Quarterly Journal of the Royal Meteorological Society
A “hurricane-like” polar low fueled by sensible heat flux:
high-resolution numerical simulations
r
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Journal:
Manuscript ID:
Wiley - Manuscript type:
Complete List of Authors:
QJ-10-0227
Research Article
Pe
Date Submitted by the
Author:
QJRMS
01-Oct-2010
er
Føre, Ivan; University of Oslo, Department of Geosciences
Kristjánsson, Jon; University of Oslo, Department of Geosciences
Kolstad, Erik; University of Bergen, Bjerknes Centre for Climate
Research
Bracegirdle, Thomas; Brittish Antarctic Survey
Røsting, Bjørn; Norwegian Meteorological Institute
Saetra, Oyvind; Norwegian Meteorological Institute, Research
Re
Keywords:
polar lows, air–sea interactions, upper level forcing, numerical
experiments
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A “hurricane-like” polar low fueled by sensible
heat flux: high-resolution numerical
simulations
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Ivan Føre, 1Jon Egill Kristjánsson, 2,3Erik W. Kolstad,
Thomas J. Bracegirdle, 5Øyvind Sætra and 5Bjørn Røsting
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Department of Geosciences, University of Oslo, PO Box 1022 Blindern, NO-0315
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Oslo, Norway
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Uni Research AS, PO Box 7810, 5020 Bergen, Norway
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Bjerknes Centre for Climate Research, PO Box 7810, 5020 Bergen, Norway
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British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 OET, UK
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The Norwegian Meteorological Institute, PO Box 43 Blindern, 0313 Oslo, Norway
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Quarterly Journal of the Royal Meteorological Society
29.09.2010
Quarterly Journal of the Royal Meteorological Society
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Abstract
During 18–21 December 2002 an unusually intense (961 hPa) polar low that resembled a
small hurricane was observed close to the sea ice edge over the Barents Sea. A series of fine
mesh (3 km) experiments with the up to date Weather Research and Forecasting (WRF)
model have been carried out. The full physics simulation was compared to sensitivity
experiments with respect to each type of surface fluxes (sensible and latent heat), their
combined effect and to condensational heating to analyse the physical properties of the polar
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low. We found that the polar low development initially was dominated by baroclinic growth
most likely under the influence of an upper-level potential vorticity anomaly. The proximity
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to the sea ice and the high surface wind speeds (about 25 ms-1) triggered extremely high
surface sensible and latent heat fluxes of about 1200 and 400 Wm-2, respectively. As the
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polar low matured maximum surface sensible and latent heat fluxes dropped to about 600
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and 300 Wm-2, respectively. The simulated polar low was similar to hurricanes and previous
case studies of similar events in that it had a clear, calm, and warm eye structure surrounded
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by moist convection organized in spiral cloud bands and that the highest surface wind speeds
were found in the eye wall. We conclude that surface fluxes are the main forcing mechanism
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as the polar low matured. However, in contrast to tropical hurricanes and some previous
polar low studies, the polar low was dominated by sensible heat fluxes, with latent heat
fluxes playing a minor role. Further, evidence of continuous strong upper level forcing in the
mature stage was found, which is not frequently reported for polar lows. Our sensitivity
experiments show for the first time that condensational heating had a negative effect on polar
low intensity.
Key words: polar lows, numerical experiments, air–sea interactions, upper level forcing
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1. Introduction
The Nordic Seas (the Greenland, Norwegian, and Barents seas) are prone to severe maritime
weather during winter, and especially to polar lows (Kolstad, 2006; Bracegirdle and Gray,
2008; Kolstad et al., 2008). Their appearances and generation mechanisms vary (e.g., see
Wilhelmsen, 1985; Bracegirdle and Gray, 2008), but a common feature of most types of
polar lows is that they are short lived (<24 hours), mesoscale (200–1000 km) cyclones with
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wind speeds above gale force (Rasmussen and Turner, 2003).
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Because some polar lows are similar to tropical hurricanes in appearance and structure, polar
lows have been referred to as “extra-tropical hurricanes” (Rasmussen, 1979), “arctic
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hurricanes” (Emanuel and Rotunno, 1989; Businger and Baik, 1991), and “hurricane-like
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polar lows” (Nordeng and Rasmussen, 1992). A common feature for both storm types is a
typically clear and calm warm eye with large gradients of wind speed and temperature at the
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eye wall and spiral-like cloud bands ending at the edge of the warm core.
Unlike tropical hurricanes, most polar lows (Businger and Baik, 1991; Nordeng and
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Rasmussen, 1992; Grønås and Kvamstø, 1995) initially form in baroclinic environments, and
upper-level lows or troughs (i.e., upper-level potential vorticity (UPV) anomalies) are
believed to be needed in order to initiate the surface developments (see Montgomery and
Farrell, 1992). As the polar low matures, the baroclinic forcing tends to weaken, and any
further development is found to be driven by condensational heating and/or surface fluxes
from the ocean. The role of upper level forcing after initiation at surface levels is still open to
debate. For simplicity we refer to polar lows that resemble hurricanes as “hurricane-like”
polar lows despite the fact that their forcing mechanisms are not identical.
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A drawback of most previous case studies of hurricane-like polar lows is that the role of
surface fluxes and condensational heating has been analyzed using old generation or
simplified models with coarser resolution than what is available today (e.g., see Rasmussen,
1979; Emanuel and Rotunno, 1989). In addition, the role of surface fluxes or condensational
heating in hurricane-like polar lows has been discussed without lack of rigorous numerical
backing (Businger and Baik, 1991; Nordeng and Rasmussen, 1992). In other polar low case
studies, such as Bresch et al. (1997), both latent heat and sensible heat fluxes were turned off
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at the same time in sensitivity experiments, precluding an analysis of the role of each
individual type of surface energy flux and condensational heating.
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In order to improve our knowledge about hurricane-like polar lows, we here investigate an
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unusually intense hurricane-like polar low development that took place during 18–21
December 2002 over the Barents Sea. Due to the scarcity of the observational data in this
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region, our study relies on high-resolution numerical modeling with the full-physics version
of the up to date Weather Research and Forecasting (WRF) model (see section 2). The full
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physics experiment (see section 4) is held against infrared satellite images and the European
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Centre for Medium-Range Weather Forecasts (ECMWF) analyses, which show the synoptic
conditions that led to the polar low development (see section 3).
