Quarterly Journal of the Royal Meteorological Society
Orographic influence of East Greenland on a Polar Low over
the Denmark Strait
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Journal:
Manuscript ID:
Wiley - Manuscript type:
Complete List of Authors:
QJ-10-0246.R2
Research Article
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Date Submitted by the
Author:
QJRMS
n/a
er
Kristjánsson, Jon; University of Oslo, Department of Geosciences
Thorsteinsson, Sigurdur; Icelandic Meteorological Office
Kolstad, Erik; Bjerknes Centre for Climate Research
Blechschmidt, Anne; Lancaster University
Keywords:
orographic influence, polar low, Denmark Strait
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Orographic influence of East Greenland
on a Polar Low over the Denmark Strait
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Jón Egill Kristjánsson1
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Sigurdur Thorsteinsson2
Erik W. Kolstad3,4
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Anne-Marlene Blechschmidt5
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Quarterly Journal of the Royal Meteorological Society
Second revision of submission to the Quarterly Journal of the Royal Meteorological Society
7 March 2011
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University of Oslo, Oslo, Norway
Icelandic Meteorological Office, Reykjavík, Iceland
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Bjerknes Centre for Climate Research, Bergen, Norway
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Now at StormGeo, Bergen, Norway
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NCAS-Weather, Lancaster University, United Kingdom
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Abstract
We present a numerical study of a polar low which hit western Iceland in January 2007, with
heavy snowfall and mean wind speeds exceeding 20 m s-1 in several locations. The
operational models at the time captured the polar low formation rather well, but there was a
large spread in their predictions of the subsequent evolution and track of the polar low. The
objective of this study is to investigate possible orographic forcing from Greenland as a
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trigger for the polar low development. In addition to an analysis of surface observations and
satellite imagery, sensitivity studies using HIRLAM were carried out with various
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degradations of Greenland’s orography, as well as with modifications to the sea surface
temperature (SST), surface roughness and the data assimilation scheme. Despite the presence
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of an upper-level trough and weak static stability in all the simulations the polar low
development was found to be very sensitive to the presence of the high mountains of eastern
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Greenland. Whereas the control run captured well the main features of the polar low,
simulations with parts of East Greenland’s orography removed obtained a southward-
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displaced polar low which moved rapidly eastward, resulting in substantially underestimated
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near-surface winds and snowfall amounts. Setting the orographic heights over all of
Greenland to zero led to the complete disappearance of the polar low. On the other hand,
artificially increasing the SST by 4 K in the Denmark Strait, reducing the orographic
roughness or replacing the four dimensional variational assimilation scheme (4D-Var) by 3DVar had only a small effect on the polar low. We suggest that hitherto unreported interactions
between the high mountains of East Greenland and polar low development over the Denmark
Strait may be more important for polar low formation than katabatic flow from valleys in
East Greenland that was highlighted in earlier studies.
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1. Introduction
Polar lows are mesoscale weather phenomena that evolve at high latitudes during the winter,
in connection with marine cold air outbreaks (MCAOs) over relatively warm seas. They
invariably occur on the cold side of the ‘polar front’, i.e., well inside the polar air mass. In the
Northern Hemisphere, the most favoured regions for polar low formation are over the
northernmost extent of the warm ocean currents (Gulf Stream, Kuroshio) and in regions with
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frequent MCAOs, such as over the Labrador, Irminger, Norwegian and Barents Seas, as well
as near Japan and over the Sea of Okhotsk (Kolstad, 2011). Recently, substantial changes in
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polar low frequency in a future warmer climate have been suggested (Kolstad and
Bracegirdle, 2008; Zahn and van Storch, 2010), drawing attention to the importance of a
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better understanding of this intriguing phenomenon.
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Polar lows usually develop in an atmosphere with weak static stability, strong surface-to-air
fluxes of sensible and latent heat and considerable low-level baroclinicity. Some polar lows
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may appear to be mainly driven by latent heat release, resembling tropical cyclones, hence
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the term ‘Arctic hurricanes’ introduced by Emanuel and Rotunno (1989). In other cases, the
life cycle of a polar low can be largely described as a shallow baroclinic wave in a
troposphere with weak static stability and a low tropopause (e.g., Reed and Duncan, 1987). In
the 1980s and early 1990s there was a debate in the scientific literature concerning this
distinction, but there is now more acceptance of the view that, depending on the atmospheric
conditions, some polar lows are mainly convective in nature while others are more baroclinic
(Rasmussen and Turner, 2003). Recently, Bracegirdle and Gray (2008) found evidence for a
gradual transition from a mainly baroclinic phase to a more convective phase during the lifecycles of polar lows over the Nordic Seas.
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Already in the 1980s it was pointed out that in order to spin up a polar low, some ‘trigger
mechanism’ was needed, i.e., some factor that helps to organize the convective elements on
scales of 1-10 km into a cyclonic system with a horizontal scale of 100-500 km. It is common
to express this trigger in terms of a pre-existing upper-level potential vorticity anomaly (e.g.,
Montgomery and Farrell, 1992; Grønås and Kvamstø, 1995). Other triggers have also been
suggested, such as for instance orographic effects related to the southern tip of Greenland
(Rasmussen, 1981) or the Antarctic peninsula (Gallée, 1995). Such links to orography seem
to be rather uncommon though and, according to a statistical analysis of polar lows off
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Northern Norway by Wilhelmsen (1985), only 2 out of 32 polar lows that were considered
had an orographic trigger.
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Greenland’s enormous ice sheet, located at high elevation, serves as a huge source of cold air,
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which is frequently drained down valleys and fjords in the form of katabatic winds or
‘piteraqs’ that frequently reach strengths of 20 m s-1 or more (e.g., Heinemann and Klein,
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2002). Klein and Heinemann (2002) suggested that convergence of the ouflowing air from
the katabatic flow in the valleys of East Greenland might be responsible for the formation of
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mesocyclones over the Denmark Strait, and found support for this view from model
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simulations. Due to its size, Greenland also has a major impact on the North Atlantic weather
and climate through its influence on the storm tracks, as shown in model studies by Petersen
et al. (2004), Junge et al. (2005) and Tsukernik et al. (2007). The mechanisms for this
interaction include various forms of flow distortion, depending on wind direction (Petersen et
al., 2005), resulting in e.g., lee vortex formation (e.g., Petersen et al., 2003), cyclone splitting
(Kurz, 2004), and phase-locking (Kristjánsson et al., 2009).
