section, a general overview of
the 3-week campaign is presented. In the section
“Campaign snapshots” we present results from
data gathered during selected flights, followed by a
section on the use of targeted observations during
the campaign. In the section “Leaving a legacy” the
future directions for observations in the Arctic are
discussed, before presenting a summary and the main
conclusions of the study in “Concluding remarks.”
OVERVIEW OF THE CAMPAIGN AND SYNOPTIC
CONDITIONS. The 3-week campaign
took place during the period 25 February–17 March
2008, with a base of operations at Andøya on the
Atlantic coast of northern Norway, at 69°N, 16°E,
some 280 km north of the Arctic circle (Fig. 1). The
main measurement platform of the field campaign
was the German research aircraft DLR Falcon,
equipped with three measurement systems: (i) probes
for in situ measurements of the turbulent fluctuations
of the three-dimensional wind, temperature, and
humidity; (ii) two lidar systems retrieving profiles
of humidity and wind; and (iii) dropsondes that
continuously measure wind, temperature, humidity,
and pressure as they fall through the atmosphere. The
technical features of the instruments are described
in appendix A. A total of 12 missions were flown,
in three cases with refueling at Spitsbergen, and the
total number of flight hours was 55. A total of 150
dropsondes were released during the 12 missions,
and all of the retrieved observations were transmitted
onto the Global Telecommunication System (GTS),
thereby influencing the operational model analyses
at the time. An overview of the flights is presented
in Table 1, while Fig. 1 shows the flight tracks and
the geographical area. All the observational data of
Fig. 1. An overview of the flights during the 3-week field
campaign 25 Feb–17 Mar 2008. The numbers refer to
flight numbers in Table 1. The campaign headquarters
were at Andenes on the coast of Norway, at 69°N,
16°E. Flights 1, 3, and 11 had refueling at Longyearbyen,
Svalbard at 78°N, 16°E, while flights 9 and 10 took off
from Keflavík, Iceland at 64°N, 23°W. The letters J, B,
and N indicate geographical locations referred to in the
text: J = Jan Mayen; B = Bear Island; N = Ny-Ålesund.
Table 1. An overview of the 12 missions that were flown during the campaign. In all 56 flight hours
were carried out, releasing 150 dropsondes.
|
Mission |
date |
Objective |
Flight |
dropsondes |
|
1 |
27 |
Svalbard |
6h |
8 |
|
2 |
28 |
Reversed |
3h |
15 |
|
3 |
1 |
Polar |
7h |
19 |
|
4 |
3 |
Polar |
4h |
20 |
|
5 |
3 |
Polar |
3h |
15 |
|
6 |
4 |
Polar |
3h |
20 |
|
7 |
6 |
Lee |
3h |
8 |
|
8 |
9 |
Barrier |
3h |
6 |
|
9 |
10 |
Orographic |
4h |
6 |
|
10 |
11 |
Targeting |
3h |
6 |
|
11 |
15 |
Cold |
7h |
14 |
|
12 |
17 |
Polar |
3h |
13 |
the campaign are made available to the international
community via the IPY Data and Information Service
(http://ipydis.org/), as described in appendix B.
Surface fluxes of sensible and latent heat, which
are important ingredients in polar lows, can be
measured in situ by flying into the lowest 50 m of
the atmosphere and exploiting the turbulent probes
on the aircraft. This was done by the DLR Falcon
in the LOFZY 2005 campaign. However, because of
the risk of misalignment of the sophisticated lidar
instruments on board, it was decided not to fly into
the turbulent surface layer. Another possible way of
obtaining the surface latent heat flux is by combining
the two lidar systems, as described by Kiemle et al.
(2007). However, this would have required flying in
the lower troposphere, which in most of the polar low
cases was very cloudy, while at the same time losing
the ability to obtain dropsonde profiles through
the whole troposphere.
Therefore, with one minor
exception north of Svalbard on 15 March, all the
flights in this campaign were carried out at altitudes
of about 8 km. That altitude is ideal both for lidar
and dropsonde profiling of the whole troposphere.
For the two polar low cases described below, surface
fluxes of sensible and latent heat were estimated from
bulk formulas using near-surface temperature and
humidity from the dropsondes, combined with sea
surface temperatures (SSTs) from model analyses
(Føre et al. 2011b). The results of these calculations
will be discussed below (note that Fig. 10 is based on
such a calculation).