This case has previously been investigated by Bracegirdle (2004), who analysed the effect of
convective rings and the role of Conditional Instability of the Second Kind (CISK) (Charney
and Eliassen, 1964; Ooyama, 1964, 1969) on the intensification of the polar low by means of
a crude axisymmetric model. In agreement with aircraft-based observations of polar lows
(Shapiro et al., 1987; Linders sand Sætra, 2010) he found the atmosphere to be neutral to
moist convection in most cases. As such conditions are not consistent with high values of
convective available potential energy, he ruled out the CISK mechanism. Other authors
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(Emanuel and Rotunno, 1988; Businger and Baik, 1991; Bresch et al., 1997; Bracegirdle and
Gray, 2009) have suggested that surface fluxes are important for maintaining the polar low
after baroclinicity forcing weakens or vanishes. The most common hurricane theory relating
surface fluxes to the intensification of the storm is the Wind Induced Surface Heat Exchange
(WISHE) (Emanuel, 1986) mechanism. In short, the WISHE is an intensification mechanism
that relates tropospheric heating in polar low developments directly to both surface energy
fluxes (Craig and Gray, 1996). In this study we use sensitivity experiments to investigate the
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role of both types of surface energy flux separately and to compare these too their combined
effect and that of condensational heating, in order to analyse the physical properties of the
polar low (see section 5).
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2. Model description and experiment design
2.1 The numerical model
In this study the version 2.2.1 of the non-hydrostatic WRF model (Skamarock et al., 2007).
The physical parameterizations for the full physics experiment were the Thompson
microphysics scheme (Thompson et al., 2004), the Betts-Miller-Janjic moist convection
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scheme (Janjic, 1994, 1996), the Yonsei University planetary boundary layer (PBL) scheme
(Skamarock et al., 2007), the Noah Land Surface model (Chen and Dudhia, 2001), the MM5
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similarity surface scheme, the Rapid Radiative Transfer Model lookup table (Mlawer et al.,
1997), and the Dudhia longwave and shortwave radiation schemes (Dudhia, 1989). The
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ECMWF T511 data (2.5° x 2.5° latitude-longitude grid) were used as initial and lateral
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boundary conditions. There were 51 vertical levels, and two-way nesting was used. All the
simulations were carried out with positive definite advection of scalars, moisture, and
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turbulent kinetic energy. The location of the ice edge was held constant throughout the
simulations. In Figure 1, the blue square shows the 9-km parent domain and the red square
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shows the 3-km nested domain. Each model experiment was initialized at 00 UTC on 17
December 2002 and ended at 12 UTC on 21 December 2002. Allowing for spin-up time,
only data from 00 UTC on 18 December and onwards were used in our analysis. Only results
from the 3-km mesh are presented.
2.2 Experiment design
A series of sensitivity experiments were carried out in order to reveal the underlying physics
of the polar low. Six experiments were carried out to test the polar low’s sensitivity to
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condensational heating and whether the sensitivity depended on the microphysics scheme
and the convection parameterization. At first, the full-physics control (CTL) experiment as
described above was carried out. Then the model was run with no convection scheme
(NoCv) followed by an experiment with the WRF Single-Moment 6-class (WSM6) cloud
physics scheme. Then the WSM6 experiment was carried out with no convection scheme
(WSM6+NoCv). Finally, the role of condensational heating was investigated by carrying out
the above experiments but with condensational heating turned off (NoCH, NoCv+NoCH,
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WSM6-NoCv+NoCH, respectively). This was done by turning off the heat contribution to
the atmospheric temperature profile given by the microphysics schemes after each
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integration time step. All other processes in the microphysics schemes were carried out as
normal. A second group of five experiments was performed to test the sensitivity to surface
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fluxes. First, an experiment with no surface fluxes (NoF) was carried out. Two similar
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experiments were conducted to test the sensitivity to removal of only sensible heat fluxes
(NoSHF) or only latent heat fluxes (NoLHF) over the sea. To isolate the role of
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baroclinicity, an experiment with no condensational heating and no surface fluxes was
performed (NoF+NoCH).
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In Table 1, a short description of each model experiment is given. The names and description
of the model experiments are based on the differences relative to the CTL experiment.
In Figure 2(a,b) the tracks of the polar low (i.e., the location of minimum surface pressure
every 3 hours) for the ECMWF analysis and the experiments listed in Table 1 are seen. The
polar low track was simulated rather well in most experiments up to 00 UTC on 20
December, but the polar low made landfall about 24 hours too early according to satellite
images (Figure 3). Only in the NoSHF and NoLHF experiments is the polar low lifetime
similar to the observed. For all experiments that include condensational heating the polar low
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track makes a northward loop before landfall (Figure 2(a,b)), while for all experiments with
condensational heating turned off, the polar low track makes a smaller loop farther south
(Figure 2(b)).
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3. Synoptic overview
This section describes the synoptic scale and mesoscale evolution during the period from 12
UTC on 17 December 2002 to 12 UTC on 21 December 2002, spanning the predevelopment stage and full life cycle of the polar low. The synoptic overview is based on
infrared images from the Advanced Very High Resolution Radiometer (AVHRR) instrument
on board the NOAA polar orbiting satellite and ECMWF analyses.
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Satellite images and mean sea level pressure (MSLP) from the ECMWF analysis show that
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the precursor of the polar low development was a synoptic-scale low (980 hPa) situated off
the coast of Northern Norway at about 12 UTC on 17 December (72°N, 12°E, Figure 3(a)).
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During the next twenty-four hours, this low moved ENE as it weakened. A satellite image
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valid at 1441 UTC on 18 December shows that the remnant of the synoptic-scale low is
located west of Novaya Zemlya at about 73°N, 45°E over the Barents Sea (Figure 3(b)).
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Closer inspection of the satellite image shows a cyclonic flow pattern and an eye-like
structure under formation, indicating that the polar low developed within the mature
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occluded synoptic-scale low. This is believed to be the first signature of the polar low (969
hPa) development, which is placed somewhat too far north in the ECMWF model (Figure
3(b)).
