Quarterly Journal of the Royal Meteorological Society
Phase-locking of a rapidly developing extratropical cyclone
by Greenland's orography
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Journal:
Manuscript ID:
Wiley - Manuscript type:
Complete List of Authors:
Research Article
Kristjansson, Jon; University of Oslo, Geosciences
Thorsteinsson, Sigurdur; Icelandic Meteorological Institute
Røsting, Bjørn; Norwegian Meteorological Institute
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Keywords:
QJ-09-0021.R1
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Date Submitted by the
Author:
Quarterly Journal of the Royal Meteorological Society
extratropical cyclones, orographic effects, Greenland
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Phase-locking of a rapidly developing
extratropical cyclone by Greenland’s
orography
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Jón Egill Kristjánsson1
Sigurdur Thorsteinsson2
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Bjørn Røsting3
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Quarterly Journal of the Royal Meteorological Society
Submitted in revised form: 13 July 2009
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Department of Geosciences, University of Oslo, P.O.Box 1022 Blindern, N-0315 Oslo, Norway. E-mail:
jegill@geo.uio.no
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Icelandic Meteorological Office, Bústadavegi 9, IS-150 Reykjavík, Iceland.
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Norwegian Meteorological Institute, P.O.Box 43 Blindern, N-0313 Oslo, Norway.
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Abstract
We present HIRLAM simulations of a deep extratropical cyclone that developed off the SE
coast of Greenland on 2-3 March 2007. The purpose of the simulations is to understand the
role of orographic forcing for the cyclone evolution, relating the results to previous model
studies. The cyclone evolution was preceded by a powerful cold air outbreak over Greenland,
starting on 27 February, manifested by a southward movement of an upper level potential
vorticity (PV) anomaly from 80°N to 60°N. In addition to a CONTROL run, starting at 00
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UTC 2 March, which captures the main features of the cyclone evolution quite well, we have
carried out simulations in which Greenland’s orography was removed (NOGREEN), as well
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as simulations with different starting times. In the NOGREEN simulation starting at 00 UTC
2 March, the cyclone deepens more rapidly than in CONTROL, due to a stronger cold
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advection on the rear side, leading to a more rapid baroclinic energy conversion. Furthermore,
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the cyclone position is shifted northward by 500 km, compared to the CONTROL run. A
very different result is found in the NOGREEN simulations that were started 24-36 hours
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earlier, as the cyclone off Greenland’s SE coast is now displaced eastwards by hundreds of
km, and more so as the run starts earlier. The results indicate a phase-locking by Greenland of
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a transient PV anomaly, indicating a mechanism for understanding cyclogenesis in this area.
Without Greenland’s orography, the PV anomaly is unconstrained, and the curvature of its
southward trajectory is larger.
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1. Introduction
Despite being one of the major mountain ranges in the Northern Hemisphere, Greenland has
received rather little attention in the meteorological literature. Petterssen (1956) was the first
one to show that the area between Greenland and Iceland, annually averaged, stands out with
a very high frequency of extratropical cyclones. The activity is particularly strong in the
winter, aided by the strong temperature contrast caused by warm waters due to the Irminger
current on the one hand and the extreme cold of the Greenland plateau on the other. The
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Icelandic Low, SW of Iceland, is a manifestation of this cyclonic activity. Model simulations
without Greenland’s presence have shown that, due to the land-sea contrast there would still
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be a strong wintertime low in the Northern Atlantic, but its position would be shifted away
from Greenland relative to its position when Greenland is present (Petersen et al., 2004; Junge
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et al., 2005). According to Serreze et al. (1997) only about 10-15% of all cyclone events in
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this region form there, with a slightly higher percentage decaying there, the majority being
cyclones moving through the region. On the other hand Tsukernik et al. (2007) found that out
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of three favoured regions for cyclone deepening over the North Atlantic, two of them are in
the vicinity of southern Greenland; one near the Icelandic Low SW of Iceland, and the other
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Quarterly Journal of the Royal Meteorological Society
south of the southern tip of Greenland. Intriguingly, numerical weather prediction (NWP)
models often seem to have rather large systematic errors in this area (Ferranti et al., 2002).
The reason for this is not fully known, but in some cases model errors in the GreenlandIceland region propagate into Europe over the following days. An example of such a
propagation was given by the explosive cyclone Gudrun, which caused great damage over the
United Kingdom, southern Sweden, Denmark and Germany on 7-8 January 2005. The
simulations of that storm that were issued in the preceding days showed considerable scatter,
and according to Ólafsson et al. (2005) that scatter could be traced back to an uncertainty in
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the initial state between Iceland and Greenland. On the other hand, Jung and Rhines (2007)
showed for the NWP model of the European Centre for Medium-Range Weather Forecasts
(ECMWF) that for wintertime cases of negative pressure drag (i.e., easterly low-level flow)
over Greenland, forecast errors in the Greenland-Iceland region did not on average propagate
into Europe on the following days. Also, targeted observations for a Scandinavian verification
region during GFDex, based on sensitive area predictions using singular vectors at the
ECMWF or the Ensemble Transform Kalman Filter (ETKF) technique at the United Kingdom
Meteorological Office (UKMO), gave varying results (Irvine et al., 2009). Clearly more
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studies of this subject are needed, and the motivation for the current study is to shed further
light on the mechanisms by which Greenland influences cyclone development in the highly
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active Greenland-Iceland area.