In this study, we describe a polar low that hit western Iceland on 11-12 January 2007. The
low, which rapidly developed over the Denmark Strait in the early hours of 11 January, was
reasonably well captured by the major numerical weather prediction models, but uncertainties
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concerning the cyclone track and strength nevertheless made accurate short-term (6-12 hour)
forecasting for Iceland very difficult (forecaster-on-duty Óli Thór Árnason, pers. comm.).
Considering the fact that the polar low developed only 400 km south of Greenland’s highest
mountain, Mt. Gunnbjørn (3700 m elevation), we have explored the possibility for orographic
forcing from that feature acting as a trigger. A series of model simulations was carried out, in
order to address the following questions:
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Was the polar low development linked to a propagating upper-level potential vorticity
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(PV) anomaly?
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What was the role of orographic forcing?
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Can we distinguish orographic forcing from noise, due to a large sensitivity of the
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initial state to random perturbations?
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In the next section, we describe the synoptic weather situation leading up to and during the
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polar low event. This is followed by a section describing the model tool that was used for the
numerical experiments, as well as the experimental setup. Section 4 deals with the results
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from the model simulations, followed by a discussion section. Finally, section 6 summarizes
the main features of the study, and presents the conclusions.
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2. Synoptic description
The polar low formation was preceded by a deep (< 960 hPa) synoptic-scale cyclone that
approached southern Iceland on 10 January 2007, moving steadily east-northeast, reaching
maximum strength of 951 hPa at 00 UTC 11 January, then gradually filling over the next 48
hours as it continued its northeasterly track past Iceland (Figure 1). In its aftermath, from 00
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UTC 11 January (Figure 1c) onwards, Arctic air was advected over the Denmark Strait and
surroundings, creating favourable conditions for polar low formation over the relatively warm
waters of the Irminger current west of Iceland.
The infrared satellite imagery showed the first sign of an incipient polar low at 05:05 UTC on
11 January near 65.5°N, 30°W (Figure 2a). At this stage no clear structure was seen, but
rather a distinct north-south oriented cloud band from about 64°N to 66°N along the 30°W
meridian (Figure 2a). Eight hours later, at 13:19 UTC (Figure 2b), a well-developed polar
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low was seen at 65.5°N, 27°W, west of the Vestfirðir peninsula in northwest Iceland. At this
stage, the winds had started to pick up from a south-southeasterly direction over western
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Iceland and it had started to snow in some areas, e.g., Keflavík airport in Southwest Iceland.
The sounding from there at 12 UTC (Figure 3a) shows high relative humidity, closely
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following the moist adiabat all the way from about 900 hPa to the tropopause, which was
very low at about 475 hPa. All these features are indications of deep moist convection in an
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Arctic air mass, characteristic of polar lows (see, e.g., Rasmussen and Turner, 2003).
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At 15:02 UTC, the polar low had deepened further (Figure 2c) and the associated
southwesterly wind field was now causing heavy snow showers over the whole western part
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of Iceland (not shown). Over the next eight hours or so, the polar low was almost stationary
(Figure 2d), so that the strongest winds were still at sea, while sustained southwesterly winds
of 10-17 m s-1 were found on the west coast of Iceland (not shown). Scatterometer-based
wind speed retrievals by QuikSCAT (Figure 4a) indicated surface wind speeds of as much as
30 m s-1 at this time, and a similar reading was obtained by the corresponding QuikSCAT
image 12 hours later (not shown). The reliability of QuikSCAT winds in the region near
Greenland has been assessed by other studies (e.g., Winterfeld et al., 2009; Kolstad, 2008 and
Renfrew et al., 2009), and they seem to agree that strong winds may be overestimated. Still,
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the spatial distribution of QuikSCAT winds seems to be reliable enough, at least for use in
case studies such as this one.
The Keflavík airport sounding at 00 UTC 12 January (Figure 3b) was distinctly different
from the one 12 hours earlier. While the air in the lowest 250 hPa of the atmosphere was still
well-mixed and rather humid, the remainder of the troposphere was now dry and much
warmer than before. This suggests that at this time strong subsidence was taking place in the
lee of Greenland in the vigorous westerly flow that was now found through the whole
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troposphere (Fig.3b) and lower stratosphere (not shown). In the early hours of 12 January, the
polar low moved slowly eastward (Figure 2e), hitting the coast of Northwest Iceland at 12
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UTC (Figure 2f). At this time a surface pressure measurement of 962 hPa was taken just
ahead of the polar low at Bjargtangar (location indicated by B in Figure 4b). This reading was
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about 10 hPa lower than in the HIRLAM analysis (Figure 1e).
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As the polar low made landfall on 12 January, it rapidly weakened (Figure 2f), but
nevertheless heavy snow showers, as well as winds exceeding 20 m s-1, were observed in
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several locations in Northwest Iceland on that day (Figure 4b). Interestingly, at 22:46 UTC
(not shown), after the polar low had dissipated, satellite images showed several new
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mesoscale vortices in the same area west of Iceland where the original polar low formed, but
none of these developed into a polar low.
3. Model and experimental setup
In order to investigate the role of various factors for the evolution of the polar low on 11-12
January 2007, several simulations were carried out using HIRLAM (High Resolution
Limited-Area Model), version 7.2. The simulations used HIRLAM analyses as initial
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conditions, and 6-hourly forecasts from ECMWF were used at the lateral boundaries. The
model grid mesh consisted of 306 × 306 horizontal grid points at 22 km grid spacing and 40
vertical levels, covering an area consisting of northern Europe, the northern North Atlantic
and the northeasternmost part of the Canadian Arctic. The HIRLAM analyses were based on
three-dimensional variational assimilation (3D-Var: Gustafsson et al., 2001; Lindskog et al.,
2001) for one experiment, while four-dimensional variational assimilation (4D-Var: Huang et
al., 2002; Gustafsson, 2006) was used for the other five experiments. The HIRLAM 4D-Var
applies a multi-incremental minimization (Veersé and Thépaut, 1998) and includes the
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simplified physical parameterization scheme of Janisková et al. (1999). Conventional
observational data as well as satellite data from the Advanced Microwave Sounding Unit A
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(AMSU-A) of the Advanced TIROS Operational Vertical Sounder (ATOVS) were
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assimilated in a 6 h assimilation cycle. The HIRLAM grid point forecast model is hydrostatic,
and it utilizes a semi-implicit, semi-Lagrangian two-time level time integration scheme
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(Undén et al., 2002). The physical parametrizations used were, e.g., the radiation scheme of
Savijärvi (1990), the CBR turbulence scheme (Cuxart et al., 2000), the Kain–Fritsch
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convection scheme (Kain, 2004), the Rasch–Kristjánsson (1998) prognostic cloud water
scheme, and the Interaction Soil-Biosphere-Atmosphere (ISBA: Noilhan and Mahfouf, 1996)
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surface scheme. Surface friction is treated using a surface roughness parameterization, which
has separate formulations over sea (Charnock’s formula), over vegetation and over orography
(Undén et al., 2002).
