Quarterly Journal of the Royal Meteorological Society
Q. J. R. Meteorol. Soc. 137: 1773–1789, October 2011 A
Orographic influence of east Greenland on a polar low over the
Denmark Strait
Jón Egill Kristjánsson,a * Sigurdur Thorsteinsson,b Erik W. Kolstadc
and Anne-Marlene Blechschmidtd
a
University of Oslo, Norway
Meteorological Office, Reykjavik, Iceland
c
Bjerknes Centre for Climate Research, Bergen, Norway
d
NCAS-Weather, Lancaster University, UK
*Correspondence to: J. E. Kristjánsson, University of Oslo, Department of Geosciences, P.O.Box 1022, Blindern, Oslo,
0315, Norway. E-mail: jegill@geo.uio.no
b Icelandic
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 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 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 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 Greenland. Whereas
the control run captured well the main features of the polar low, simulations with
parts of east Greenland’s orography removed gave a southward-displaced polar
low which moved rapidly eastward, resulting in substantially underestimated nearsurface 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 3D-Var 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
c 2011 Royal Meteorological Society
highlighted in earlier studies. Copyright Key Words:
polar low; Denmark Strait; orographic influence
Received 27 October 2010; Revised 7 March 2011; Accepted 24 March 2011; Published online in Wiley Online
Library 24 May 2011
Citation: Kristjánsson JE, Thorsteinsson S, Kolstad EW, Blechschmidt A-M. 2011. Orographic influence
of east Greenland on a polar low over the Denmark Strait. Q. J. R. Meteorol. Soc. 137: 1773–1789.
DOI:10.1002/qj.831
1. Introduction
Polar lows are mesoscale weather phenomena that evolve at
high latitudes during the winter, in connection with marine
c 2011 Royal Meteorological Society
Copyright 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
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the northernmost extent of the warm ocean currents (Gulf
Stream, Kuroshio) and in regions with 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 polar low
frequency in a future warmer climate have been suggested
(Kolstad and Bracegirdle, 2008; Zahn and von Storch,
2010), drawing attention to the importance of a better
understanding of this intriguing phenomenon.
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 may appear to be mainly driven by latent heat
release, resembling tropical cyclones, hence 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 life cycles of polar lows over the Nordic Seas.
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 upperlevel 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 northern
Norway by Wilhelmsen (1985), only two out of 32 polar
lows that were considered had an orographic trigger.
Greenland’s enormous ice sheet, located at high elevation,
serves as a huge source of cold air, 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, 2002). Klein and Heinemann
(2002) suggested that convergence of the outflowing air
from the katabatic flow in the valleys of east Greenland
might be responsible for the formation of mesocyclones
over the Denmark Strait, and found support for this view
from model simulations. Due to its size, Greenland also has
a major impact on the North Atlantic weather and climate
through its influence on 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
c 2011 Royal Meteorological Society
Copyright numerical weather prediction models, but uncertainties
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,
personal communication). 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:
• Was the polar low development linked to a
propagating upper-level potential vorticity (PV)
anomaly?
• What was the role of orographic forcing?
• Can we distinguish orographic forcing from noise,
due to a large sensitivity of the initial state to random
perturbations?
In the next section, we describe the synoptic weather
situation leading up to and during the 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 set-up. Section 4 deals with the results from
the model simulations, followed by a discussion section.
Finally, section 6 summarizes the main features of the study,
and presents the conclusions.
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 0000 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 0000 UTC 11 January (Figure 1(c)) 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 0505 UTC on 11 January near
65.5◦ N, 30◦ W (Figure 2(a)). 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 2(a)). Eight hours later, at 1319 UTC (Figure 2(b)), a
well-developed polar 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 Iceland and it had started to snow in
some areas, e.g. Keflavı́k airport in southwest Iceland. The
sounding from there at 1200 UTC (Figure 3(a)) shows high
relative humidity, closely 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 Arctic air mass, characteristic
of polar lows (e.g. Rasmussen and Turner, 2003).