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This study deals with an explosive cyclone development in this area that took place on 2-3
March 2007, during the Greenland Flow Distortion experiment (GFDex). GFDex was a 3-
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week field campaign, operated from Keflavík, Iceland (Renfrew et al., 2008), with the
objective of gathering hitherto almost non-existing in situ data of the various weather
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phenomena that are particular to this region, such as tip jets, barrier winds, mesocyclones and
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lee cyclogenesis. These data, in turn, will be used to improve the understanding of these
phenomena, including, e.g., associated air-sea flux interactions (Petersen and Renfrew, 2009),
with the purpose of: (1) improving weather forecasts over northern Europe on time scales of
0-5 days; and (2) better assessing the role for the ocean circulation of the intense surface
fluxes from the ocean to the overlying atmosphere (Sproson et al., 2008; Våge et al., 2008).
In a companion paper (McInnes et al., 2009), an analysis of the mesoscale structure of the 2-3
March cyclone is presented, based on observations gathered from dropsondes and in-situ
measurements by aircraft. The purpose of the current study is to understand the driving
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mechanisms behind the cyclone evolution. In particular, we focus on the role of Greenland’s
orography and place the current study in a theoretical perspective, paying attention to
previous model studies of Greenland cyclogenesis. For this purpose we will present results
from numerical experiments, where in addition to a control run, we have artificially removed
Greenland’s orography, and used different initial times. Section 2 outlines the experimental
setup in more detail, while section 3 presents a description of the synoptic conditions. The
numerical experiments are dealt with in sections 4 and 5, followed by a discussion in section 6
and a summary with conclusions in section 7.
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2. Model and Experimental Setup
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Seven experiments, using the NWP model HIRLAM, version 7.0, were performed over an
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area covering Northern Europe and the northern North Atlantic to understand the mechanisms
of the cyclogenesis on 2-3 March 2007. The simulations used HIRLAM analyses as initial
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conditions, whereas 6-hourly forecasts from ECMWF were used at the lateral boundaries. The
model grid mesh consisted of 306 x 306 horizontal gridpoints at 22 km grid spacing and 40
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levels.
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The HIRLAM analyses are based on three-dimensional variational assimilation (3D-Var;
Gustafsson et al., 2001, Lindskog et al., 2001). 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 experiments were based on the model physics (version 7.0) used operationally at the
Swedish Meteorological and Hydrological Institute (SMHI). The physical parameterizations
include the radiation scheme of Savijärvi (1990), the Cuxart et al. (2000) turbulence scheme,
the Kain-Fritsch convection scheme (Kain, 2004), the Rasch-Kristjánsson (1998) prognostic
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cloud water scheme, and the Interaction Soil-Biosphere-Atmosphere (ISBA; Noilhan and
Mahfouf, 1996) surface scheme.
3. Synoptic description
During the first week of the GFDex campaign, 19-25 February 2007, the upper-level flow
pattern was characterized by a blocking high over central Greenland (not shown), meaning
that the extratropical cyclone activity across the North Atlantic was shifted southwards to
about 45°N. Reverse-shear flow conditions with wind speed decreasing with height in the
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lower troposphere prevailed over the area of interest, covering southern Greenland, the
Denmark Strait and Iceland. For instance, a reverse tip jet of up to 50 m s-1 at 950 hPa was
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observed by a dropsonde near the southern tip of Greenland on 21 February (Renfrew et al.,
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2009), and on the following day, an easterly low-level jet of 35 m s-1 was observed just south
of Iceland. The upper-level flow pattern in this region changed drastically over the last few
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days of February and the first 3 days of March (26 February – 3 March 2007), as the blocking
high over Greenland was first displaced southwards, and then disappeared. This flow
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transition occured in connection with a major cold air outbreak from the NW, entering NW
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Greenland on 27 February and then progressing southeastwards and later SSW (Figure 1a-c).
As shown by McInnes et al. (2009), this cold air outbreak was associated with a dramatic
southward movement of an upper-level PV anomaly, most clearly seen at the 290 K isentropic
surface. As the mid-tropospheric blocking high disappeared, the previously calm conditions at
these levels were replaced by gradually intensifying westerly winds. Even though the
conditions in the lowest part of the troposphere were still dominated by northeasterly flow
along the east coast of Greenland, the shift to westerly winds aloft was still very significant,
because it created favourable conditions for lee cyclogenesis off SE Greenland, as studied in
idealized conditions by Petersen et al. (2003; 2005) and for real cases by Kristjánsson and
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McInnes (1999) and Skeie et al. (2006). Indeed, at the surface, a weak low had already started
to appear at 65°N, 38°W on 28 February, and was more clearly seen on the following day
(Figures 2a-b).