On 17-18 December a sharp baroclinic zone bounding the polar and arctic air masses
develops over the Barents Sea (Figure 4(a,b)). Closer inspection of the satellite image
(Figure 3(b)) shows shallow moist convection taking place in the low-level cold air outbreak
to the west and higher level stratiform clouds in the ascending air masses east and north of
the low. During the same time period the upper-level low seen north of Novaya Zemlya
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(80°N, 60°E, Figure (4a)) prior to the polar low development moves WSW and is positioned
east of Svalbard at 12 UTC on 18 December (77°N, 30°E, Figure 4(b)).
At 0213 UTC on 19 December the satellite image shows an intense polar low with a welldefined cloud-free eye at about 74°N, 47°E (Figure (3c)). By now, the stratiform clouds are
cyclonically wrapped around the polar low eye. The ECMWF model still places the polar
low too far north, but confirms a deepening of the low (963 hPa). The polar low is located on
the warm side of the N-S oriented baroclinic zone (Figure (4c)). Because of the cold air
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advection to the rear of the polar low, the upper-level low has moved southeast and its center
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is now found southwest of the polar low, at about 73°N, 40°E (Figure (3c)). The tilt between
the lows clearly demonstrates the baroclinic nature of the evolution. The interaction between
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the upper level low (i.e., upper potential vorticity anomaly) and the polar low will be
analysed through a PV perspective (section 4.3)
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Twenty-four hours later the polar low bears a striking resemblance to a tropical cyclone,
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with spiral cloud bands surrounding a cloud-free eye seen at about 73°N, 49°E (Figure (3d)).
The cyclonical deep convection surrounding the eye suggests that moist convection and
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associated condensational heating may be an essential forcing mechanism for the polar low
(see section 5.2). Still, at this time MSLP shows that the polar low has weakened (966 hPa),
and is located too far north in the ECMWF model. Based on the small size of the observed
eye it would have been expected to see tighter pressure gradients at the central part of the
simulated polar low (Figure 3(d)). Therefore, it is not unlikely that the ECMWF model
underestimates the intensity of the polar low by several hPa at this time. By now the polar
low develops inside the arctic air masses, and the absence of the baroclinic zone so clearly
evident at previous times (Figure 4(c,d)) suggests that baroclinic energy conversion is no
longer important. The polar low proximity to the sea ice results in large air-sea temperature
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differences downwind of the ice edge. Evidence of this is seen as shallow cumulus
convection spiraling towards the polar low (Figure 4(d)). This suggests that surface energy
fluxes may also play an essential role in the polar low development (see sections 4.1 and
5.1).
Further development shows that the polar low diminished in size (Figure 3(e)). Still, spirallike clouds are evident surrounding a clear but smaller eye seen at about 73°N, 51°E. The
ECMWF model places the weaker polar low (970 hPa) too far south. The weak pressure
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gradients at the central part of the simulated polar low suggest that the model underestimates
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the polar low intensity at this time. During the past twelve hours, the vertical structure
between the upper-level low (500 hPa height, Figure 4(d,e)) and the polar low (MSLP,
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Figure 3(d,e)) is essentially equivalent-barotropic, having almost no tilt with height.
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The satellite image valid at 0728 UTC on 21 December shows that the polar low is decaying
west of Novaya Zemlya at approximately 72°N, 50°E (Figure (3f)). The 06 UTC analysis
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clearly underestimates the low (985 hPa) as it makes landfall east of the observed low at this
time. As this takes place the upper-level low diminishes and moves east of Novaya Zemlya
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(75°N, 60°E, Figure (4f)). Later this day at about 11 UTC, satellite images show that the
remnants of the polar low make landfall about 5 hours later than forecasted, close to the
southern tip of Novaya Zemlya (not shown).
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4. Evolution of the polar low in the control
experiment
In this section, we analyse the control experiment (CTL). As the synoptic-scale low (972
hPa) enters the Barents Sea the highest surface wind speeds (about 20 ms-1) are found on the
cold side north of 73°N (Figure 5(a)). Twelve hours later the synoptic-scale low has
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deepened (970 hPa) and is seen at about 72°N, 41°E, with surface wind speed reaching up to
25 ms-1 west of the low (Figure 5(b)). The high vertical velocity seen at about 74°N, 48°E
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(Figure 6(b)) indicates an area of condensational heating. According to satellite images the
simulated polar low develops northeast of the observed one (74°N, 45°E). As a result of the
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cyclonic flow of the synoptic low (Figure 5(a,b)) a baroclinic zone separating arctic and
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polar air masses develops over the Barents Sea (Figure 6(a,b)), with an outbreak of arctic air
masses west of the developing polar low.
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At 00 UTC on 19 December a polar low with an eye-like structure and closed isobars
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centered at 74°N, 47°E is seen (Figure 5(c)). At surface level the developing eye consists of
calm (< 5 ms-1) and relatively warm (-4°C) air surrounded by colder air masses. Maximum
wind speeds of about 25 ms-1 are seen in the eye wall. At this time the polar low position and
baroclinic structure (Figure 6(c)) are similar to the ECMWF analyses (Figure 4(c)). Thus,
baroclinic energy conversion is most likely an important physical mechanism in early stages
of the development. The roughly parallel lines of vertical velocity seen in the cold air
outbreak to the west of the low show shallow moist convection that is consistent with
satellite images (Figure 3(c)).
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By 12 UTC on 19 December the simulated polar low had a pronounced hurricane-like
appearance with a clear, calm, and warm eye located at 74°N, 47°E; circular isobars; and the
highest surface wind speeds (~20 ms-1) in the eye wall (Figure 5(d)). At this time surface
wind speeds had decreased, especially west and south of the polar low, despite the deepening
of the polar low (961 hPa). By now, the warm-core polar low develops in a less baroclinic
environment inside the arctic air masses, suggesting that baroclinic forcing has weakened
(Figure 6(c,d)), which may explain the weaker surface wind speed. The high vertical
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velocities seen close to the core of the low coincide with areas of latent heat release
indicating that condensational heating may be an important forcing mechanism (Figure
6(c,d)).
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Twelve hours later the polar low has moved southwest while it weakened to 966 hPa (Figure
5(e)). A distinct eye is still present centered at about 73°N, 44°E, but according to satellite
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observations the simulated polar low is located about 4° too far west. Despite the weakening
of the polar low, surface wind speed at the western side of the eye has increased (~25 ms-1).