At 1502 UTC, the polar low had deepened further
(Figure 2(c)) and the associated southwesterly wind field was
now causing heavy snow showers over the whole western
part of Iceland (not shown). Over the next eight hours or
so, the polar low was almost stationary (Figure 2(d)), so
that the strongest winds were still at sea, while sustained
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
Orographic Influence of East Greenland on a Polar Low
(a)
(b)
(c)
(d)
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(e)
Figure 1. HIRLAM analyses of sea-level pressure (hPa, isolines) and temperature at 700 hPa (K, shaded) at (a) 1200 UTC 10 January 2007; (b) 0000
UTC 11 Jan 2007; (c) 1200 UTC 11 Jan 2007; (d) 0000 UTC 12 Jan 2007; (e) 1200 UTC 12 Jan 2007.
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 (Quick Scatterometer)
(Figure 4(a)) indicated surface wind speeds of as much as
30 m s−1 at this time, and a similar reading was obtained
c 2011 Royal Meteorological Society
Copyright 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. Kolstad, 2008; Renfrew et al., 2009; Winterfeldt et al.,
2010), and they seem to agree that strong winds may be
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
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(a)
(b)
(c)
(d)
(e)
(f)
Figure 2. NOAA AVHRR infrared (channel 4) images at (a) 0505 UTC 11 January 2007; (b) 1319 UTC 11 Jan 2007; (c) 1502 UTC 11 Jan 2007; (d) 2310
UTC 11 Jan 2007; (e) 0455 UTC 12 Jan 2007; (f) 1256 UTC 12 Jan 2007. The white line (red in the online version) in (b) indicates the position of the
cross-sections in Figures 11 and 12. This figure is available in colour online at wileyonlinelibrary.com/journal/qj
overestimated. Still, the spatial distribution of QuikSCAT
winds seems to be reliable enough, at least for use in
case-studies such as this one.
The Keflavı́k airport sounding at 0000 UTC 12 January
(Figure 3(b)) 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 troposphere
(Figure 3(b)) and lower stratosphere (not shown). In the
early hours of 12 January, the polar low moved slowly
c 2011 Royal Meteorological Society
Copyright eastward (Figure 2(e)), hitting the coast of northwest Iceland
at 1200 UTC (Figure 2(f)). 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 4(b)).
This reading was about 10 hPa lower than in the HIRLAM
analysis (Figure 1(e)).
As the polar low made landfall on 12 January, it
rapidly weakened (Figure 2(f)), but nevertheless heavy
snow showers, as well as winds exceeding 20 m s−1 , were
observed in several locations in northwest Iceland on that
day (Figure 4(b)). Interestingly, at 2246 UTC (not shown),
after the polar low had dissipated, satellite images showed
several new mesoscale vortices in the same area west of
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
Orographic Influence of East Greenland on a Polar Low
(a)
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(b)
Figure 3. Skew-T diagrams displaying radiosonde soundings from Keflavı́k, Iceland (64.0◦ N, 22.6◦ W) at (a) 1200 UTC 11 January 2007; (b) 0000 UTC
12 Jan 2007. (Figures obtained from the University of Wyoming).
Iceland where the original polar low formed, but none of
these developed into a polar low.