As an increasingly baroclinic zone was established over Greenland, with strong westerly flow
at upper levels, gradually an interaction was enabled with the cyclonic activity to the south of
Greenland and Iceland: A slow moving equivalent-barotropic 989 hPa low, which was located
near 55°N, 35°W on 27 February - 1 March, served to advect warm air at low levels toward
the lee of SE Greenland (Figures 2a-c), while at the same time, the cold air outbreak from the
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north continued, with the isotherms becoming increasingly aligned with the shape of
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Greenland (Figures 2a-c). As a result of this clash of air masses, presumably aided by
vorticity generation due to the westerly flow upstream, very favourable conditions had arisen
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for a major cyclone development off SE Greenland. Indeed, here a rapid cyclogenesis took
place on 2 March, reaching peak intensity around 12 UTC 3 March near 60°N, 39°W (Figures
2d-f).
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Operational models at the time predicted the general course of events quite well, even in the
medium range. However, the position and strength of the cyclone were less well predicted,
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with considerable scatter between the models and from one forecast to the other. Figure 3
illustrates this, as it shows predicted geopotential heights at 925 hPa at the time of maximum
strength, at 12 UTC 3 March, from 4 consecutive forecasts (issued at 12 hour intervals) of the
Icelandic HIRLAM system at the time. The position of the low is quite accurate in the 12 h
forecast, while in the 48 h forecast the low is too far east by about 400 km. The results of the
36 h and 24 h forecasts are in between the other two, the latter being quite close to the 12 h
forecast. A very similar result to that of Figure 3 was obtained for sea-level pressure (not
shown).
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The purpose of the numerical simulations dealt with in the following sections is to identify the
role of Greenland’s orography for the cyclogenesis on 2-3 March 2007. We, seek, e.g., to
answer the question of to what extent this cyclonic development merits the term ‘lee
cyclogenesis’. We will also seek to understand the role of different parts of Greenland’s
orography for the formation of vorticity off SE Greenland.
4. Simulations starting at 00 UTC 2 March 2007
We first present simulations starting at 00 UTC 2 March 2007, i.e., 36 hours before the time
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of the maximum intensity of the low. This is a typical forecasting time for which high
resolution limited-area models have their main applicability, while for the medium range of 2-
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7 days the global models are the models of choice. The standard HIRLAM simulation from
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this starting time will be referred to as CONTROL from now on, while a corresponding
simulation carried out with Greenland’s orography reduced to 0 m above sea level will be
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termed NOGREEN, in analogy with earlier studies, e.g., Kristjánsson and McInnes (1999).
The NOGREEN run is carried out starting from an analysis that is based on the presence of
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Greenland, while the removal of orography is done before the initialization. Conceivably,
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therefore, the model might encounter spurious noise arising from the sudden removal of the
mountains, and such noise might then considerably affect the results. However, in agreement
with the earlier studies of Kristjánsson and McInnes (1999), using NORLAM and Skeie et al.
(2006), using MM5, we did not encounter spurious noise in the wind fields or the pressure
fields. To be on the safe side, we nevertheless deliberately avoid the first few hours of
simulation time in our analysis of NOGREEN, and mainly focus on results after 24-48 hours
of simulation time, to avoid any potential side-effects of the mountain removal. Previous
studies referred to in the introduction, as well as the current study, reveal strong and
consistent signals by Greenland’s orography, presumably far exceeding in magnitude any
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signals caused by random forecast errors. A more rigorous approach would have been to carry
out a quantitative comparison between the differences between CONTROL minus
NOGREEN runs on the one hand and the spread of an ensemble of model simulations, on the
other. Such an approach is recommended for future studies, as well as running two full data
assimilation streams in parallel, with and without Greenland’s orography, to see the full extent
of Greenland’s influence in NWP simulations.
In the CONTROL run, the cyclone evolution is fairly similar to what happened in reality
(Figure 4a-b), although at +36 h at 12 UTC 3 March the low pressure centre is displaced NE
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by 300 km compared to the analysis, and the central pressure is 3 hPa too high (Figures 4b
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and 3f). We note an upper level PV anomaly that has moved southward over Greenland, and
is now wrapping around the low centre (Figure 4b).
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In NOGREEN, as in Kristjánsson and McInnes (1999), the cyclone evolution is more
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vigorous than in CONTROL. As in that study, this is caused by a stronger, more coherent cold
air outbreak to the rear of the cyclone, aiding the baroclinic energy conversion from eddy
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potential to eddy kinetic energy through rising warm air and sinking cold air. The difference
is already very evident at +12 h (not shown) and +18 h (Figures 5a,c). Note also that the
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upper-level PV anomaly that is advancing southwards is considerably stronger and located
further NE in NOGREEN (Figure 4c) than in CONTROL (Figure 4a). Consequently, at +18
h, the cyclone in NOGREEN has already deepened to 973 hPa and is located NW of Iceland
at 65.5°N, 27°W, while in CONTROL the central pressure at this time is 977 hPa, with a
broad low centre SW of Iceland, centered at 61.5°N, 32°W, i.e., 500 km farther SSW. Over
the following 24 hours, the cyclone deepens further in CONTROL, while in NOGREEN its
life-cycle is more rapid, and it starts filling already after 06 UTC 3 March. One interpretation
of these results is that they indicate a combination of a baroclinic forcing tending to create a
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low NW of Iceland and an orographic forcing tending to create a low several hundreds of km
further SW. Such a location, east of Cape Farewell, is consistent with flow from a
northwesterly direction impinging on southern Greenland, according to idealized simulations
by Petersen et al. (2005: Fig. 13b). As Fig. 5a indicates, the upstream wind direction at 700
hPa (height of the highest mountains) was from the NW on 2 March, supporting this notion.