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According to scatterometer winds (not shown) the control experiment successfully simulated
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the wind speed west of the low, but east of the low the wind speed is underestimated (Figure
5(e)). The spiral-like moist convection ending at the eye of the polar low (Figure 6(e)) shows
similar structure to the cloud bands seen in the satellite images (Figure 4(d)). Consistently
with the ECMWF analyses the absence of a baroclinic zone (Figure 6(e)) and evidence of
strong cold air outbreak suggest that surface fluxes may be essential in the development at
this time (see section 5).
At 12 UTC on 20 December the MSLP shows that the polar low had made landfall at about
72°N, 52°E (Figure 5(f)). Although the behavior of the simulated low did not exactly
duplicate that of the observed low, the two systems had enough features in common up to 00
Quarterly Journal of the Royal Meteorological Society
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UTC on 20 December to regard the modeled low as representative of the observed system up
to this time. This assumption is the basis for the analysis that we now present.
4.1 Surface fluxes
In Figure 7 simulated surface sensible heat (SH) fluxes are shown in the left column and
latent heat (LH) fluxes in the right column, both for the CTL experiment. The magnitude of
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the simulated surface fluxes is controlled by roughness length, surface wind speed, relative
humidity, and air–sea temperature differences (Chen and Dudhia, 2001).
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At early stages of the polar low development high surface wind speed and strong cold air
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outbreak of arctic air masses west of the polar low (Figure 5(c)) trigger extreme surface
fluxes. Similar conditions explain the high surface fluxes seen farther west downwind of the
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sea ice edge (74–76°N). The small area of almost no surface fluxes centered at about 74°N,
47°E marks the calm and relatively warm eye of the polar low (Figure 7(a,b)). At the eye
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wall maximum sensible heat SH fluxes are about 1200 Wm-2 while the LH fluxes are about
400 Wm-2.
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At 12 UTC on 19 December the pronounced warm and calm eye (74°N, 47°E) of the polar
low (e.g., see Figure 5(d)) is clearly evident with negligible SH and LH fluxes (Figure
7(c,d)). Maximum SH and LH fluxes at the eye wall have slightly weakened to about 1000
Wm-2 and 350 Wm-2, respectively. The drop in surface wind speed during the last 12 hours
(Figure 5(c,d), respectively) explains the lower surface fluxes. The increase in surface fluxes
seen west of the polar low, between 30°E and 35°E, is probably explained by the slightly
higher surface wind speed in this area.
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Twelve hours later, despite increased surface wind speed at the western side of the eye wall
(Figure 5(d,e)), maximum SH and LH fluxes have now weakened to about 600 Wm-2 and
300 Wm-2, respectively (Figure 7(e,f)). The reduction in surface fluxes is probably explained
by reduced air–sea temperature differences. As the distance from the sea ice edge has
increased, air masses are further warmed (Figure 5(d,e)) by the extreme SH fluxes before
reaching the polar low. This results in less air–sea temperature differences and thus a drop in
surface fluxes. The area of low surface fluxes east of the polar low is most likely caused by
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the strong reduction in surface wind speed in this area (Figure 5(e)).
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According to Lui et al. (2006) the updraft- and downdraft-induced circulations set up by the
development of cloud streets influence the spatial distribution of surface fluxes. Below cloud
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streets, surface fluxes are suppressed (Figure 7(e,f) and 6(e), respectively). The increased
flux between the cloud streets is caused by a downdraft of relatively dry air (not shown here)
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resulting in higher surface wind speed (Lui et al., 2006). Close inspection of Figure 5(e)
shows traces of lines of high and low surface wind speeds south and west of the polar low
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that correlate with the simulated cloud streets in the same area (Figure 6(e)).
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The relative magnitude of surface energy fluxes in our simulations is very much at odds to
hurricanes, which typically have SH fluxes of 150-200 Wm-2 and maximum LH fluxes of
about 1000 Wm-2 (Trenberth and Fasullo, 2008). Still, the surface energy fluxes seen in this
study are consistent with previous numerical simulations of surface fluxes close to the sea
ice edge. A numerical case study of a hurricane-like polar low over the Norwegian Sea by
Nordeng and Rasmussen (1992) yielded SH fluxes of about 500 Wm-2 and LH fluxes of
about 250 Wm-2. Bresch et al. (1997) studied an intense polar low close to the sea ice edge
over the western Bering Sea with the MM5 model. Their simulations indicated maximum
SH fluxes of 1000 Wm-2 and LH fluxes of 300 Wm-2. In a numerical study of an arctic front
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near Bjørnøya (Bear Island near Svalbard), for which measurements from a coast guard ship
indicated winds in excess of hurricane force, Grønås and Skeie (1999) found SH fluxes of
1300 Wm-2 in their simulations. These values were matched in the numerical polar low study
of Mailhot et al. (1996), who found SH fluxes of 1400 Wm-2 near the edge of the sea ice. As
will be seen in section 5, the extremely high SH fluxes in our case inevitably influence the
simulated behavior of the polar low, including its sensitivity to both surface fluxes and
condensational heating.
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4.2 Control experiment through a PV perspective
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In this sub-section, the polar low development is analysed through the PV paradigm
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originally developed for cyclogenesis by Hoskins at al. (1985) and adjusted to polar lows by
Montgomery and Farrell (1992).
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In Figure 8(a), PV and 500 hPa heights at 12 UTC on 18 December are shown along with
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MSLP for the CTL experiment. A strong (3-4 PV unit [PVU]) upper level potential vorticity
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(UPV) anomaly is seen on the western side of the figure, related to the upper-level low
shown in Figure 2(b).
A cross-section of PV and potential temperature shows (Figure 8(b)) a rather well-mixed
atmosphere with weak static stability up to the tropopause (2 PVU, ~8 km). The crosssection goes through the area which develops into our polar low during the next 12 hours.
Over the sea ice north of 75.3°N stable arctic air masses are seen below 4 km height. The
low-level PV (LPV) anomaly at about 500–2000 m height, located between 74°N and 76°N,
is most likely caused by condensational heating (Figure 6(b)) and is believed to be a
signature of the developing polar low.