3. Model and experimental set-up
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 conditions,
and 6-hourly forecasts from the European Centre for
Medium-range Weather Forecasts (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 north-easternmost 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 fourdimensional variational assimilation (4D-Var: Huang et al.,
2002; Gustafsson, 2006) was used for the other five
experiments. The HIRLAM 4D-Var applies a multiincremental minimization (Veersé and Thépaut, 1998) and
includes the simplified physical parametrization scheme
of Janisková et al. (1999). Conventional observational data
as well as satellite data from the Advanced Microwave
Sounding Unit A (AMSU-A) of the Advanced TIROS
Operational Vertical Sounder (ATOVS) were assimilated
in a 6 h assimilation cycle. The HIRLAM grid-point forecast
model is hydrostatic, and it utilizes a semi-implicit, semiLagrangian two-time-level time integration scheme (Undén
et al., 2002). The physical parametrizations used were, for
example, the radiation scheme of Savijärvi (1990), the
Cuxart–Bougeault–Redelsperger (CBR) turbulence scheme
(Cuxart et al., 2000), the Kain–Fritsch convection scheme
(Kain, 2004), the Rasch–Kristjánsson (1998) prognostic
cloud water scheme, and the Interaction Soil-BiosphereAtmosphere (ISBA: Noilhan and Mahfouf, 1996) surface
scheme. Surface friction is treated using a surface roughness
parametrization, which has separate formulations over sea
c 2011 Royal Meteorological Society
Copyright (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 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
Figure 5(a)–(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 5(a),
we see that this would be expected to mainly influence the
flow over Greenland, and to a lesser extent over Iceland.
We also investigated the sensitivity to the data assimilation
scheme. In the simulation called 3DVAR, the fourdimensional data assimilation was replaced by the threedimensional variational data assimilation scheme.
Finally, a simulation was carried out in which the
sea-surface temperatures (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.
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
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
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(a)
(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 2030 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 = Keflavı́k airport; R = Reykjavı́k.
the Nordic Seas region. Kolstad et al. (2009) chose to express
the criterion in terms of the potential temperature difference
between 700 hPa and the surface: the so-called MCAO index.
The majority of polar lows investigated by Blechschmidt
et al. (2009) reached a temperature difference SST-T 500 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-T 500 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 6(a))
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 polar
low initiation in the satellite imagery (Figure 2(a)), much
larger values were found, and in the area of the incipient
polar low the values were higher than 48 K (Figure 6(b)), in
excellent agreement with Blechschmidt et al..’s suggestion.
The temperature difference then decreased somewhat, but
nevertheless values well over 40 K persisted for another
c 2011 Royal Meteorological Society
Copyright 30 hours in the area around the polar low (Figure 6(c)
and (d)), thereafter gradually decreasing as the polar low
dissipated (not shown).
As discussed in the introduction, another crucial
ingredient in polar low developments is the existence of an
upper-level PV anomaly that can serve as a trigger. In order
to investigate whether such a trigger was present, we studied
the analysed upper-level PV every 6 hours from 0000 UTC on
8 January, i.e. almost 2.5 days before the polar low formation
and until it dissipated at 1800 UTC on 12 January. A subset of
these results, along with the height of the 500 hPa pressure
level is shown in Figure 7(a)–(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 (Figure 7(a) and (b)). The westernmost branch of
this anomaly, which was associated with a trough at 500 hPa,
was located at 70◦ N, 50◦ W at 1200 UTC on 9 January
(Figure 7(a)), then gradually moved southeastwards and
increased in strength, so that at 0000 UTC 11 January, just
about the time when the polar low started forming, a rather
sharp trough was found along the coast of eastern Greenland
at 65◦ N, 40◦ W (Figure 7(c)). This is in very good agreement
with Fig. 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 (Figure 7(d) and (e)). The upper-level features
gradually became more vertically aligned with the polar low
at the surface (comparing Figures 1(d) and 7(e)), 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. 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
In order to understand the possible role of Greenland’s
orography in triggering the polar low, we start by
investigating the evolution of sea-level pressure in the six
simulations (Figures 8–10). 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 8(a)), large
deviations from this are found in the three simulations
with degraded Greenland orographies (Figure 8(b)–(d)). In
the NOGREEN case, the result (Figure 8(b)) 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 8(b), 9(b) and
10(b)). This indicates that the very existence of the polar low
is dependent on the presence of 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 8(c), 9(c) and 10(c) we compare the results of the
NOEAST simulation to those of CONTROL. In the NOEAST
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
Orographic Influence of East Greenland on a Polar Low
(a)
(b)
(c)
(d)
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Figure 5. Orographic heights above sea level (m) in simulations (a) CONTROL; (b) NOGREEN; (c) NOEAST; (d) NOGUNN.