In order to evaluate the interpretation provided above, we show in Figure 6 results from
potential vorticity inversion applied to the analysis fields, based on the methodology of Davis
(1992) and Thorsteinsson et al. (1999). As discussed in McInnes et al. (2009), at 12 UTC 3
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March the positive upper level PV anomaly produces a large cyclonic anomaly at 900 hPa (253 m in geopotential height), and Figure 6 shows the contributions from the upper level PV
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anomalies 12 hours earlier, at the time of maximum deepening of the low. Also at this time it
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is seen that the southward propagating upper level PV anomaly induces a large negative
height perturbation (of -230 m) just SE of Greenland. A qualitatively similar result is found
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12 hours earlier (not shown), although the amplitude of the negative height anomaly off SE
Greenland is weaker at that time, i.e., -170 m at 900 hPa. Even though a corresponding PV
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inversion for the NOGREEN run has not been conducted, the results of Figure 6 together with
the PV structures in Figure 4 strongly suggest that the Greenland orography induces a ‘phase-
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locking’ between the orographic forcing of Greenland and the dynamic forcing of the
propagating upper level PV anomaly, meaning that the trajectory of the PV anomaly is
strongly influenced by Greenland, while the strength of the anomaly is less affected.
In figures 5c-d we see in NOGREEN a clear indication of the cold air that has moved
southwards over Greenland quickly descending toward the developing low west of Iceland,
creating an intense cyclone development. In CONTROL, on the other hand (Figures 5a-b), the
interaction with the cold air is much weaker, and consequently the low deepens considerably
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less. Comparing the low deepening in the two runs at the surface and at 700 hPa, we see an
interesting difference: At the surface, the low in the CONTROL run, despite a slow deepening
initially, after 36-48 hours has practically the same central pressure as in NOGREEN or even
lower, but, conversely, at 700 hPa the NOGREEN low is consistently about 120 m deeper
than in CONTROL, corresponding to about 15 hPa on a constant height surface. We
hypothesize that this big difference is due to the orographic forcing (in CONTROL and in
reality), which tends to create a “warm” low, while in NOGREEN the more developed
baroclinic low has a cold core, and therefore becomes stronger with height. To demonstrate
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the difference between the CONTROL and NOGREEN flow patterns, we show in Figure 7
cross sections through the troposphere over southern Greenland, aligned approximately
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parallel to the upstream wind direction. In the cross section from the CONTROL run (Figure
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7a), the advance of the cold air is greatly impeded by the orography, and hence the cold
advection behind the cold front (near 61°N, 40°W) is rather weak. Importantly, one can also
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discern a downward transport of high-θ air from above, in the lee of the mountain. In
NOGREEN (Figure 7b), by comparison, in much of the figure (especially near 64°N, 36°W) a
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very strong low-level cold advection behind the cold front is seen by the combination of the
alignment of the isentropes and the wind pattern.
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The significance of the southwesterly shift of the cyclone position due to Greenland’s
orography can be appreciated from Figure 8, which shows the surface fluxes of latent and
sensible heat from CONTROL and NOGREEN at +24 h simulation time. It has been
suggested by Pickart et al. (2003) that strong surface fluxes associated with barrier flow in
this region could have a major impact on the oceanic circulation, including the formation of
deep water. Therefore, it is of particular interest to understand how Greenland’s orography
influences these fluxes. First, we notice that in the area east of Cape Farewell the fluxes are
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much stronger in NOGREEN than in CONTROL, with sensible heat fluxes exceeding 600 W
m-2, i.e., about 200 W m-2 larger than in CONTROL, while for the latent heat flux the
corresponding figures are 300 W m-2 in NOGREEN and 260 W m-2 in CONTROL.
Conversely, NW of Iceland, the CONTROL run displays the typical barrier flow conditions,
so often experienced during GFDex (Renfrew et al., 2008) with strong surface fluxes in the
cold northeasterly low-level flow (360 W m-2 sensible heat and 240 W m-2 latent heat),
whereas the fluxes in NOGREEN are negligible, due to the weak winds at this time.