In addition to a CONTROL run in which all the model features were as described above, a
series of sensitivity simulations was carried out. Firstly, three simulations were made to
investigate the sensitivity to the orography of Greenland; in NOGREEN all orographic
heights over Greenland were set to 0 m above sea level; in NOEAST the orographic heights
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over the easternmost part of Greenland were set to 0 m, while other parts of Greenland were
left intact; in NOGUNN only orographic heights around Mt. Gunnbjørn in eastern Greenland
were set to 0 m. The different orographic height fields are shown in Figures 5a-d.
To explore a possible link between the orographic effects and the model’s formulation of
orographic roughness, a simulation was carried out (SMOOTH), in which the model’s
orographic roughness length was reduced to 1% of the nominal values over all land areas.
From Figure 5a, we see that this would be expected to mainly influence the flow over
Greenland, and to a lesser extent over Iceland.
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We also investigated the sensitivity to the data assimilation scheme. In the simulation called
3DVAR, the 4-dimensional data assimilation was replaced by the 3-dimensional variational
data assimilation scheme.
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Finally, a simulation was carried out in which the SSTs in a rectangular area (65°-68°N, 25°35°W) west of Northwest Iceland were increased by 4 K. This simulation will be referred to
as SST+4.
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4. Results from the HIRLAM analyses and simulations
4.1 Static stability and upper-level conditions
As deep convection is one of the main ingredients of mature polar lows, the low-level static
stability is a good indicator of polar low potential (e.g., Kolstad, 2006; 2011). Empirical data
suggest that the temperature difference between the surface (i.e., SST) and at 500 hPa (T500)
tends to be well above 40 K upon polar low formation (e.g., Noer and Ovhed, 2003) in the
Nordic Seas region. Kolstad et al. (2009) chose to express the criterion in terms of the
potential temperature difference between the 700 hPa and the surface; the so-called MCAO
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index. The majority of polar lows investigated by Blechschmidt et al. (2009) reached a
temperature difference SST - T500 of 48 K. Compared to polar lows in other parts of the
Nordic Seas, the strongest anomalies of this parameter were found for polar lows that
developed near Greenland. In Figure 6 we show SST - T500 from HIRLAM analyses at
different times. Starting with the evening of 10 January, about 12 hours before the formation
of the polar low, we see (Figure 6a) that the values all around Iceland were in the range 30 –
40 K, except for a small area near 65°N, 35°W and another one further southwest, with
values between 40 and 45 K. Twelve hours later, just one hour after the first clear signs of a
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polar low initiation in the satellite imagery (Figure 2a), much larger values were found, and
in the area of the incipient polar low the values were higher than 48 K (Figure 6b), in
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excellent agreement with Blechschmidt et al.’s suggestion. The temperature difference then
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decreased somewhat, but nevertheless values well over 40 K persisted for another 30 hours in
the area around the polar low (Figures 6c and 6d), thereafter gradually decreasing as the polar
low dissipated (not shown).
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As discussed in the introduction, another crucial ingredient in polar low developments is the
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existence of an upper-level PV anomaly that can serve as a trigger. In order to investigate
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whether such a trigger was present, we studied the analyzed upper-level PV every 6 hours
from 00 UTC on 8 January, i.e., almost 2 ½ days before the polar low formation and until it
dissipated at 18 UTC on 12 January. A subset of these results, along with the height of the
500 hPa pressure level is shown in Figures 7a-f. In the days preceding the polar low event,
there was a rather weak (1-2 PVU) west-east oriented upper-level PV anomaly over
Greenland near 70°N (Figures 7a,b). The westernmost branch of this anomaly, which was
associated with a trough at 500 hPa, was located at 70°N, 50°W at 12 UTC on 9 January
(Figure 7a), then gradually moving southeastwards and increasing in strength, so that at 00
UTC 11 January, just about the time when the polar low started forming, a rather sharp
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trough was found along the coast of eastern Greenland at 65°N, 40°W (Figure 7c). This is in
very good agreement with Figure 6 (top right) of Blechschmidt et al. (2009). Over the
following 24 hours the trough and the associated PV anomaly continued their cyclonic
progression and deepened further, possibly due to mutual interaction with the developing
polar low (Figures 7d, 7e). The upper-level features gradually became more vertically aligned
with the polar low at the surface (comparing Figures 1d and 7e), as expected in a baroclinic
development. In summary, it is clear that both the conditions at the surface and near the
tropopause were favourable for a polar low development west of Iceland on 11 January 2007.
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Consequently, one might expect the polar low development to be a foregone conclusion and
that it would require drastic changes in the initial or boundary conditions to significantly alter
the course of events.
4.2 Sensitivity runs
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In order to understand the possible role of Greenland’s orography in triggering the polar low,
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we start by investigating the evolution of sea-level pressure in the six simulations (Figures 8 -
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10). First at +24 h, we see that while the CONTROL run has a distinct polar low in
approximately the correct position at 66°N, 29°W (Figure 8a), large deviations from this are
found in the three simulations with degraded Greenland orographies (Figure 8b-d). In the
NOGREEN case, the result (Figure 8b) is strikingly similar to the results of previous studies
by Kristjánsson and McInnes (1999) and Skeie et al. (2006), displaying a strong dipole of
orographically enhanced surface pressure over Northeast Greenland and a corresponding
reduction of as much as – 25 hPa in the region around the Denmark Strait. There is no polar
low present at either +24 h or at +36 h in the NOGREEN simulation (Figures 8b, 9b and
10b). This indicates that the very existence of the polar low is dependent on the presence of
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Greenland’s orography, in a similar way as the ‘residual low’ in the study by Kristjánsson
and McInnes (1999).
The removal of all of Greenland’s orography is a very drastic perturbation, so we now
investigate to what extent only parts of the orography may play a role. Hence, in Figures 8c,
9c and 10c we compare the results of the NOEAST simulation to those of CONTROL. In the
NOEAST simulation, the sea-level pressure is several hPa higher than in CONTROL in the
area where the polar low was located both in reality and in CONTROL (Figures 8c and 9c).