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 8(c) and 9(c)).
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.
A more moderate orographic modification is imposed in
the NOGUNN simulation (Figure 5(d)), but even so, the
polar low is greatly weakened also in this case (Figures 8(d)
and 9(d)). The pressure signals are quite similar to those in
NOEAST, but with a slightly smaller amplitude.
The results from the SMOOTH simulation (not shown)
exhibit very small differences 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 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
c 2011 Royal Meteorological Society
Copyright 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 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 coincidence. It is conceivable that the
atmospheric state near the initial time on 10 January 2007
was so sensitive to perturbations that almost any random
perturbation of the initial state or the boundary conditions
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
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(a)
(b)
(c)
(d)
Figure 6. The temperature difference between the sea surface and 500 hPa (K) from HIRLAM analyses at different times: (a) 1800 UTC 10 January 2007;
(b) 0600 UTC 11 Jan 2007; (c) 1800 UTC 11 Jan 2007; (d) 0600 UTC 12 Jan 2007.
would have yielded a strong 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. 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
scheme (3DVAR), we find the largest impact at the time
of analysis, diminishing with increasing forecast length
(Table I; Figures 8(e) and 9(e)). This can be interpreted
as being caused by the importance of making use of the
observations at the right time in the analysis, 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 8(e). Nevertheless, apart from a
somewhat delayed deepening (Table I), 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 heat fluxes
of about 50–100 W m−2 each in the area of enhanced
c 2011 Royal Meteorological Society
Copyright SSTs (not shown), only rather modest changes in the polar
low evolution were obtained in the SST + 4 simulation,
compared to CONTROL (Figures 8(f) and 9(f)). 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 5 hPa, relative to CONTROL at +36 h in the former case
and by 8 hPa in the latter case (Table I). 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 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
different results: Linders et al. (2011), using an axisymmetric
model, found a rather weak sensitivity to SST variations
of −0.6 hPa maximum deepening per degree warming,
while Adakudlu and Barstad (2011) found a much larger
sensitivity of −2 hPa/K in their simulations of a Barents Sea
polar low during the 2008 International Polar Year–THe
Observing system Research and Predictability EXperiment
(IPY-THORPEX) Andøya campaign. By comparison, our
results correspond to a varying sensitivity of −0.5, −0.6 and
−1.0 hPa/K, respectively for the SST + 4, SST + 8 and SST
+ 12 experiments.
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
Orographic Influence of East Greenland on a Polar Low
(a)
(b)
(c)
(d)
(e)
(f)
1781
Figure 7. HIRLAM analyses of geopotential height at 500 hPa (isolines, every 50 m) and potential vorticity at 300–500 hPa (shaded) : (a) 1200 UTC 9
January 2007; (b) 1200 UTC 10 Jan 2007; (c) 0000 UTC 11 Jan 2007; (d) 1200 UTC 11 Jan 2007; (e) 0000 UTC 12 Jan 2007; (f) 1200 UTC 12 Jan 2007.
In order to explore the results of the sensitivity runs
in more detail, Figure 10 shows near-surface winds and
accumulated precipitation, in addition to sea-level pressure
at 0000 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 (Figure 10(a), (e) and (f))
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
c 2011 Royal Meteorological Society
Copyright in quite good agreement with observations (not shown),
taking into account the lack of observations over the sea
and the well-known 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 10(f)),
but otherwise the main features are very similar to those in
the CONTROL run (Figure 10(a)). 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
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
1782
J. E. Kristjánsson et al.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 8. Sea-level pressure at 1200 UTC 11 January 2007 (+24 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 is 4 hPa in (a) and 2 hPa in the
other panels.