A further simulation was performed, in which all of Greenland’s orography was kept intact,
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except for the area near Mt. Gunnbjørn (at 68.92°N, 29.88°W), which at 3700 m is
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Greenland’s highest mountain. Being located only some 500 km NNW of Iceland, this
mountain area undoubtedly has a strong influence on barrier flow along the southern part of
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Greenland’s east coast, and it conceivably also influences cyclone evolution in this area. This
idea has gained support from a recent modeling study of a polar low development in January
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2007, which demonstrated high sensitivity to the presence or absence of this mesoscale
orographic feature (Kristjánsson et al., 2009). As opposed to the polar low study, however,
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is related to orographic effects associated with the southern Greenland mountain massif,
rather than vorticity generation by isolated mountains near the east coast of Greenland.
5. Simulations starting at 00 UTC 1 March 2007 or at 12 UTC 28
February 2007
Even though the results of the simulations CONTROL and NOGREEN in the previous
section showed a strong signal, indicating a major influence of Greenland’s orography on the
evolution of the cyclone, there are several unresolved issues. For instance, in section 3 it was
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Page 13 of 38
noted that the cyclone evolution on 2-3 March happened after a series of events, starting on 27
February, involving a dramatic cold-air outbreak over Greenland, associated with a sharply
defined upper level PV anomaly. Due to the long duration of this transition period, it is not
unlikely that the atmosphere at 00 UTC 2 March was already pre-conditioned to form an
intense cyclone somewhere in the Greenland-Iceland region. This would mean that the results
of the previous section do not tell the whole story about Greenland’s influence on the cyclone
event, but rather only describe Greenland’s influence in the explosive development phase.
The role of Greenland’s orography for the equally important pre-conditioning phase is
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addressed in this section, in which we present results from simulations starting 24-36 h
earlier, i.e., at 00 UTC 1 March or at 12 UTC 28 February. We start by looking at the results
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from 00 UTC 1 March 2007. Once again, we present results from one simulation with a
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standard orography (GREEN_1March) and another with all orographic heights over
Greenland set to 0 m (NOGREEN_1March).
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Already at 00 UTC 2 March we see a large difference between the two simulations: In
GREEN_1March, the coldest air stays over Greenland due to the blocking of the airflow,
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(explained in more detail in the next section) and there is only a weak tendency for the cold
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air to be advected east (Figure 9a), behind a developing low (not shown). In
NOGREEN_1March, by comparison, the cold advection behind the low near Jan Mayen is
much stronger, and the cold air curves cyclonically toward Jan Mayen (Figure 9c). This large
difference in the progression of the cold air mass NW of Iceland is a result of the undisturbed
southeastward progression of the cold air in the NOGREEN_1March run, while in the
CONTROL and GREEN_1March simulations the cold air advance comes to a halt near the
Denmark Strait, because of the blocking and coldness of Greenland’s mountain plateau.Over
the next 24-36 h, in GREEN_1March the events follow a similar route to that of CONTROL,
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with rapid cyclogenesis between Greenland and Iceland, with warm air from the southeast
being advected over Iceland in a rising motion and then curling around the low, while cold air
from Greenland comes around the low from the northwest, sinking in the process (not shown).
The main difference compared to CONTROL is the location of the low, which in
GREEN_1March (Figure 9b) is displaced some 100-200 km further NE than in CONTROL,
as was also the case for operational models at the time. In NOGREEN_1March, on the other
hand, due to the more rapid progression of the cold air mass, the two contrasting air masses
clash over Iceland, and cyclogenesis takes place here instead (Figure 9d). This huge shift in
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cyclone position of some 750 km highlights the strong control exerted by Greenland on the
southward propagation of the upper level PV anomaly, and hence on the position of the
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developing cyclone. With such a long integration time, possible contributions from the growth
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of errors / perturbations in the initial state can not be ruled out. Only an ensemble approach, as
suggested in the first paragraph of section 4 would enable a fully objective separation of such
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effects from those due to the removal of Greenland’s orography.
In Figure 10 we show results from simulations that were started a further 12 h earlier than
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GREEN_1March and NOGREEN_1March; we term these runs GREEN_28Feb and
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NOGREEN_28Feb, respectively. While the two simulations with Greenland’s orography
intact, GREEN_28Feb and GREEN_1March, show similar results (Figures 10a-b vs. 9a-b and
Figure 11a vs. 11b), there are large differences between NOGREEN_28Feb and
NOGREEN_1March. In NOGREEN_28Feb (Figures 10c-d), the cold air outbreak behind the
Jan Mayen low extends even further east than in NOGREEN_1March (Figures 9c-d), and as a
result, the cyclone development which takes place over the following 36 hours is displaced
eastwards by about 550 km, being east of Iceland at 65°N, 10°W at 12 UTC 3 March in
NOGREEN_28Feb (Figure 11c), whereas the corresponding low in NOGREEN_1March is
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located near 64°N, 20°W (Figure 11d), as compared to 61.5°N, 32°W in CONTROL (Figure
4b).
6. Discussion
In Schär’s (2002) regime diagram for the flow interactions with the major mountain ranges,
three regimes were identified: “quasi-geostrophic solutions” for small Rossby numbers (Ro =
U / (fL); U being the wind speed, f the Coriolis parameter and L the length scale of the
mountain), a “flow over” regime for large Rossby numbers and small values of the non-
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dimensional height, Nh/U (N being the Brunt-Vaisala frequency, h the mountain height and U
the wind speed), and thirdly, a “flow around” regime for large values of Nh/U. Greenland was
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placed at the boundary between “quasi-geostrophic solutions” and the “flow around” regime.