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An opposite signal is found north of Iceland, where the high mountains of East Greenland
cause the sea-level pressure to be several hPa higher in CONTROL than in NOEAST,
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possibly through vortex stretching in the lee. A more moderate orographic modification is
imposed in the NOGUNN simulation (Figure 5d), but even so, the polar low is greatly
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weakened also in this case (Figures 8d and 9d). The pressure signals are quite similar to those
in NOEAST, but with a slightly smaller amplitude.
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The results from the SMOOTH simulation (not shown) exhibit very small differences
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compared to those of CONTROL. This means that the orographic effect indicated by
NOGREEN, NOEAST and NOGUNN is not related to the enhanced surface friction of the
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mountains, but rather to the general flow distortion induced by them.
Returning now to the discussion at the end of section 4.1, all the experiments except
NOGREEN exhibit large similarities in both the upper-level PV and tropospheric static
stability (not shown) to the corresponding fields in the analysis and the CONTROL run (cf.
Figures 6-7). This indicates that the polar low is a shallow disturbance, most pronounced in
the lowermost part of the troposphere, and that the features in the upper-level PV and static
stability are not strongly affected by the polar low development. Rather, they help set the
stage for such a development. In NOGREEN, on the other hand, the PV anomaly moves
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much more rapidly eastwards than in the other simulations (not shown), which does not allow
the polar low enough time to develop. Such an eastward propagation was explored in detail
by Kristjánsson et al. (2009) in a case of a synoptic-scale lee cyclone over the Denmark
Strait.
Before considering orographic influence as the true cause of the results from simulations
NOGREEN, NOEAST and NOGUNN, one must also consider the possibility that these
results are a pure co-incidence. It is conceivable that the atmospheric state near the initial
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time on 10 January 2007 was so sensitive to perturbations that almost any random
perturbation of the initial state or the boundary conditions would have yielded a strong
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response. In order to explore this possibility, and thereby avoid a misinterpretation of the
results shown so far, results from the experiments 3DVAR and SST+4 will now be discussed.
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It turns out that in both cases, the sensitivity is quite small compared to the sensitivity of
modifying East Greenland’s orography. First, in the case of changes to the data assimilation
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scheme (3DVAR), we find the largest impact at the time of analysis, diminishing with
increasing forecast length (Table 1; Figures 8e and 9e). This can be interpreted as being
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which is done better in 4D-Var than in 3D-Var. Further into the simulation the boundaries
have an increasing impact on the simulation, with reduced importance of how the
observations were used initially. In this simulation alone, and in contrast to what was
observed, a secondary polar low southwest of Iceland developed on 11 January, causing the
positive anomaly in that area seen in Figure 8e. Nevertheless, apart from a somewhat delayed
deepening (Table 1), the main polar low developed similarly to that in the CONTROL run.
As polar lows are partly driven by surface fluxes, increasing the SST by 4 K as in simulation
SST+4 has the potential to significantly deepen the polar low, through increased fluxes of
both sensible and latent heat. However, despite an enhancement of the sensible and latent
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heat fluxes of about 50-100 W m-2 each in the area of enhanced SSTs (not shown), only
rather modest changes in the polar low evolution were obtained in the SST+4 simulation,
compared to CONTROL, (Figures 8f and 9f). We then repeated the experiment but with even
larger perturbations of the SST, i.e., 8 K and 12 K, respectively. A considerably larger
response was then found, with the polar low deepening by 2 hPa at +36 h in the former case
and by 8 hPa in the latter case (Table 1). However, clearly an 8 K or 12 K enhancement of the
SST in such a small area is a much larger perturbation than the uncertainty in the initial state
would represent, and even a 4 K enhancement is probably excessive (e.g., Garand, 2003). For
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more detailed studies of the sensitivity of polar low development to SST, we refer the reader
to two recent studies of the influence of SST on polar low development, which show widely
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different results: Linders et al. (2011), using an axi-symmetric model found a rather weak
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sensitivity to SST variations of – 0.6 hPa maximum deepening per degree warming, while
Adakudlu and Barstad (2011, this issue) found a much larger sensitivity of – 2 hPa / K in
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their simulations of a Barents Sea polar low during the 2008 IPY-THORPEX Andøya
campaign. By comparison, our results correspond to a varying sensitivity of – 0.5, – 0.6 and –
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1.0 hPa / K, respectively for the SST+4, SST+8 and SST+12 experiments.
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In order to explore the results of the sensitivity runs in more detail, Figure 10 shows nearsurface winds and accumulated precipitation, in addition to sea-level pressure at 00 UTC 12
January (+36 h simulation time). Looking first at the sea-level pressure, we see that only the
runs CONTROL, 3DVAR and SST+4 (Figures 10a, 10e and 10f) have a well developed polar
low in approximately the correct position near Vestfirðir peninsula. All three simulations
have 12 h accumulated precipitation between 4 and 16 mm over large areas of western and
southern Iceland, west of the Vestfirðir peninsula, as well as in the westerly flow over the
warm waters off the south coast. These results are in quite good agreement with observations
(not shown), taking into account the lack of observations over the sea and the well known
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underestimation from conventional precipitation measurements in windy conditions with dry
snow. In the SST+4 run the low is about 2 hPa deeper than in CONTROL, and the associated
wind and precipitation fields are somewhat stronger than in CONTROL (Figure 10f), but
otherwise the main features are very similar to those in the CONTROL run (Figure 10a).
Much larger differences are found in the simulations with degraded orography: Firstly, as
noted in connection with Figures 8 and 9, the polar low is completely absent in the
NOGREEN run, and we note in Figure 10b the greatly suppressed precipitation over Iceland.
The near-surface wind is influenced to a lesser extent than the precipitation, because in the
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absence of Greenland’s orography and the polar low, an unrealistically strong northwesterly
airflow emanating from Greenland impinges on Iceland, creating jets along the coast and in
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the lee of Iceland (Figure 10b). In the NOEAST run the polar low, in addition to being far too
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weak takes a much too southerly course, hitting Reykjavik in southwest Iceland at 18 UTC on
11 January (not shown), and moving rapidly eastward. Therefore, in Figure 10c, the heaviest
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precipitation and strongest winds are located offshore with unrealistic dry and calm
conditions over much of West and Southwest Iceland. In the NOGUNN simulation, only a
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trough forms west of Iceland, while unrealistically a closed low forms north of Iceland, in
agreement with the positive sea-level pressure anomaly north of Iceland in Figures 8d and 9d.
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This secondary feature is clearly seen in the precipitation and wind patterns north of Iceland
in Figure 10d, while over Northwest Iceland the weather is relatively calm and dry, in stark
contrast to what was actually observed at this time.