we note in Figure 10(b) the greatly suppressed precipitation
over Iceland. The near-surface wind is influenced to a lesser
extent than the precipitation, because in the 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
the lee of Iceland (Figure 10(b)). In the NOEAST run the
polar low, in addition to being far too weak, takes a much
c 2011 Royal Meteorological Society
Copyright too southerly course, hitting Reykjavik in southwest Iceland
at 1800 UTC on 11 January (not shown), and moving
rapidly eastward. Therefore, in Figure 10(c), the heaviest
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 trough forms west of Iceland, while unrealistically a
closed low forms north of Iceland, in agreement with the
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
Orographic Influence of East Greenland on a Polar Low
(a)
(b)
(c)
(d)
(e)
(f)
1783
Figure 9. Sea-level pressure at 0000 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 is 4 hPa in (a) and 2 hPa in the
other panels.
positive sea-level pressure anomaly north of Iceland in
Figures 8(d) and 9(d). This secondary feature is clearly seen
in the precipitation and wind patterns north of Iceland in
Figure 10(d), while over northwest Iceland the weather is
relatively calm and dry, in stark contrast to what was actually
observed at this time.
orography of 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
5. Discussion
low-level vorticity deficit in the region where the polar low
forms? These questions are relevant not just for polar lows,
The results presented above show that the topography of but for synoptic-scale cyclones as well.
In Figure 11, we explore the cold air outbreak associated
Greenland had a crucial influence on the development of
the polar low. The NOGREEN simulation, in which the with the polar low in the CONTROL run. It turns out that
c 2011 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
1784
J. E. Kristjánsson et al.
Table I. Simulated sea-level pressure in the centre of the polar low near northwest Iceland on 11–12 January 2007.
hPa
+24 h
+30 h
+36 h
+42 h
CONTROL
NOGREEN
NOEAST
NOGUNN
3DVAR
SMOOTH
SST + 4
SST + 8
SST + 12
975
–
(975)
(977)
978
975
975
973
967
975
–
(972)
(976)
977
975
973
970
963
974
–
(976)
(978)
975
973
972
969
966
974
–
–
–
975
974
973
972
973
The initial time for all the simulations is 1200 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.
the cold air below 800 hPa from east Greenland was advected
toward the south-southeast more or less along the section,
and this transport was amplified between 0000 UTC and
0600 UTC on 11 January (Figure 11(a) and (b)), during the
incipient stage of the polar low. At 1200 UTC, when the polar
low was rapidly developing, we note that below 800 hPa the
cold air outbreak had come to a halt (Figure 11(c)), while
there 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 northerly flow and sinking motion. These
indications of a lee effect associated with Greenland are much
less pronounced when we take away the east Greenland
orography (Figure 12(a) and (b)). Furthermore, compared
to NOEAST (Figure 12(a)) and NOGUNN (Figure 12(b)),
we note that above 850 hPa the potential temperature
is higher in CONTROL (Figure 11(c)), especially in the
northern part of the section, i.e. in the vicinity of the
developing polar low.
Twelve hours later (Figure 11(d)), corresponding to the
time of the sounding in Figure 3(b), 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 11(d))
the mixed layer containing snow showers is gradually
shallower due to a ridge of high pressure here (Figure 1(e)).
A very different situation is found in the NOEAST and
NOGUNN simulations (Figure 12(c) and (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 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 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 1800 UTC on 11 January. Firstly,
in the CONTROL run (Figure 13(a)), there is at 500 hPa a
pronounced trough along Greenland’s east coast northwest
of Iceland, while the 850 hPa temperature field shows a
well-defined tongue of warm air west of Iceland (where the
c 2011 Royal Meteorological Society
Copyright polar low develops), as well as another tongue of cold air
stretching eastward from the coast of Greenland at about
65◦ N, 35◦ W. In the NOGREEN simulation (Figure 13(b)),
the 500 hPa trough is much stronger than 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 (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
low (Montgomery and Farrell, 1992).