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We have investigated the value of the non-dimensional height at the time preceding the lee
cyclone development studied here, i.e., at 00 UTC 2 March 2007. It turns out that the values
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in the lower troposphere, in the unperturbed flow upstream of the west coast of Greenland, are
typically of the order 5 or so (not shown), suggesting that conditions are favourable for flow
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splitting, corresponding to the “flow around” regime, e.g., according to the idealized studies
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by Petersen et al. (2003; 2005). The same result was obtained by McInnes et al. (2009), who
based their estimates of non-dimensional height on soundings from Egedesminde on the west
coast of Greenland (68.7°N, 52.9°W) at 00 UTC 2 March, taking the variation with height
into account in their calculation of N, in accordance with Reinecke and Durran (2008). This
large value of Nh/U suggests a strong blocking effect of Greenland in this case.
Figure 12 schematically summarizes the proposed interaction between Greenland’s orography
and lee cyclone formation. With northwesterly flow impinging on the mountains of southern
Greenland, low-level vorticity is produced east of Cape Farewell. As the upper level PV
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anomaly enters from the north, a cyclonic development is induced off the coast of SE
Greenland. These two systems are independent of each other, except for the fact that, due to
topographic blocking, Greenland’s cold massif distorts the PV anomaly, so that it becomes
shaped almost like Greenland itself (McInnes et al., 2009). The result of these three
interacting factors: a) The upper level PV forcing; b) the modification of the PV anomaly by
the cold Greenland plateau; c) the orographic production of cyclonic vorticity in the lee off SE
Greenland, is the evolution of an explosive cyclone between Cape Farewell and Iceland. This
location is thus the result of a phase-locking between free dynamic forcing and a combination
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of orographic and thermal forcing due to Greenland. It is no co-incidence that this location,
which is very close to the climatological Icelandic Low, coincides with the favoured location
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for lee cyclone formation in the idealized simulations of Petersen et al. (2003; 2005) with a
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westerly component over a Greenland-like mountain. Figure 13 illustrates the eastward
progression of the southward trajectory of the upper level PV anomaly, as the initial time is
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shifted further back in time. Also, the different low positions resulting from this shift are
indicated. We suggest that the southward propagation of the PV anomaly over Greenland in
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the days preceding the cyclogenesis on 2-3 March 2007, is greatly weakened when
Greenland’s orography is removed already on 28 February or 1 March. Instead the PV
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anomaly takes a more easterly route, so that when it eventually interacts with the low-level
baroclinicity, causing cyclogenesis, that development takes place over the middle of the North
Atlantic Ocean, rather than in the “favoured region” SW of Iceland.
7. Summary and Conclusions
An explosive cyclone deepening between Greenland and Iceland on 2-3 March 2007, during
GFDex, has been studied with the aid of model simulations using HIRLAM. The purpose of
the simulations is to investigate the role of Greenland’s orography for the cyclogenesis, hence
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complementing earlier case studies and studies with idealized flow conditions. The study was
motivated by rather frequent forecast failures in this region, with model errors in some cases
(e.g., Ólafsson et al., 2005) propagating downstream towards Scandinavia and the British Isles
on the subsequent days. More investigations are needed to reveal the cause of these model
errors, but the present study, as well as, e.g., the investigation by Kristjánsson et al. (2009)
indicate that model forecasts of cyclogenesis in the region off E Greenland are extremely
sensitive to the orography of Greenland.
In the days preceding the event, a blocking high over Greenland gave way to gradually
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increasing westerly flow at upper levels, as a strong upper level PV anomaly propagated
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southwards over Greenland, accompanied by intense cold air advection in the lower
troposphere. Simulations with and without Greenland’s orography were carried out, starting at
different initial times.
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In the first set of simulations, starting at 00 UTC 2 March, as the cyclone development
commenced, a CONTROL simulation with the standard model settings gave a fairly similar
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evolution to that which was observed, although the position and strength of the cyclone were
somewhat in error, as was also the case for operational models at the time. The run without
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Greenland’s orography (NOGREEN) produced a deeper, more coherent cyclone, in
agreement with earlier studies (Kristjánsson and McInnes, 1999; Skeie et al., 2006; Tsukernik
et al., 2007), due to a stronger, unimpeded cold air advection in the rear of the cyclone.