5. Discussion
The results presented above show that the topography of Greenland had a crucial influence on
the development of the polar low. The NOGREEN simulation, in which the orography of
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Greenland was set to zero, did not produce any trace of a polar low, while the polar low was
greatly weakened in the simulations with a degradation of East Greenland’s orography;
NOEAST and NOGUNN. Is this because the MCAO from Greenland onto the warm sea
surface fails to materialize in these three simulations or is it because the absence of
orographic features leads to a low-level vorticity deficit in the region where the polar low
forms? These questions are relevant not just for polar lows, but for synoptic-scale cyclones as
well.
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In Figure 11, we explore the cold air outbreak associated with the polar low in the
CONTROL run. It turns out that the cold air below 800 hPa from East Greenland was
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advected toward the south-southeast more or less along the section, and this transport was
amplified between 00 UTC 11 January and 06 UTC on 11 January (Figures 11a,b), during the
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incipient stage of the polar low. At 12 UTC, when the polar low was rapidly developing, we
note that below 800 hPa the cold air outbreak had come to a halt (Figure 11c), while there
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was (not shown) an increased westerly flow of warmer air perpendicular to the cross section.
In the northern part of the section, between about 650 hPa and 850 hPa there is a tendency for
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northerly flow and sinking motion. These indications of a lee effect associated with
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Greenland are much less pronounced when we take away the East Greenland orography
(Figures 12a,b). Furthermore, compared to NOEAST (Figure 12a) and NOGUNN (Figure
12b), we note that above 850 hPa the potential temperature is higher in CONTROL (Figure
11c), especially in the northern part of the section, i.e., in the vicinity of the developing polar
low.
Twelve hours later (Figure 11d), corresponding to the time of the sounding in Figure 3b, we
note that there is now strong rising motion in the northern part of the section (near 66°N,
25°W), in association with the polar low. Here the troposphere is well-mixed, all the way up
to 500 hPa, while further south (to the right in Figure 11d) the mixed-layer containing snow
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showers is gradually shallower due to a ridge of high pressure here (Figure 1e). A very
different situation is found in the NOEAST and NOGUNN simulations (Figures 12c,d),
which below 800 hPa display strong cold advection throughout and a strongly stratified
troposphere above 750 hPa (especially in NOEAST), in the absence of the polar low. In
NOGUNN, a pronounced sinking motion is found in the southern part of the section, but that
is unrelated to Greenland.
Having seen the large sensitivity of the polar low evolution to Greenland’s orography, despite
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the favourable conditions both at the surface and at upper levels, one may ask to what extent
these conditions are modified in the simulations with degraded orography. Therefore, we
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show and compare in Figure 13, from the six simulations, the upper-level height field and the
low-level temperature at the time of maximum strength of the polar low at 18 UTC on 11
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January. Firstly, in the CONTROL run (Figure 13a), there is at 500 hPa a pronounced trough
along Greenland’s east coast northwest of Iceland, while the 850 hPa temperature field shows
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a well defined tongue of warm air west of Iceland (where the polar low develops), as well as
another tongue of cold air stretching eastward from the coast of Greenland at about 65°N,
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35°W. In the NOGREEN simulation (Figure 13b), the 500 hPa trough is much stronger than
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in CONTROL, while the cold air advection at 850 hPa is far more advanced than in the
CONTROL run. In NOGREEN, the warm tongue found in CONTROL is replaced by a
wedge of cold air that is being effectively advected from Greenland. This big difference in
the ability of cold air to advance from Greenland toward Iceland was also found in earlier
studies (Kristjánsson and McInnes, 1999; Kristjánsson et al., 2009). As a consequence of
Greenland’s high elevation the cold air there can only be brought toward Iceland after
warming it adiabatically, which makes it warmer than the surroundings, thereby lowering the
surface pressure. In this way the cold air over Greenland becomes isolated in a way that
would not happen in the absence of the high elevation, but with the same degree of coldness
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(as in NOGREEN). This also to some extent explains the tendency of Greenland to create
cyclones in its vicinity. While previous model studies removing Greenland’s orography have
demonstrated a strong influence on synoptic-scale cyclones off Southeast Greenland (e.g.,
Kristjánsson and McInnes, 1999; Skeie et al., 2006), in this study we have obtained a similar
result for a polar low. The features that distinguish such a ‘polar lee low’ from the lee lows
studied earlier are: (i) that it forms in the Arctic air mass poleward of the main baroclinic
zone, rather than due to interaction with the main baroclinic zone; (ii) that the static stability
is weak and the tropopause low, resulting in a mesoscale system, rather than a synoptic-scale
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low (Montgomery and Farrell, 1992).
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Interestingly, in NOEAST and NOGUNN, which failed completely in simulating a polar low
with any resemblance to that observed, the 500 hPa height field is not dramatically different
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from the corresponding fields in CONTROL. The trough along East Greenland at 500 hPa in
CONTROL is replaced by a deeper, more circular low in both NOEAST (Figure 13c) and
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NOGUNN (Figure 13d), probably due to stronger cold advection in this area, as the obstacle
provided by the orography of eastern Greenland is removed. On the other hand, in both
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CONTROL, with warmer air off northeast Iceland than in CONTROL, probably in
connection with the secondary meso-cyclone there, and colder air west of the Vestfirðir
peninsula which is where the polar low is present in CONTROL, but absent in NOEAST and
NOGUNN at this time (viz. Figures 8 and 9).
In 3DVAR, on the other hand (Figure 13e), which had a surface pressure field similar to
CONTROL, the 500 hPa trough over east Greenland is somewhat weaker than in
CONTROL, whereas the 850 hPa temperature field is similar to that in CONTROL. In the
SST+4 run the 500 hPa height field (Figure 13f) is almost identical to that of CONTROL,
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while the 850 hPa temperature field shows a slightly more pronounced warm tongue west of
Iceland, as might be expected from the enhanced surface fluxes.
To summarize, the large sensitivity to the orography of East Greenland does not seem to be
caused by different pressure fields at upper levels. Rather the differences there are likely to be
caused by the different low-level flows that result from the differences in orography. The
sensitivity of the polar low development to East Greenland’s orography appears to be a lee
effect associated with northerly flow interacting with the steep orography associated with
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Greenland’s highest mountains near 69°N, 30°W, northwest of Iceland.