To complement the aircraft observations, the campaign
was augmented by surface observations: Two
Norwegian coast guard vessels released radiosondes
on demand within the area of interest. Additional
6-hourly radiosonde releases were carried out at the
Norwegian Arctic stations Jan Mayen, Bear Island,
and Ny-Ålesund. In collaboration with the integrated
Arctic Ocean Observing System (iAOOS), drifting
buoys supplied observations of sea-level pressure and
near-surface winds. Finally, two unmanned aerial systems
were deployed at Spitsbergen for measurements
of the Arctic atmospheric boundary layer. The larger
one, a CryoWing with a wingspan of 3.8 m and a maximum
takeoff weight of 30 kg (http://uas.norut.no/
UAV_Remote_Sensing/CryoWing.html), operated by
the Northern Research Institute (NORUT) in Tromsø,
was stationed close to Longyearbyen. The smaller one,
a Small Unmanned Meteorological Observer (SUMO;
Reuder et al. 2009)—a lightweight micro-UAS with
a wingspan of 80 cm and a takeoff weight of 580 g,
developed and operated by the Geophysical Institute,
University of Bergen (UiB)—was used as a “recoverable
radiosonde” both on land and during a cruise around
Spitsbergen from the helicopter deck of the Norwegian
Coast Guard vessel KV Svalbard.
Following the mildest December–February (DJF)
period on record in northern Norway, the weather
pattern changed dramatically during the last week of
February, such that two out of the three weeks of the
campaign were dominated by cold Arctic air masses
over the area of interest (shown in Fig. 1). This is shown
in Fig. 2, expressed by the dimensionless MCAO index,
which is approximately equivalent to the difference
between the SST and the potential temperature at
700 hPa in kelvins (Bracegirdle and Kolstad 2010) and
is therefore a good measure of the vertical stability in
the lower troposphere. Typical MCAO index values
during polar lows range from 2 to 20, with an average
in the 5–10 range. The figure displays the daily mean
MCAO index from 1 February to 30 March 2008
defined along the 71°N latitude line in the Atlantic
sector. During the first 10 days of the campaign
(25 February–6 March) the MCAO index values
were about 5–10 units higher than the climatological
mean in this region (about −5 to 0 during winter), as a
cold upper-level low dominated the flow pattern (not
shown). This was the period when several polar low
developments took place, including the 3–4 March
polar low, which is extensively described below. Seven
flight missions with a total of 35 flight hours were
flown in this 11-day period. On 7 March, the atmospheric
flow patterns started to change, with warmer
air masses advancing into the Norwegian and Barents
Seas over the following days. Consequently, the attention
was shifted to the Greenland–Iceland area, where
northerly flow prevailed, and the next three missions
were carried out in that region (Table 1), studying
barrier flow generated by Greenland’s orography, as
well as wind patterns generated by isolated mountains
in Iceland (see, e.g., Ólafsson and Ágústsson 2007).
Then, starting on 15 March, a sustained period with
high MCAO index values, at times more than 10 units
higher than the climatological mean, occurred in most
of the region east of the 0° meridian.
Fig. 2. The MCAO index for each NCEP reanalysis grid
point along latitude 71°N in the Atlantic sector from
1 Feb to 30 Mar 2008. The dark blue cells on the far left
are due to the presence of sea ice. The dashed black
lines delineate the campaign period 25 Feb–17 Mar
2008. The climatological average of the MCAO index
during the winter in this region is about −5 to 0.
Months before the campaign, a flight plan strategy
document for the campaign was written (Barstad
et al. 2008). Different weather scenarios were worked
out and flight plans were made. During the campaign
a group of about 30 scientists and technicians
from several institutions and as many as 11 countries
had their headquarters at the Andøya Rocket
Range (http://rocketrange.no), only 4 km from
the airport at Andøya where the DLR Falcon was
based. A core group of about five scientists carried
out the day-to-day flight planning and were in close
dialog with the DLR crew and technicians.
For the flight planning, in addition to rapid Internet access,
the scientists also had direct access to up-to-date
analysis and forecast products of the Norwegian Meteorological
Institute (NMI), making extensive use
of the graphical software DIANA. In addition, they
were in occasional contact with on-duty forecasters
at the NMI regional office in nearby Tromsø, who
have for the last 5–10 years had a special focus on
polar low forecasting.
CAMPAIGN SNAPSHOTS. We now describe
some results from selected f lights during the
campaign, giving a flavor of the different weather
phenomena that were encountered, as well as some
of the associated weather forecasting issues.