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At 00 UTC on 19 December the UPV anomaly has moved southeast and is seen centered at
74.5°N, 36°E (Figure 8(c)). Figure 8(d) shows a stratospheric down-folding of high PV
values (2-5 PVU) located between 71°N and 73°N reaching down to about 4 km height. A
warm surface potential temperature anomaly (Ө) and a strong (2-5 PVU) LPV anomaly
confined below 2 km height are seen centered at the polar low eye (74.2°N). At this time an
phase of high PV air (1-2 PVU) is seen between the polar low and the UPV anomaly. The
high PV values (1-5 PVU) below 1 km height seen over the sea ice (>75.4°N) indicate very
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stable arctic masses.
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Twelve hours later the UPV anomaly has intensified as it moved east and is now located
south of the polar low. Condensational heating appears to erode the upper-level PV anomaly
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and prevent it from becoming vertically aligned with the surface polar low (as seen in Plant
et al., 2003 and Bracegirdle and Gray, 2009). Because of this, the polar low is placed below
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the outer edge of the UPV anomaly (Figure 8(e)). As the UPV anomaly moves eastward the
polar low is steered by the upper level cyclonical flow set up by the UPV anomaly (not
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shown here), explaining the polar low northward track (Figure 8(c,e)) and also in Figure
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2(a). This most likely explains the polar low northward loop-like track seen in Figure 2(a).
The cross-section now shows a wider and more intense UPV anomaly (Figure 8(f)). The
high PV air (2-5 PVU, Figure 8(f)) seen in the phase tilt between the UPV anomaly and the
polar low, at about 2-3 km height, is not believed to be a sign of stratospheric downfolding
but is most likely caused by strong condensational heating at the eye wall (Figure 6(d)). A
slightly weaker Ө anomaly is centered at the polar low eye (74.2°N). The high gradients of
potential temperature seen above the surface show the height of the planetary boundary layer
(PBL). On top of the PBL (~2km), centered at the polar low core, a strong (2-5 PVU) LPV
anomaly is seen. The air over the ice (>75.3°N) is now less stable than before, because
Quarterly Journal of the Royal Meteorological Society
18
during the last twelve hours southerly flow of modified air masses east of the polar low
(Figure 5(c,d)) results in less stable air over the sea ice (>75.3°N).
The model diagnostic cannot completely clarify the exact coupling mechanisms. However,
based on the above discussion we note that the polar low development fits well with the
conceptual model of polar low cyclogenesis by Montgomery and Farrell (1992) and Grønås
and Kvamstø (1995). In short, the UPV anomaly sets up a Ө anomaly (Figure 8(d,e)) that
through mutual interaction and phase-locking with the UPV anomaly was able to form a
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significant low-level circulation (e.g., see Figure 5(c,d)) and create an LPV anomaly (Figure
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8(d,f)) by condensational heating in organized convection (e.g., see Figure 6(c,d)).
Condensational heating maintained a phase tilt between the polar low and the UPV anomaly
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by destruction of PV aloft. The PV so produced then added its contribution to the low-level
circulation, further intensifying the polar low (Figure 5(c,d)). The intensifying UPV anomaly
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and increased PV values seen in the phase tilt are viewed as evidence of a phase lock and
mutual intensification between the UPV anomaly and the polar low (Figure 8(d,f)). It should
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be mentioned that evidence of a phase lock between the UPV anomaly and polar low was
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persisted until the polar low made landfall, suggesting that upper level forcing is important
throughout the polar low lifetime.
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5. Sensitivity experiments
This section investigates polar low sensitivity to surface fluxes and condensational heating in
order to clarify its forcing mechanisms. The rate of intensification of the polar low is
described using plots of minimum central surface pressure as a function of time. As in the
CTL experiment, most of the experiments simulated the polar low track rather well up to
early 20 December, but they predicted landfall about 24 hours too early, according to
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satellite observations (Figure 3). Only experiments removing sensible heat (NoSHF) or
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latent heat (NoLHF) fluxes reproduced well the time of landfall at Novaya Zemlya. Thus,
our conclusions will be based on the time period from 00 UTC on 18 December to 00 UTC
on 20 December.
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5.1 The role of surface fluxes
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In this sub-section we investigate the role of surface fluxes using sensitivity experiments
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described in section 2 (Table 1). As shown in Figure 9, the ECMWF analysis and the CTL
experiment show almost identical surface pressures throughout. A difference of only 1–2
hPa is seen between the experiments. Viewing the experiments as a whole, the CTL
experiment simulates the deepest low (Figure 9). During the polar low deepening period, up
to about 06 UTC on 19 December, the polar low is not very sensitive to surface flux
modifications (NoF, NoSHF, NoLHF). After this time the simulated polar low intensity is
increasingly sensitive to the removal of sensible heat fluxes, but latent heat fluxes seem to
play a minor role (NoSHF versus NoLHF, respectively). This indicates that moist supply
from the sea, available for condensational heating through moist convection, is an important
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20
energy source for this polar low case. The very importance of sensible heat fluxes is clearly
seen by comparing the NoSHF experiment with experiments omitting both energy surface
fluxes (NoF). Surprisingly, they show similar intensities, which illustrates the dominant role
of sensible heat fluxes in our case (Figure 9). A stunning observation in Figure 9 is that
condensational heating seems to have a slightly detrimental effect (1–2 hPa) on polar low
intensity (NoF+NoCH versus NoF, respectively) after the polar low has reached its peak
intensity. The role of condensational heating will be analysed in the sub-section below.
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Several authors have shown (Harrold and Browning, 1969; Duncan, 1977; Bresch et al.,
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1997) that baroclinic growth in polar low developments is able to proceed only when the
atmosphere possesses reduced stability at lower levels. In their studies the baroclinic zone
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was confined to lower atmospheric levels, which is affected by surface fluxes. However, in
our case the polar low initially developed in the major baroclinic zone bounding the polar
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and arctic air masses. This may explain why the polar low intensity is not sensitive to surface
flux modifications (NoF, NoSHF) or condensational heating (NoF+NoCH) until
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baroclinicity weakens after 00 UTC on 19 December (Figure 6). After this time the polar low
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intensity is increasingly dependent on surface fluxes (Figure 9), suggesting a gradual
transition from a polar low driven by baroclinic instability to one driven by sensible heat
fluxes.