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6. Summary and conclusions
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A polar low that struck the western part of Iceland on 11-12 January 2007 has been
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investigated using available observations in the area, model analyses and dedicated
simulations with the HIRLAM numerical weather prediction model. The polar low developed
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in an Arctic air mass in the aftermath of a deep synoptic-scale cyclone moving northeast past
Iceland. The polar low had the characteristic features of polar lows in this area, previously
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documented by Blechschmidt et al. (2009); i.e., a temperature difference between the surface
and 500 hPa of more than 48 K and an upper-level PV anomaly approaching from the
northwest.
The control model run gave a polar low evolution that was quite close to the observed one.
Artificially enhancing the sea surface temperature in the area of polar low development by 4
K had a small effect on the polar low, and this was also the case for a simulation in which the
orographic roughness was reduced by a factor of 100, as well as a run in which the initial
state was based on 3D-Var, instead of the operational 4D-Var data assimilation scheme.
Conversely, in three simulations in which the orography of Greenland was degraded, the
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polar low was greatly weakened or even absent. Interestingly, the simulations revealed a
particular sensitivity to the area of eastern Greenland northwest of Iceland, the site of
Greenland’s highest mountain (Mt. Gunnbjørn at 3700 m), only about 50 km from
Greenland’s east coast. Cross-sections through the air masses between Greenland and Iceland
revealed features that are known to characterize lee cyclone formation, such as flow away
from Greenland and adiabatic warming of sinking air. In the absence of East Greenland’s
orography, these “lee cyclone” features are absent, and there is much stronger cold advection
between Iceland and Greenland, while the upper-level flow is quite similar in all the
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simulations. This leads us to conclude that the orography provides a trigger that is needed, in
order to spin-up the low-level circulation into a vigorous polar low. We further hypothesize
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that flow distortion associated with this part of Greenland, possibly in the form of subsidence
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with associated adiabatic warming and vortex stretching, may play a larger role in polar low
formation east of Greenland than katabatic flows, previously suggested to be a trigger by
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Klein and Heinemann (2002). More studies are needed to test this hypothesis, and to explore
in more detail the exact mechanism for the orographic triggering. Unfortunately, no Denmark
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Strait polar lows were captured during the Greenland Flow Distortion experiment (Renfrew et
al., 2008), which would otherwise have been a useful testbed for such a hypothesis.
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While this study has sought to provide new insight into the trigger mechanisms for polar low
developments, it has not explicitly dealt with forecasting improvements. A possible follow-up
would be to investigate the importance of model resolution for simulations of such ‘polar lee
lows’, because it is clear that the ability to resolve Mt. Gunnbjørn is resolution dependent. A
recent case study by McInnes et al. (2011, this issue) found a significant sensitivity to model
resolution in the polar low simulations, but in that study orography was not of importance.
Another issue worth exploring is the sensitivity to the choice of lateral boundary conditions,
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as the different prediction centres may have different representations of Greenland’s
orography.
7. Acknowledgments
This study was supported by the Norwegian Research Council’s project “THORPEX-IPY:
Improved forecasting of adverse weather in the Arctic - present and future” (grant no.
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175992). The first author would like to thank Hans von Storch for helpful suggestions that led
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to significant improvements in the experimental setup. QuikSCAT data were obtained from
the Physical Oceanography Distributed Active Archive Center (PO.DAAC) at the NASA Jet
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Propulsion Laboratory, Pasadena, CA (http://podaac.jpl.nasa.gov). We acknowledge help
from the Swedish Meteorological and Hydrological Institute (SMHI), concerning computer
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power, HIRLAM and graphics. We thank Laura Rontu for advice concerning the setup of the
SMOOTH experiment. Two anonymous reviewers are thanked for constructive comments
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that led to improvements of the manuscript. This is publication no. XXXXXXX from the
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Bjerknes Centre for Climate Research.
8. References
Adakudlu, M., and I. Barstad, 2011: Impacts of sea-surface temperature on polar lows: A
numerical study. Subm. to Q. J. R. Meteorol. Soc., this issue.
Blechschmidt, A.-M., S. Bakan, and H. Graßl, 2009: Large-scale atmospheric circulation
patterns during polar low events over the Nordic seas. J. Geophys. Res., 114, D06115,
doi:10.1029/2008JD010865.
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Bracegirdle, T. J., and S. L. Gray, 2008: An objective climatology of the dynamical forcing
of polar lows in the Nordic Seas. Int. J. Climatology, 28, 1903-1919.
Cuxart, J., P. Bougeault, J.-L. Redelsperger, 2000: A turbulence scheme allowing for
mesoscale and large-eddy simulations. Q. J. R. Meteorol. Soc. 126: 1–30.
Emanuel, K. A., and R. Rotunno, 1989: Polar lows as arctic hurricanes. Tellus, 41A, 1-17.
Gallée, H. 1995: Simulation of the mesocyclonic activity in the Ross Sea, Antarctica. Mon.
Wea. Rev., 123, 2051-2069.
Garand, L., 2003: Toward an integrated land-ocean surface skin temperature analysis from
Fo
the variational assimilation of infrared radiances. J. Appl. Meteor., 42, 570-583.
Grønås, S., and N. G. Kvamstø, 1995: Numerical simulations of the synoptic conditions and
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development of Arctic outbreak polar lows. Tellus, 47A, 797-814.
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Gustafsson, N., L. Berre, S. Hörnquist, X.-Y. Huang, M. Lindskog, B. Navascués, K. S.
Mogensen, and S. Thorsteinsson, 2001: Three-dimensional variational data
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assimilation for a limited area model. Part I: General formulation and the background
error constraint. Tellus, 53A, 425-446.
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Gustafsson, N., 2006: Status and performance of HIRLAM 4D-Var. HIRLAM Newsletter, 51,
8-17.
iew
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Heinemann, G., and T. Klein, 2002: Modelling and observations of the katabatic flow
dynamics over Greenland. Tellus, 54A, 542-554.
Huang, X.-Y., X. Yang, N. Gustafsson, K. S. Mogensen, and M. Lindskog, 2002: Fourdimensional variational data assimilation for a limited area model. HIRLAM
Technical Report 57, 44 pp. (Available from HIRLAM-5, c/o Per Undén, SMHI, S60176 Norrköping, Sweden)
22
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Janisková, M., J.-N. Thépaut, and J.-F. Geleyn, 1999: Simplified and regular physical
parameterizations for incremental four-dimensional variational assimilation. Mon.
Wea. Rev., 127, 26-45.
Junge, M. M., R. Blender, K. Fraedrich, V. Gayler, U. Luksch, and F. Lunkeit, 2005: A world
without Greenland: Impacts on the Northern Hemisphere winter circulation in lowand high-resolution models. Clim. Dyn., 24, 297-307.