Fig. 3. The weather situation at 1200 UTC 27 Feb
2008, depicted by an infrared satellite image (NOAA-16
channel 4) overlaid with fronts and symbols from the
Norwegian Meteorological Institute’s subjective analysis,
sea-level pressure from ECMWF analysis (black
contours), and QuikSCAT-derived 10-m winds (wind
barbs). The QuikSCAT wind data are from the time
window 0900–1500 UTC. The thick dashed black lines
outline the section shown in Fig. 4, while the red dotted
line A–B indicates the position of the cross section in
Fig. 5. The ice edge is shown by a dotted black line.
Orographic winds at Spitsbergen. On the first full day of
the campaign, 26 February, a decision was made to fly
a double mission on the following day, with the objective
of measuring 1) air mass contrasts across a stationary
front over the Barents Sea and 2) terrain-induced
disturbances in the vicinity of the main Svalbard
islands, Spitsbergen and Nordaustlandet (separated by
a dotted line in Fig. 3). Figure 3 displays the weather
situation for 1200 UTC 27 February 2008, showing a
stationary front oriented northwest–southeast over
Bear Island (74°18ʹN, 19°06ʹE), dividing the air masses
in the Barents Sea. Relatively warm air resides to the
southwest, while the northeast sector is dominated by
Arctic air moving westward, leading to cloud street
formation over the relatively warm ocean. When the
cold and stable air in the northeasterly flow impinges
on the Spitsbergen orography, some lifting is expected
as the air—at some vertical level—flows over the
mountains.
Linear theory for flow over topography
(Smith 2002) indicates that whenever stable air rises
(sinks), a positive (negative) pressure anomaly is expected
at the surface. As surface air approaches this
positive anomaly upstream of the mountains, it is
forced to slow down, and rotational effects yield a
diversion of the air to the left of the mountains (in the
Northern Hemisphere). This is evident in Fig. 3 where
the model analysis shows a tight sea-level pressure
field, and elevated Quick Scatterometer (QuikSCAT)
winds (15 m s−1) observed near the southern tip of
Spitsbergen. Fortunately, we were able to obtain exceptionally
good coverage and range with the lidars
on this day. The humidity lidar scan (Fig. 4, upper
panel) shows the much lower near-surface specific
humidity over the sea near Svalbard (at 400–500 km
in the figure) than over the open ocean. At 2–6 km it
shows structures possibly associated with mesoscale
circulations.
The gaps in the display are mainly due
to clouds, which cause a rapid saturation of the laser
beam. From the wind lidar scans (Fig. 4, lower panel),
the most spectacular feature is the near-surface wind
speed exceeding 20 m s−1 near the southern tip of
Spitsbergen (just before 1216 UTC in the figure), as
well as in outflow areas from the fjords on the west
coast of the island (e.g., the Hornsund fjord soon after
1216 UTC and the Van Mijen fjord just before the end
of the section). The low-level wind direction from the
lidar scans (not shown) is east-northeasterly, parallel to
these fjords. These observations are particularly interesting
because the observed winds are several meters
per second stronger than those predicted by the operational
NWP models at the time.
The large data gap in
the wind lidar near 600 km is due to a lack of aerosol
backscatter in the pristine Arctic troposphere.
Fig. 4. Lidar measurements along the track of flight
1 (track shown in Fig. 3) from northern Norway to
Spitsbergen between 1100 and 1228 UTC 27 Feb 2008.
(top) Specific humidity (g kg−1); (bottom) horizontal
wind speed (m s−1). The blue mark just after 400 km
indicates the location of the ice edge, while the red
mark denotes where the aircraft comes in over land
slightly north of the southern tip of Spitsbergen. The
black mark near the end of the lower panel indicates
the Van Mijen fjord. The blue bar below the two panels
has tick marks for turning points of the flight track
(cf. Fig. 3).
After refueling in Longyearbyen at Spitsbergen,
the DLR Falcon took off at 1330 UTC and then
flew through the Hinlopen Strait (from A to B in
Fig. 3), which separates the islands of Spitsbergen
and Nordaustlandet in the Svalbard Archipelago.
Five dropsondes were released. Figure 5 shows a
vertical cross section where the lidar scans indicate
wind speeds up to 20 m s−1 in the strait exit.
This is
corroborated by dropsonde data showing maximum
strength at about 200-m height in the same location.
From idealized simulations, Gaberšek and Durran
(2004) found that increased gap wind along with a
heating of exiting air must be due to a net descent in
the gap.