5.2 The role of condensational heating
This sub-section investigates the role of condensational heating in the polar low development
using sensitivity experiments described in section 2 (Table 1).
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In short, all simulations showed a deepening of the polar low (approximately 9 hPa day-1) up
to about 09 UTC on 19 December (Figure 10). The NoCv experiment shows almost identical
surface pressure as the CTL experiment throughout, while the WSM6+NoCv experiment
simulates a 1-2 hPa weaker polar low (Figure 10). Thus, the polar low simulation is not
sensitive to convection parameterization but is slightly sensitive to the treatment of cloud
microphysical processes. In Figure 10, the most striking feature occurs after 12 UTC on 19
December, in a period when baroclinicity all but vanishes (Figure 6) and the polar low was
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found to be driven by sensible heat fluxes. In this period, condensational heating has a
negative effect on polar low intensity. In the experiments omitting condensational heating
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(NoCH, NoCv+NoCH, WSM6-NoCv+NoCH) a 1-4 hPa deeper polar low is simulated
relative to their counterparts including its effect (C, NoCv, WSM6+NoCv). To our
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knowledge this is the first study to report a negative effect on intensity of condensational
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heating on a warm core polar low. In the satellite images, deep, convective, spiral-like clouds
ending at the eye wall are seen (Figure 1(c–e)). It would therefore have been reasonable to
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expect condensational heating to have a positive effect on polar low intensity. Several
previous case studies of polar lows (e.g., Nordeng and Rasmussen, 1992; Mailhot, 1996;
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Bresch et al., 1997) have suggested condensational heating as a main driving factor behind
the development. The following section attempts to explain the negative effect of
condensational heating on polar low intensity.
5.3 A comparison of the CTL, NoCH and NoSHF
experiments
This sub-section discusses the unexpected effect of condensational heating on the polar low
intensity through a PV perspective. This is accomplished by analyzing the N-S oriented
cross-sections of PV and potential temperature through the polar low center at 12 UTC on 19
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22
December comparing the CTL, NoCH and NoSHF experiments. At this time, the simulations
showed large deviations in minimum surface pressure (Figure 10 and 11). A short discussion
about the role of the organization of convection is also given. It should be mentioned that the
discussion in this subsection was found to fit all pairs of experiments with and without
condensational heating (Table 1).
In Figure 11(a,c), PV at 500 hPa and the height of the 500 hPa surface are shown along with
MSLP for the NoCH and NoSHF experiments, respectively. The UPV anomaly is slightly
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more intense and smaller in the NoCH experiment (Figure 11(a)), but it is simulated
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approximately at the same location as in the NoSHF and CTL experiments (73°N, 49°E,
Figure 11(a,c) and 8(e)). The absence of condensational heating in the NoCH experiments
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causes the UPV anomaly to be vertically aligned with the polar low (Figure 11(a)). As a
result, the polar low in the NoCH experiment is positioned farther south (~73°N) than in the
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CTL and NoSHF experiments (~74°N, Figure 8(e) and 11(c)). As shown in Figure 2(b) the
polar low has a more southerly track when condensational heating is omitted in the
experiments.
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Figure 11(b,d) shows the cross-sections of PV and the potential temperature for the NoCH
and NoSHF experiments, respectively. Both experiments show similar features as in the
CTL experiment (Figure 8(f)), i.e. a UPV anomaly located between 70°N and 74°N reaching
down to about 4 km height. In both simulations, a surface Ө anomaly is centered at the polar
low eye (red dot). Relatively constant potential temperature above the surface shows the
well-mixed planetary boundary layer (PBL). The high PV air (1-5 PVU) seen at the top of
the PBL, centered at the polar low core, shows the LPV anomaly. As for the CTL
experiment, the high PV air (1–5 PVU) stretching between the LPV and UPV anomalies
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(Figure 11(b,d)) is viewed as evidence of a phase lock between the anomalies (see
Montgomery and Farrell,1992).
Downwind of the sea ice strong convection triggered by the extreme sensible heat fluxes
deepens the well mixed PBL, resulting in a gradual destabilization and warming of the lower
atmosphere (Figure 8(f), 11(b)). In the NoSHF experiment a relatively thick (~2 km) PBL is
seen south of the polar low even if sensible heat fluxes are omitted (Figure 11(d)). This
suggests that condensational heating may also play a role in the development of the PBL.
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Still, the importance of sensible heating in warming and deepening of the PBL is clearly seen
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in the NoCH experiment, where the polar low is positioned further south in a warmer PBL
which reaches up to the tropopause (Figure 11(b)). Because of this, the polar low core is
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warmer and deeper and the lower atmosphere is less statically stable than in the CTL
experiment (Figure 11(b) and 8(f), respectively).
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According to Hoskins et al. (1985), the capability of a UPV anomaly to induce circulation at
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surface levels is dependent upon the static stability of the lower troposphere. This can be
understood with the aid of the Rossby penetration depth (H), which is related to the static
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stability of the troposphere through the relation H~fL/N, where f is the coriolis parameter, N
(Brunt-Väisälä frequency) is a measurement of atmospheric static stability, and L is the
horizontal scale of the UPV anomaly (Rasmussen and Turner, 2003). Due to the weaker
static stability in the NoCH experiment the atmospheric conditions are more favorable for
interactions (small H) between the UPV anomaly and the polar low than in the CTL
experiment (Figure 11(b) and 8(f), respectively). This may explain the slightly more intense
UPV anomaly and the deeper polar low in the NoCH experiment (Figure 11(a) versus 8(f),
respectively). Bresch et al. (1997) and Mailhot et al. (1996) showed that by turning off
surface fluxes in their experiments the lower atmosphere became too stable (small H) for the
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24
UPV anomaly to induce polar low development, which is not the case in our study (NoSHF,
Figure 9). The evidence that the UPV anomaly phase locked with the polar low in the
NoSHF experiment (Figure 11(d)), regardless of a statically stable atmosphere (small H),
support our hypothesize that the UPV anomaly has strong influence on surface levels.
However, as the sensitivity experiments showed (Figure 9) sensible heat fluxes are the main
energy source as the polar low matured.