Kain, J. S., 2004: The Kain–Fritsch convective parameterization: An update. J. Appl.
Meteorol. 43: 170–181.
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Klein, T., and G. Heinemann, 2002: Interaction of katabatic winds and mesocyclones near the
eastern coast of Greenland. Meteorol. Appl., 9, 407-422.
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Kolstad, E. W., 2006: A new climatology of favourable conditions for reversed-shear polar
lows. Tellus, 58, 344-354.
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Kolstad, E. W., 2008: A QuikSCAT climatology of ocean surface winds in the Nordic seas:
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Identification of features and comparison with the NCEP/NCAR reanalysis. J.
Geophys. Res., 113: D11106.
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Kolstad, E. W., and T. J. Bracegirdle, 2008: Marine cold-air outbreaks in the future: an
assessment of IPCC AR4 model results for the Northern Hemisphere. Clim. Dyn., 30,
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871-885.
Kolstad, E. W., T. J. Bracegirdle, and I. A. Seierstad, 2009: Marine cold-air outbreaks in the
North Atlantic: temporal distribution and associations with large-scale atmospheric
circulation. Clim. Dyn., 33, 187-197.
Kolstad, E. W., 2011: A global climatology of favourable conditions for polar lows. Subm.
Q. J. Roy. Meteorol. Soc., this issue.
Kristjánsson, J. E., and H. McInnes, 1999: The impact of Greenland on cyclone evolution in
the North Atlantic. Q. J. Roy. Meteorol. Soc., 125, 2819-2834.
23
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Kristjánsson, J. E., S. Thorsteinsson, and B. Røsting, 2009: Phase-locking of a rapidly
developing extratropical cyclone by Greenland’s orography. Q. J. R. Meteorol. Soc.,
135, 1986-1998.
Kurz, M., 2004: On the dynamics of the splitting process near southern Greenland. Meteorol.
Z., 13, 143-148.
Linders, T., Ø. Saetra, and T. J. Bracegirdle, 2011: Limited polar low sensitivity to SST. Q. J.
R. Meteorol. Soc., 137, 58-69.
Lindskog, M., N. Gustafsson, B. Navascués, K. S. Mogensen, X.-Y. Huang, X. Yang, U.
Fo
Andrae, L. Berre, S. Thorsteinsson, and J. Rantakokko, 2001: Three-dimensional
variational data assimilation for a limited area model. Part II: Observation handling and
rP
assimilation experiments. Tellus, 53A, 447-468.
ee
McInnes, H., J. Kristiansen, J. E. Kristjánsson, and H. Schyberg, 2011: The role of horizontal
resolution for polar low simulations. Subm. Q. J. R. Meteorol. Soc., this issue.
rR
Montgomery, M. T., and B. F. Farrell, 1992: Polar low dynamics. J. Atmos. Sci., 49, 24842505.
ev
Noer, G. and M. Ovhed, 2003: Forecasting of polar lows in the Norwegian and the Barents
Sea. 9th meeting of the EGS Polar Lows Working Group, Cambridge, UK.
iew
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2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
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46
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48
49
50
51
52
53
54
55
56
57
58
59
60
Noilhan, J., and J.-F. Mahfouf, 1996: The ISBA land surface parameterisation scheme.
Global and Planetary Change 13: 145–159.
Petersen, G. N., H. Ólafsson, and J. E. Kristjánsson, 2003: Flow in the lee of idealized
mountains and Greenland. J. Atmos. Sci., 60, 2183-2195.
Petersen, G. N., J. E. .Kristjánsson, and H. Ólafsson, 2004: Numerical simulations of
Greenland’s impact on the Northern Hemisphere winter circulation. Tellus, 56A, 102111.
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Petersen, G. N., J. E. Kristjánsson, and H. Ólafsson, 2005: The effect of upstream wind
direction on atmospheric flow in the vicinity of a large mountain. Quart. J. Roy.
Meterol. Soc., 131, 1113-1128.
Rasch, P.J., and J. E. Kristjánsson, 1998: A comparison of the CCM3 model climate using
diagnosed and predicted condensate parameterizations. J. Climate, 11, 1587–1614.
Rasmussen, E., 1981: An investigation of a polar low with a spiral cloud structure. J. Atmos.
Sci., 38, 1785-1792.
Rasmussen, E., and J. Turner, 2003: Polar lows. Mesoscale weather systems in the polar
Fo
regions. Cambridge University Press, 612 pp.
Reed, R. J., and C. N. Duncan, 1987: Baroclinic instability as a mechanism for the serial
rP
development of polar lows: A case study. Tellus, 39A, 376-384.
ee
Renfrew, I. A., G. W. K. Moore, J. E. Kristjánsson, H. Ólafsson, S. L. Gray, G. N. Petersen,
K. Bovis, P. R. A. Brown, I. Føre, T. Haine, C. Hay, E. A. Irvine, A. Lawrence, T.
rR
Ohigashi, S. Outten, R. S. Pickart, M. Shapiro, D. Sproson, R. Swinbank, A. Woolley,
and S. Zhang, 2008: The Greenland Flow Distortion experiment. Bull. Am. Meteorol.
ev
Soc., 89, 1307-1324.
Renfrew, I. A., S. D. Outten, and G. W. K. Moore, 2009: An easterly tip jet off Cape
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Farewell, Greenland. I: Aircraft observations. Q. J. Roy. Meteorol. Soc., 135, 19191933.
Savijärvi, H., 1990: Fast radiation parameterization schemes for mesoscale and short-range
forecast models. J. Appl. Met., 29, 437-447.
Skeie, R., J. E. Kristjánsson, H. Ólafsson, and B. Røsting, 2006: Dynamical processes related
to cyclone evolution near Greenland. Met. Zeitschrift, 15, 147-156.
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Tsukernik, M., D. N. Kindig, and M. C. Serreze, 2007: Characteristics of winter cyclone
activity in the northern North Atlantic: Insights from observations and regional
modeling. J. Geophys. Res., 112, D03101, doi:10.1029/2006JD007184.
Undén, P., L. Rontu, H. Järvinen, P. Lynch, J. Cavlo, et al., 2002: HIRLAM-5 Scientific
Documentation. Technical Report HIRLAM-5 project, c/o Per Undén, SMHI, SE60176 Norrköping, Sweden, 144 pp.
Veersé, F. and J.-N. Thépaut, 1998: Multiple-truncation incremental approach for fourdimensional variational data assimilation. Q. J. R. Meteorol. Soc., 124, 1889-1908.