This is the situation in the Hinlopen Strait,
Fi g . 5. Cross section through the Hinlopen Strait between 1400 and 1430 UTC 27 Feb 2008, showing retrieved wind speed (color; m s−1) from doppler lidar scans overlaid with dropsonde wind arrows (one long barb is 5 m s−1; a short barb is 2.5 m s−1). Horizontal axis indicates the distance (km) in the flight direction. The black, dashed–dotted lines show potential tempera- ture (every 4 K) interpolated from the dropsonde data. The cross section’s position is indicated in Fig. 3 as a red broken line. The dashed gray line is the silhouette of the large-scale mountains adjacent to the strait. The thick solid line indicates the sea ice extent.
as observed in the dropsonde dataset: the potential temperature surfaces descend through the strait, as shown in Fig. 5, and at the same time, the wind speed increases. In addition to mechanisms indicated by Gaberšek and Durran (2004), the open waters on the lee side of the mountains produce positive buoyancy, which may have a significant impact on the f low. A more in-depth investigation of this case is presented by Barstad and Adakudlu (2011).
27 February, the campaign team was keeping a close eye on that area on 28 February, when a new disturbance was expected to develop there, while a weatherr system was moving toward Lofoten (67°30ʹN,
7°33ʹE) from the southwest. Interest- ingly, these two systems, which had dif ferent structures and dif ferent behavior, were dynamically linked, as seen in Fig. 6a. Figure 6b shows
that the frontal zone is reversed, with the coldest air residing to the south. The reversal is caused by advec- tion of cold air around an upper-level cold low over
Fig. 6. (top) A surface analysis from the Norwegian Meteorological Institute at 1200 UTC 28 Feb 2008, showing sea level pressure (blue, every 2 hPa) and
500–1000-hPa thickness (red, dashed, every 20 m). The green line and the numbers 1– 4 refer to the cross sec- tion in the lower panel. (bottom) A north-northeast– south-southwest-oriented cross section of potential temperature (K) and horizontal wind (arrows) from two of the dropsondes from the third flight leg shown in the upper panel, between 1244 and 1306 UTC. The wind from the other two dropsondes is not shown be- cause of gaps in the data from these sondes.
Fig. 7. NOAA AVHRR IR satellite images overlaid with sea-level pressure analyses (every 5 hPa) from the operational Norwegian HIRLAM system and QuikSCAT winds, showing a) a 1200 UTC 3 Mar 2008 analysis combined with the IR image at 1221 UTC; b) a 1500 UTC 3 Mar 2008 analysis, combined with the IR image at
1601 UTC; c) a 0300 UTC 4 Mar 2008 analysis, combined with the IR image at 0307 UTC; and d) a 1200 UTC
4 Mar 2008 analysis, combined with the IR image at 1128 UTC. Red dots indicate dropsonde release positions discussed in the text. Note that the dropsonde points do not coincide exactly in time with the satellite images. The QuikSCAT wind data are from the time window 0900 –1500 UTC.
Greenland, while the warm air on the north side has been advected from a southerly direction over the relatively warm waters of the eastern North Atlantic. Associated with the reversed temperature gradient, there was a strong easterly thermal wind, causing a southeasterly upper-level jet of 34 m s−1 (Fig. 6b).
Polar low on 3–4 March. In the aftermath of a deep synoptic-scale cyclone, which dissipated off the coast of northern Norway on 2 March 2008, a convergence zone g radua l ly formed a long t he 0 °E mer id ia n, stretching from the sea-ice edge at around 80°N to
about 74°N (Fig. 7a). At the same time, as indicated by the cloud structure in Fig. 7a, another frontal zone, most likely a remnant of a previous frontal system, stretched eastward along 74°N latitude to about 25°E. Based on the available NWP guidance, a polar low was expected to develop near the intersection of these two zones, and it was decided to launch two f light missions on 3 March.
In the northerly f low west of the north–south- oriented frontal zone, evidence of a cold air outbreak is seen as cloud streets developing downwind of the ice edge (Fig. 7a). Farther downwind, near 70°N,
a cold front is clearly visible at the leading edge of the cold air outbreak, west of 5°E (Fig. 7a), near the island of Jan Mayen (indicated by the letter J in Fig. 1). According to t he operationa l N WP models, t his system was not expected to develop further, possibly due to the much lower SSTs in that area.
The surface analysis in Fig. 7a shows the incipient surface low at about 73.5°N and 2°E, with a minimum pressure of about 990 hPa. The first f light took off at
1000 UTC, releasing 20 dropsondes in a “butterf ly” pattern (f light 4 in Fig. 1). This pattern was carefully designed to 1) capture the air mass contrasts by f lying along the temperature gradients associated with the conf luence zones, 2) capture the center of the incipi- ent cyclone, and 3) facilitate the use of the lidar instru- ments by f lying relatively long straight f light legs in the area outside the polar low. A similar strategy was followed on the two subsequent f light patterns into the polar low (f lights 5 and 6 in Fig. 1).