The negative effect of condensational heating on the polar low intensity could also be related
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to the organization of convection in the experiments. As discussed above the NoCH
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experiment showed a more optimal atmosphere for deep convection at the core of the polar
low. Further, in the NoCH experiment cloud streets failed to develop and the model
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simulated convection was mostly organized at the core of the polar low and less on its sides
(Figure 12(a,b), respectively). This suggests that warm air seclusion of heated air masses by
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the extreme sensible heat fluxes explains the development of the warm core, rather than
condensational heating. Nordeng and Rasmussen (1992) discussed that the convection must
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be organized in such a way that air masses warmed and moistened by surface fluxes must
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ascend at the vicinity of the polar low and less on its sides in order to have a significant
effect on the large scale flow. If convection were not only organized at the vicinity of the
polar low, as in the CTL experiment, the pressure gradient would weaken, resulting in a
weaker polar low development (Figure 5(d) versus 11(a)). This is consistent with Van
Delden (1989), who argued that deepening of a cyclone is strongly inhibited if the diabatic
heating is located dynamically too far away from the cyclone centre.
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6. Summary and concluding remarks
An unusually deep (961 hPa) mesoscale polar low resembling a hurricane that developed
over the Barents Sea during 18-21 December 2002 has been investigated using satellite
images, ECWMF analyses and high resolution (3 km mesh) WRF simulations. Synoptic
analyses reveal that the polar low developed close to the sea ice edge in the aftermath of an
eastward moving synoptic scale low west of Novaya Zemlya on 18 December. The polar low
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initially developed in the major baroclinic zone bounding polar and arctic air masses. As the
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polar low matured, baroclinicity vanished and moist convection intensified. The satellite
images revealed deep spiral cloud bands surrounding a cloud free eye, which is frequently
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observed in tropical hurricanes. The low dissipated as it made landfall at Novaya Zemlya
(75°N, 60°E) during the morning on 21 December.
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The full-physics experiment (CTL) produced an intense cyclone (961 hPa) that had
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characteristics similar to the observed polar low, but made landfall about 24 hours too early,
according to satellite images. The simulated eye-like structure, with a clear, calm and warm
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core with highest surface wind speed (<25ms-1) in the eye wall is consistent with simulations
of previous case studies of similar events (Businger and Baik, 1991; Nordeng and
Rasmussen, 1992). Similar to satellite observations the model successfully simulated cloud
streets. The polar low close proximity to the arctic sea ice and high surface wind speed
(25ms-1) triggered strong cold-air advection resulting in extreme surface energy fluxes.
Maximum sensible and latent heat fluxes during the intensification stage were about 1200
Wm-2 and 400 Wm-2, respectively. As the polar low matured maximum SH and LH fluxes
decreased to about 600 Wm-2 and 300 Wm-2, respectively. The PV perspective (e.g., see
Montgomery and Farrell, 1992) indicates that the interaction between an eastward moving
Quarterly Journal of the Royal Meteorological Society
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UPV anomaly and deep baroclinicity provide the necessary mechanism to spin-up the polar
low, in agreement with the scenario proposed by Grønås and Kvamstø (1995). Similar
interactions have been suggested in previous studies of similar events (Businger and Baik,
1991; Nordeng and Rasmussen, 1992). In our case we found evidence of upper level forcing
throughout the simulated polar low lifetime, which is not frequently reported.
Several high-resolution (3 km mesh) sensitivity experiments were conducted with the up to
date WRF model to elucidate the underlying physics of the polar low. Based on the analysis
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of the sensitivity experiments, we arrive at the following conclusions followed with a short
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discussion concerning the polar low:
• initially baroclinic energy conversion is the dominant forcing mechanism;
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The complex interaction between surface fluxes, condensational heating and
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baroclinic dynamics makes is difficult to separate their role in the intensification
stage of the simulated polar low. Still, the polar low developed in the major
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baroclinic zone bounding polar and arctic air masses, which most likely explains way
the polar low intensity was not sensitive to surface energy fluxes and condensational
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heating modifications until baroclinicity weakened.
• as the polar low matures sensible heat fluxes are the main energy source, with a
minor role of latent heat fluxes;
We suggests that the WISHE mechanism, as presented for polar lows in Craig and
Gray (1996), is a likely forcing mechanism that gradually becomes more important as
the polar low matured. However, in contrast to previous case studies of some polar
lows and tropical hurricanes the polar low in this study was driven by sensible heat
fluxes with a minor role of latent heat fluxes. Four points indicate the dominance of
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WISHE: (1) baroclinicity vanished, (2) small CAPE in the vicinity of the polar low
(not shown), (3) the large sensible heat fluxes, (4) results of sensitivity experiments.
• condensational heating has a negative effect on the mature polar low intensity;
For the first time we report a negative effect of condensational heating on a warm
core polar low. As the polar low has reached its peak intensity sensitivity
experiments revealed that condensational heating has a negative effect on polar low
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intensity. Two factors may explain this unusual results: (1) By turning of
condensational heating the simulated polar low required a more optimal atmosphere
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for stronger upper level forcing. (2) The simulated convection was organized at the
vicinity of the polar low and less in its sides, which is favorable for deepening of the
cyclone.
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Polar lows have previously been classified by their appearance in satellite images, their
origin and their forcing mechanisms. Still, hurricane-like polar lows are not captured by
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today’s classification scheme (e.g., see Wilhelmsen, 1985; Bracegirdle and Gray, 2008).
Bracegirdle and Gray (2008) recommended that hurricane like polar lows should form their
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own class in new classification schemes. As seen in this paper, they may have characteristics
that are unusual compared to other polar low cases. In order to generalize our findings more
case studies need to be carried out. For future work we especially urge other authors to carry
out sensitivity experiments on each type of surface flux (SH and LH) separately, and
condensational heating in order to fully understand their roles in polar low developments.
Acknowledgment
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
Quarterly Journal of the Royal Meteorological Society
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future’ (grant no. 175992).The authors are grateful to WRF-help and Greg Thompson at
NCAR, for support on configurations of sensitivity experiments and the set-up of the WRF
model. The authors wish to specially thank Gunnar Wollan and Simen Gaure for support on
implementation of WRF at the University of Oslo. The main author wishes especially to
thank Bjørn Egil Nygård and Øyvind Hodneborg for support with Matlab and WRF.
Especially thanks go to colleagues at MetOs for motivating support and discussions.