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Wilhelmsen, K., 1985: Climatological study of gale-producing polar lows near Norway.
Tellus, 37A, 451-459.
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Winterfeldt, J., A. Andersson, C. Klepp, S. Bakan, and R. Weisse, 2009: Comparison of
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HOAPS, QuikSCAT and buoy wind speed in the Eastern North Atlantic and the
North Sea. IEEE Transactions on Geoscience and Remote Sensing, 48 (1): 338-348,
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doi:10.1109/TGRS.2009.2023982.
Zahn, M., and H. von Storch, 2010: Decreased frequency of North Atlantic polar lows
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+30
+36
+42
CONTROL
975 hPa
975 hPa
974 hPa
974 hPa
NOGREEN
-
-
-
-
NOEAST
(975 hPa)
(972 hPa)
(976 hPa)
-
NOGUNN
(977 hPa)
(976 hPa)
(978 hPa)
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3DVAR
978 hPa
977 hPa
975 hPa
975 hPa
SMOOTH
975 hPa
973 hPa
974 hPa
SST+4
975 hPa
973 hPa
972 hPa
973 hPa
SST+8
973 hPa
970 hPa
969 hPa
972 hPa
SST+12
967 hPa
963 hPa
966 hPa
973 hPa
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Table 1: Simulated sea-level pressure in the centre of the polar low near NW Iceland on 1112 January 2007. The initial time for all the simulations is: 12 UTC 10 January 2007. In
NOGREEN no polar low developed, therefore no values are given. In NOEAST and
NOGUNN, only a rapidly eastward-moving polar low some 200 km further south was
obtained, and the values in parentheses are for this polar low.
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e)
Figure 1: HIRLAM analyses of sea-level pressure (hPa, isolines) and temperature at 700 hPa
(K, shaded) at: a) 12 UTC 10 January 2007; b) 00 UTC 11 January 2007; c) 12 UTC 11
January 2007; d) 00 UTC 12 January 2007; e) 12 UTC 12 January 2007.
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70°N
70°N
40°W
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40°W
e)
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f)
Figure 2: NOAA AVHRR infrared (ch.4) images at: a) 05:05 UTC 11 January 2007; b) 13:19
UTC 11 January 2007; c) 15:02 UTC 11 January 2007; d) 23:10 UTC 11 January 2007; e)
04:55 UTC 12 January 2007; f) 12:56 UTC 12 January 2007. The red line indicates the
position of the cross-sections in Figures 11 and 12.
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a)
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Figure 3: Skew-T diagrams displaying radiosonde soundings from Keflavík, Iceland (64.0°
N, 22.6°W) at: a) 12 UTC 11 January 2007; b) 00 UTC 12 January 2007. (Figures obtained
from the University of Wyoming).
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R
K
b)
Figure 4: Observed wind speeds (m s-1) associated with the polar low: a) QuikScat level 2
winds (coloured arrows; resolution of 12.5 km; obtained from Remote Sensing Systems)
around 20:30 UTC on 11 January 2007; b) Maximum observed sustained (10 minute average)
winds at 10 m height at various locations in Iceland on 12 January 2007. Letters V, K and R
refer to geographical locations mentioned in the text; V=Vestfirðir peninsula; B=Bjargtangar;
K=Keflavík airport; R=Reykjavík.
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Figure 5: Orographic heights above sea level (m) in simulations: a) CONTROL; b)
NOGREEN; c) NOEAST; d) NOGUNN.
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Figure 6: The temperature difference between the sea surface and 500 hPa (K) from
HIRLAM analyses at different times: a) 18 UTC 10 January 2007; b) 06 UTC 11 January
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2007; c) 18 UTC 11 January 2007; d) 06 UTC 12 January 2007.
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e)
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Figure 7: HIRLAM analyses of geopotential height at 500 hPa (isolines, every 50 m) and
potential vorticity at 300-500 hPa (shaded) : a) 12 UTC 9 Jan 2007; b) 12 UTC 10 Jan 2007;
c) 00 UTC 11 Jan 2007; d) 12 UTC 11 Jan 2007; e) 00 UTC 12 Jan 2007; f) 12 UTC 12 Jan
2007.
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Figure 8: Sea-level pressure at 12 UTC 11 January 2007 (+24 h): a) CONTROL; b)
CONTROL minus NOGREEN; c) CONTROL minus NOEAST; d) CONTROL minus
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Quarterly Journal of the Royal Meteorological Society
NOGUNN; e) CONTROL minus 3DVAR; f) CONTROL minus SST+4. The contour interval
is 4 hPa in a) and 2 hPa in the other panels.
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Figure 9: Sea-level pressure at 00 UTC 12 January 2007 (+36 h): a) CONTROL; b)
CONTROL minus NOGREEN; c) CONTROL minus NOEAST; d) CONTROL minus
NOGUNN; e) CONTROL minus 3DVAR; f) CONTROL minus SST+4. The contour interval
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is 4 hPa in a) and 2 hPa in the other panels.
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e)
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Figure 10: Sea-level pressure (black isolines, every 4 hPa), wind speed at 10 m height (blue
dashed isolines, every 4 m s-1) and accumulated precipitation over 12 h preceding the
simulation time (colour shading) at 00 UTC 12 January 2007 (+36 h): a) CONTROL; b)
NOGREEN; c) NOEAST; d) NOGUNN; e) 3DVAR; f) SST+4.
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Figure 11: Potential temperature (red isolines, every 2 K) and the velocity component along
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the section (black arrows) in the cross-section between 68°N, 26°W and 61°N, 22°W (cf.
Figure 2b), from the CONTROL run at: a) 00 UTC 11 Jan 2007 (+12 h); b) 06 UTC 11 Jan
2007 (+18 h); c) 12 UTC 11 Jan 2007; (+24 h); d) 00 UTC 12 Jan 2007 (+36 h).
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Figure 12: Potential temperature (red isolines, every 2 K) and velocity wind component along
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Figure 2b) at 12 UTC 11 Jan 2007 (+24 h), from: a) run NOEAST; b) run NOGUNN, and at
00 UTC 12 Jan 2007 (+36 h), from: c) run NOEAST; d) run NOGUNN.
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Figure 13: Simulated 500 hPa heights (isolines, every 60 m) and 850 hPa temperature (colour
shading, K) at 18 UTC 11 Jan 2007 (+30 h) in: a) CONTROL run; b) Run NOGREEN; c)
Run NOEAST; d) Run NOGUNN; e) Run 3DVAR; f) Run SST+4.
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