Figure 7a indicates the positions of five dropsondes released during one of these legs, crossing the north– south-oriented convergence zone from east to west, and Fig. 8a shows the potential temperature from this section based on the dropsonde data. The most prominent feature is the sha llow front, conf ined below 700 hPa, with the advancing cold air on the western side. Because of geostrophic adjustment in the reversed-shear f low with a southerly thermal wind, there is an associated northerly low-level jet with a wind speed of 26 m s−1 at 940 hPa, detected by sonde number 3 (Fig. 8b).
The second f light took off at 1430 UTC and lasted for 3 h, releasing dropsondes in three straight f light legs. A close inspection of the satellite image about midway into the f light (Fig. 7b) reveals the formation
of an eyelike feature in the polar low, surrounded by cyclonic motion near 73°N, 2°E. The second f light leg stretches from southwest to northeast near the center of the developing polar low (see Fig. 7b). A potential temperature section from this leg (not shown) reveals a warm anoma ly bet ween 400 and 800 hPa ver y close to the cyclone center. Such a downfolding of the potential temperature at upper troposphere level can be an indication of an upper-level PV anomaly (Hoskins et al. 1985). It can also be an indication of descending air.
Overnight, the developing polar low intensified and moved southeast, so that by 0307 UTC a fully developed polar low was clearly discernible in satel- lite imagery (Fig. 7c). A third f light was launched on
4 March at 1000 UTC, lasting 3 h 15 min, dropping a total of 20 sondes, and seeking to capture the struc- ture of the mature stage of the polar low. Figure 7d shows the polar low at 1128 UTC on 4 March, about midway through this mission. Now, the center of the polar low consists of organized convective cells, with a spiral-like cloud band curving to the northeast being the only remnant of the convergence zone that was so well defined one day earlier. Farther south, the Arctic front that was seen near Jan Mayen in Fig. 7a has now reached the Norwegian coast. The first f light leg, re- leasing nine dropsondes, was a straight cut through the low in a direction from northeast to southwest (see marks in Fig. 7d). Figure 9a shows the relative humidity from these observations. At the mature stage the polar low is characterized by towers of moist air, with relative humidity values above 80% observed all the way up to 500 hPa. A very interesting feature in Fig. 9a is the dry upper-level air found in sonde 4. This sonde was dropped close to what looks like an eye in the satellite image in Fig. 7d. Figure 9b shows the potential temperature in the section through the cyclone. On the cold side (left) the tropopause is located at about 500 hPa, which is also the height of the convective towers seen in Fig. 9a. The central core of the cyclone (dropsonde 4) is characterized by warm air and weak static stability, which, combined with the dry feature seen in Fig. 9a, may indicate intrusion of stratospheric air near the center of the cyclone. A corresponding cross section of specific humidity (not shown) confirms this dryness, with values of, for example, below 0.4 g kg−1 at 750 hPa, as compared to about 1.0 g kg−1 in the surrounding dropsondes. At
1800 UTC, a few hours after the f light on 4 March, the polar low made landfall at 64°N, 10°E. As it did so,
20 m s−1 winds and snowfall were observed at Hitra island, about 100 km farther west-southwest.
As mentioned earlier, Føre et al. (2011b) used the dropsonde data from 3–4 March to calculate surface f luxes of sensible and latent heat. Already, during the incipient stage on 3 March (Fig. 7a), they found sensible heat f luxes of about 250 W m−2 and latent heat f luxes of about 200 W m−2 in the cold air outbreak to the west (rear) of the polar low. On the follow- ing day when the polar low had reached maturity, the sensible and latent heat f luxes had increased to about 300 and 350 W m−2, respectively. These latter values can be compared to the direct measurements by Brümmer et a l. (20 09) a nd t he obser vationa l estimates by Shapiro et al. (1987). The former study had maximum sensible heat f luxes similar to those of Føre et al. (2011b), but the maximum latent heat f lux in Brümmer et al. (2009) of 520 W m−2 was much higher than obtained for the 4 March 2008 case. These differences may be related to the somewhat stronger winds in Brümmer’s case yielding generally
higher f luxes, combined with the fact that the sea surface temperatures in the Norwegian Sea south of
70°N are much higher than in the Barents Sea, which would tend to boost the sensible heat f lux. The f lux estimates of Shapiro et al. (1987) of 500 W m−2 for sensible as well as for latent heat f lux are considerably higher than those of Føre et al. (2011b), and that can be attributed to the stronger winds in Shapiro’s case:
35 m s−1, as compared to about 25 m s−1 on 4 March
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