Further, the cyclone in NOGREEN was shifted some 500 km to the NNE, compared to
CONTROL. Interestingly, the difference in central pressure between the two simulations,
which is about 15 hPa at 700 hPa, is negligible at the surface, meaning that the orographic
forcing of low-level vorticity E of the southern tip of Greenland, is largest at the surface. In
the second set of simulations, starting 24 or 36 hours earlier, results similar to CONTROL
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were found when Greenland’s orography was intact, while the simulations without
Greenland’s orography (termed NOGREEN_1March and NOGREEN_28Feb, respectively)
failed to produce a cyclone between Greenland and Iceland. Instead, the cyclone formation
took place progressively further east, as the initial time was shifted back in time. The eastward
shift from NOGREEN to NOGREEN_28Feb, which started 36 h earlier was about 1000 km,
while in NOGREEN_1March, starting 24 h earlier than NOGREEN, the cyclone was located
about halfway between these two extremes. This dramatic shift highlights the role of
Greenland’s orography in the pre-conditioning phase, which has not been dealt with in earlier
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studies. This sheds new light on the results of Held (1983) and Petersen et al. (2004), who in
general circulation model experiments without Greenland’s orography found a shift of the
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Icelandic Low to the east. In combination with the findings of Petersen et al. (2003; 2005),
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our results provide a possible mechanism for understanding those earlier results.
Acknowledgments
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This study has received support from the Norwegian Research Council through the project
“THORPEX-IPY: Improved forecasting of adverse weather in the Arctic – present and
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future” (grant no. 175992). We thank the anonymous reviewers for constructive and thorough
comments that led to significant improvements of the manuscript. Duing the course of this
work we have also benefited from illuminating discussions with Haraldur Ólafsson, Ian
Renfrew, Guðrún Nína Petersen, Thomas Spengler and Melvyn Shapiro. We thank scientists
at SMHI for their help in setting up and running the HIRLAM system including graphics.
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Figure Captions
Figure 1: HIRLAM analyses of 700 hPa temperature (K) at: a) 12 UTC 28 February 2007; b)
00 UTC 2 March 2007; c) 12 UTC 3 March 2007.
Figure 2: HIRLAM analyses of sea-level pressure (hPa, isolines) and 700 hPa temperature (K,
shaded) at: a) 00 UTC 1 March 2007; b) 12 UTC 1 March 2007; c) 00 UTC 2 March 2007; d)
12 UTC 2 March 2007; e) 00 UTC 3 March 2007; f) 12 UTC 3 March 2007.
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Figure 3: Operational HIRLAM forecasts of 925 geopotential height (in tens of m, drawn
every 20 m) at valid time 12 UTC 3 March 2007: a) 48 h forecast from 12 UTC 1 March
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2007; b) 36 h forecast from 00 UTC 2 March 2007; c) 24 h forecast from 12 UTC 2 March
2007; d) 12 h forecast from 00 UTC 3 March.
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Figure 4: HIRLAM simulations of sea-level pressure (every 4 hPa, isolines) and potential
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vorticity (PVU, shaded) at 300-500 hPa in simulations starting at 00 UTC 2 March 2007: a)
CONTROL run at +18 h; b) CONTROL run at +36 h; c) NOGREEN run at +18 h; d)
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Figure 5: Simulated 700 hPa geopotential height (m, isolines) and 700 hPa temperature (K,
shaded) in simulations starting at 00 UTC 2 March 2007: a) CONTROL run at +18 h; b)
CONTROL run at +36 h; c) NOGREEN run at +18 h; d) NOGREEN run at +36 h.
Figure 6: Contribution to the 900 hPa deepening (every 20 m) from an upper level positive
PV anomaly (blue lines) and an upper level negative PV anomaly (red lines) on 3 March 00
UTC. The height of the 900 hPa surface (every 40 m) is shown as green dashed curves.
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Figure 7: Cross sections of potential temperature (K, isolines) and vertical velocity (Pa s-1,
arrows) at 00 UTC 3 March 2007: a) At +24 h in the CONTROL run; b) at +24 h in the
NOGREEN run.
Figure 8: Simulated sensible (W m-2, shaded) and latent (W m-2, isolines) heat fluxes from the
surface to the atmosphere from simulations starting at 00 UTC 2 March 2007: a) At +24 h
from the CONTROL run; b) At +24 h from the NOGREEN run.
Figure 9: Simulated 850 hPa temperature (K, shaded) in simulations starting at 00 UTC 1
Fo
March 2007: a) At +24 h from GREEN_1March; b) at +60 h from GREEN_1March; c) at +24
h from NOGREEN_1March; d) at +60 h from NOGREEN_1March.
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Figure 10: Simulated 850 hPa temperature (K, shaded) in simulations starting at 12 UTC 28
ee
February 2007: a) At +36 h from GREEN_28Feb; b) at +72 h from GREEN_28Feb; c) at +36
h from NOGREEN_28Feb; d) at +72 h from NOGREEN_28Feb.
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Figure 11: HIRLAM simulations of sea-level pressure (hPa, isolines) and potential vorticity
(PVU, shaded) at 300-500 hPa at 12 UTC 3 March 2007 (as in Figures 4b,d): a) in simulation
ev
GREEN_28Feb starting at 12 UTC 28 February 2007 (+72 h): b) in simulation
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GREEN_1March starting at 00 UTC 1 March (+60 h); c) in simulation NOGREEN_28Feb
starting at 12 UTC 28 February 2007 (+72 h); d) in simulation NOGREEN_1March starting at
00 UTC 1 March 2007 (+60 h).
Figure 12: A schematic figure of the hypothesized relationship between the orographic and
baroclinic influence on cyclone formation off SE Greenland, as found in simulations
CONTROL and NOGREEN. The arrow on the left represents the flow impinging on the
Greenland orography, Lbar represents the purely baroclinic low, while Loro is the
orographically forced lee cyclone.