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Table 1: A brief summary of the characteristics of each model experiment.
Abbreviation
Description
CTL
The default model experiment.
NoCH
Condensational heating was turned off.
NoCv
The convection scheme was turned off.
NoCv+NoCH
The convection scheme and condensational heating were turned
off.
WSM6+NoCv
The WRF single-moment 6-class microphysics scheme was used
and the convection scheme was turned off.
WSM6+NoCv+NoCH
NoF
NoLHF
The sensible heat flux was turned off over the sea.
The latent heat flux was turned off over the sea.
Condensational heating as well as sensible and latent heat fluxes
over the sea were turned off.
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NoSHF
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and the convection scheme and condensational heating were
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Surface fluxes of sensible heat and latent heat were turned off
over the sea.
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Figure 1. The computational domains. The blue square indicates the parent domain (9 x 9
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Figure 2. Tracks of minimum centred surface pressure at 3-hour intervals for each model
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experiment. The track starts at 00 UTC 18 December and ends when the polar low makes
landfall. The sea ice edge is marked with a gray curve. The southern part of Novaya Zemlya
is seen in the lower right corner. The following experiments are shown: (a) ECMWF (green),
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CTL (blue), NoF (red), NoSHF (dark green), NoLHF (purple), and NoF+NoCH (yellow). (b)
ECMWF (green), CTL (blue), NoCH (red), NoCv (light green), NoCv+NoCH (purple),
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Figure 3. NOAA satellite images (infrared channel 4) documenting the temporal
development of the polar low. Superimposed on the satellite images is the ECMWF mean
sea level pressure (MSLP, yellow lines, hPa) at roughly the same times. Satellite images are
shown for at (a) 1052 UTC 17 December along with MSLP from the 12 UTC analysis, (b)
1441 UTC 18 December along with MSLP from the 12 UTC analysis, (c) 0213 UTC 19
December along with MSLP from the 00 UTC analysis, (d) 0202 UTC 20 December along
with MSLP from the 00 UTC analysis, (e) 1158 UTC 20 December along with MSLP from
the 12 UTC analysis, and (f) 0728 UTC 21 December along with MSLP from the 06 UTC
analysis. Coastlines are shown in black.
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PL
Figure 4. Height of the 500 hPa surface from the ECMWF analysis (in meters, blue
contours) and 500–1000 hPa thickness (in meters, brown contours) for (a) 12 UTC on 17
December, (b) 12 UTC on 18 December, (c) 00 UTC on 19 December, (d) 00 UTC on 20
December, (e) 12 UTC on 20 December, and (f) 12 UTC on 21 December. The polar low
central positions are marked PL.
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Figure 5. Mean sea level pressure (MSLP, black contours, hPa), temperature at 2 m (2°C
interval, red contours), and 10-m wind speed (in ms-1, color shading) from the CTL
experiment. Black arrows show 10-m wind direction. The sea ice edge is shown with a thick,
white curve. The following times are shown: (a) 00 UTC on 18 December, (b) 12 UTC on 18
December, (c) 00 UTC on 19 December, (d) 12 UTC on 19 December, (e) 00 UTC on 20
December, and (f) 12 UTC on 20 December.
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Figure 6. Vertical wind speed (ms-1, color shading) at 850 hPa surface and 500–1000 hPa
thickness (40-m interval, black contours) form the CTL experiment. The sea ice edge is
shown with a thick, gray curve. (a) 00 UTC on 18 December, (b) 12 UTC on 18 December,
(c) 00 UTC on 19 December, (d) 12 UTC on 19 December, (e) 00 UTC on 20 December,
and (f) 12 UTC on 20 December.
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Figure 7. Surface sensible heat (left column) and latent heat (right column) fluxes (Wm–²)
form the CTL experiment. The sea ice and landmasses are seen as areas of almost zero
fluxes. (a, b) 00 UTC on 19 December, (c, d) 12 UTC on 19 December, and (e, f) 00 UTC on
20 December. Note the differences in y-axis scale between sensible heat (left column) and
latent heat (right column) fluxes.
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Figure 8. Left column: Potential vorticity (color shading) and geopotential height (blue
contours, 40-m interval) at 500 hPa and sea level pressure (5 hPa black contours) from the
CTL experiment. The black dotted line shows position of cross-section to the right. The sea
ice edge is shown with a gray curve. Right column: Cross-section of PV and potential
temperature from the CTL experiment. 1 PVU unit is equivalent to SI units 1 x 10-6 m2 s-1 K
kg-1. (a, b) 12 UTC on 18 December, (c, d) 00 UTC on 19 December, and (e, f) 12 UTC on
19 December. Red dots mark the polar low positions.
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980
ECMWF
978
CTL
976
NoF
974
NoSHF
972
NoLHF
970
NoF+NoCH
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Figure 9. A 3-hour time evolution (date and hour) of minimum sea level pressure (hPa) was
utilized in the following experiments: ECMWF (blue), CTL (red), NoF (Green), NoSHF
(purple), NoLHF (black), and NoF+NoCH (yellow). The tracking starts at 00 UTC on 18
rR
December and ends at 12 UTC on 20 December.
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Figure 10. A 3-hour time evolution (date and hour) of minimum sea level pressure (hPa) for
ee
the following experiments: CTL (red), NoCH (black), NoCv (purple), NoCv+NoCH (blue),
WSM6+NoCv (yellow), and WSM6+NoCv+NoCH (green). The tracking starts at 00 UTC
rR
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Figure 11. Left column: Potential vorticity (color shading) and geopotential height (blue
contours, 40-m interval) at 500 hPa surface at 12 UTC on 19 December from the (a) NoCH
ev
and (c) NoSHF experiments. The black solid line shows sea level pressure (5 hPa contours).
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shown with a gray curve. Right column: Cross-section of PV and potential temperature at 12
UTC on 19 December from the (b) NoCH and (d) NoSHF experiments. 1 PVU unit is
equivalent to SI units 1 x 10-6 m2 s-1 K kg-1. The red dot marks the polar low’s position.
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a
b
Figure 12. Vertical wind speed (ms-1, color shading) at 850 hPa surface form the (a) CTL
Fo
and the (b) NoCH experiment valid at 12 UTC on 19 December. The sea ice edge is shown
with a thick, gray curve.
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