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 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 13(c)) and NOGUNN (Figure 13(d)), 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 NOEAST and NOGUNN the
850 hPa temperature field is significantly different from
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
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
Orographic Influence of East Greenland on a Polar Low
(a)
(b)
(c)
(d)
(e)
(f)
1785
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 0000 UTC 12 January 2007 (+36 h): (a) CONTROL; (b) NOGREEN; (c) NOEAST;
(d) NOGUNN; (e) 3DVAR; (f) SST + 4.
CONTROL, but absent in NOEAST and NOGUNN at this
time (viz. Figures 8 and 9).
In 3DVAR, on the other hand (Figure 13(e)), 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
c 2011 Royal Meteorological Society
Copyright is similar to that in CONTROL. In the SST + 4 run the
500 hPa height field (Figure 13(f)) is almost identical to
that of CONTROL, 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.
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
1786
J. E. Kristjánsson et al.
(a)
(b)
(c)
(d)
Figure 11. Potential temperature (red isolines, every 2 K) and the velocity component along the section (black arrows) in the cross-section between
68◦ N, 26◦ W and 61◦ N, 22◦ W (cf. Figure 2(b)), from the CONTROL run at: (a) 0000 UTC 11 January 2007 (+12 h); (b) 0600 UTC 11 Jan 2007 (+18 h);
(c) 1200 UTC 11 Jan 2007 (+24 h); (d) 0000 UTC 12 Jan 2007 (+36 h).
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 Greenland’s highest
mountains near 69◦ N, 30◦ W, northwest of Iceland.
6.
Summary and conclusions
A polar low that struck the western part of Iceland on
11–12 January 2007 has been investigated using available
observations in the area, model analyses and dedicated
simulations with the HIRLAM numerical weather prediction
model. The polar low developed 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 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.
c 2011 Royal Meteorological Society
Copyright 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 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 simulations.
This leads us to conclude that the orography provides
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
Orographic Influence of East Greenland on a Polar Low
1787
(b)
(a)
(c)
(d)
Figure 12. Potential temperature (red isolines, every 2 K) and velocity wind component along the section (black arrows) in the cross-section between
68◦ N, 26◦ W and 61◦ N, 22◦ W (cf. Figure 2(b)) at 1200 UTC 11 January 2007 (+24 h) from: (a) run NOEAST; (b) run NOGUNN, and at 0000 UTC 12
Jan 2007 (+36 h) from: (c) run NOEAST; (d) run NOGUNN.
a trigger that is needed in order to spin up the lowlevel circulation into a vigorous polar low. We further
hypothesize that flow distortion associated with this part
of Greenland, possibly in the form of subsidence 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
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 Strait polar lows were captured during the
Greenland Flow Distortion experiment (Renfrew et al.,
2008), which would otherwise have been a useful test bed
for such a hypothesis.
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 resolutiondependent. A recent case study by McInnes et al. (2011)
found a significant sensitivity to model resolution in the
polar low simulations, but in that study orography was not of
c 2011 Royal Meteorological Society
Copyright importance. Another issue worth exploring is the sensitivity
to the choice of lateral boundary conditions, as the different
prediction centres may have different representations of
Greenland’s orography.
Acknowledgements
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. 175992). The first author would like to thank Hans
von Storch for helpful suggestions that led to significant
improvements in the experimental set-up. QuikSCAT
data were obtained from the Physical Oceanography
Distributed Active Archive Center (PO.DAAC) at the
NASA Jet Propulsion Laboratory, Pasadena, California
(http://podaac.jpl.nasa.gov). We acknowledge support from
the Swedish Meteorological and Hydrological Institute
(SMHI), concerning computer power, HIRLAM and
graphics. We thank Laura Rontu for advice concerning
the set-up of the SMOOTH experiment. Two anonymous
reviewers are thanked for constructive comments that led to
improvements of the manuscript.
Q. J. R. Meteorol. Soc. 137: 1773–1789 (2011)
1788
J. E. Kristjánsson et al.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 13. Simulated 500 hPa heights (isolines, every 60 m) and 850 hPa temperature (colour shading, K) at 1800 UTC 11 January 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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