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Figure 13: A schematic figure of the interaction between the southward moving upper level
PV anomaly and the cyclogenesis in the Greenland-Iceland region: The arrows indicate the
path taken by the PV anomaly in the different simulations, while the L-s indicate the sea-level
low pressure centres in simulations CONTROL (LCON), NOGREEN (L0),
NOGREEN_1March (L1) and NOGREEN_28Feb (L2), respectively.
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b)
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Figure 1: HIRLAM analyses of 700 hPa temperature (K) at: a) 12 UTC 28 February 2007; b)
00 UTC 2 March 2007; c) 12 UTC 3 March 2007.
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f)
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Figure 2: HIRLAM analyses of sea-level pressure (hPa, isolines) and 700 hPa temperature (K,
shaded) at: a) 00 UTC 1 March 2007; b) 12 UTC 1 March 2007; c) 00 UTC 2 March 2007; d)
12 UTC 2 March 2007; e) 00 UTC 3 March 2007; f) 12 UTC 3 March 2007.
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a)
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Figure 3: Operational HIRLAM forecasts of 925 hPa geopotential height (in tens of m, drawn
every 20 m) at valid time 12 UTC 3 March 2007: a) 48 h forecast from 12 UTC 1 March; b)
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36 h forecast from 00 UTC 2 March; c) 24 forecast from 12 UTC 2 March; d) 12 h forecast
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Figure 4: HIRLAM simulations of sea-level pressure (every 4 hPa, isolines) and potential
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vorticity at 300-500 hPa (PVU, shaded) in simulations starting at 00 UTC 2 March 2007: a)
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CONTROL run at +18 h; b) CONTROL run at +36 h; c) NOGREEN run at +18 h; d)
NOGREEN run at +36 h.
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Figure 5: Simulated 700 hPa geopotential height (m, isolines) and 700 hPa temperature (K,
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shaded) in simulations starting at 00 UTC 2 March 2007: a) CONTROL run at +18 h; b)
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CONTROL run at +36 h; c) NOGREEN run at +18 h; d) NOGREEN run at +36 h.
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ee
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Figure 6: Contribution to the 900 hPa deepening (every 20 m) from an upper level positive
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PV anomaly (blue lines) and an upper level negative PV anomaly (red lines) on 3 March 00
UTC, based on HIRLAM analyses. The height of the 900 hPa surface (every 40 m) is shown
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as green dashed curves.
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b)
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Figure 7: Cross sections of potential temperature (K, isolines) and vertical velocity (Pa s-1,
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arrows) at 00 UTC 3 March 2007: a) At +24 h in the CONTROL run; b) at +24 h in the
NOGREEN run.
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b)
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Figure 8: Simulated sensible (W m-2, shaded) and latent (W m-2, isolines) heat fluxes from the
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surface to the atmosphere from simulations starting at 00 UTC 2 March 2007: a) At +24 h
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from the CONTROL run; b) At +24 h from the NOGREEN run.
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Figure 9: Simulated 850 hPa temperature (K, shaded) in simulations starting at 00 UTC 1
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March 2007: a) At +24 h from GREEN_1March; b) at +60 h from GREEN_1March; c) at +24
h from NOGREEN_1March; d) at +60 h from NOGREEN_1March.
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Figure 10: Simulated 850 hPa temperature (K, shaded) in simulations starting at 12 UTC 28
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February 2007: a) At +36 h from GREEN_28Feb; b) at +72 h from GREEN_28Feb; c) at +36
h from NOGREEN_28Feb; d) at +72 h from NOGREEN_28Feb.
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Figure 11: HIRLAM simulations of sea-level pressure (hPa, isolines) and potential vorticity
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(PVU, shaded) at 300-500 hPa at 12 UTC 3 March 2007 (as in Figures 4b,d): a) in simulation
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GREEN_28Feb starting at 12 UTC 28 February 2007 (+72 h): b) in simulation
GREEN_1March starting at 00 UTC 1 March (+60 h); c) in simulation NOGREEN_28Feb
starting at 12 UTC 28 February 2007 (+72 h); d) in simulation NOGREEN_1March starting at
00 UTC 1 March 2007 (+60 h).
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Figure 12: A schematic figure of the hypothesized relationship between the orographic and
baroclinic influence on cyclone formation off SE Greenland, as found in simulations
CONTROL and NOGREEN. The arrow on the left represents the flow impinging on the
Greenland orography, Lbar represents the purely baroclinic low, while Loro is the
orographically forced lee cyclone.
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Figure 13: A schematic figure of the interaction between the southward moving upper level
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path taken by the PV anomaly in the different simulations, while the L-s indicate the sea-level
low pressure centres in simulations CONTROL (LCON), NOGREEN (L0),
NOGREEN_1March (L1) and NOGREEN_28Feb (L2), respectively.
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