QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY
Q. J. R. Meteorol. Soc. 135: 1986–1998 (2009)
Published online 1 October 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/qj.497
Phase-locking of a rapidly developing extratropical cyclone
by Greenland’s orography
Jón Egill Kristjánsson,a * Sigurdur Thorsteinssonb and Bjørn Røstingc
a
Department of Geosciences, University of Oslo, Blindern, Oslo, Norway
b
Icelandic Meteorological Office, Reykjavik, Iceland
c
Norwegian Meteorological Institute, Blindern, Oslo, Norway
ABSTRACT: We present HIRLAM simulations of a deep extratropical cyclone that developed off the southeast 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 0000 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 as simulations with different starting times. In the NOGREEN simulation starting
at 0000 UTC 2 March, the cyclone deepens more rapidly than in CONTROL, due to a stronger cold advection on the
rear side, leading to a more rapid baroclinic energy conversion. Furthermore, 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 earlier, as the cyclone off Greenland’s southeast 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 a transient PV anomaly, indicating a
mechanism for understanding cyclogenesis in this area. Without Greenland’s orography, the PV anomaly is unconstrained,
c 2009 Royal Meteorological Society
and the curvature of its southward trajectory is larger. Copyright KEY WORDS
cyclone evolution; cold air outbreak; PV anomaly; orographic forcing
Received 26 January 2009; Revised 13 July 2009; Accepted 16 July 2009
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 Icelandic
Low, southwest 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 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 et al., 2005).
According to Serreze et al. (1997) only about 10–15% of
all cyclone events in this region form there, with a slightly
higher percentage decaying there, the majority being
cyclones moving through the region. On the other hand,
∗
Correspondence to: Jón Egill Kristjánsson, Department of Geosciences, University of Oslo, PO Box 1022, Blindern, N-0315 Oslo,
Norway. E-mail: jegill@geo.uio.no
c 2009 Royal Meteorological Society
Copyright Tsukernik et al. (2007) found that out 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 southwest of Iceland, and the
other 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 Greenland–Iceland
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 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
PHASE-LOCKING OF AN EXTRATROPICAL CYCLONE BY GREENLAND
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 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
active Greenland–Iceland area.
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 three-week field campaign, operated from
Keflavı́k, Iceland (Renfrew et al., 2008), with the objective of gathering hitherto almost non-existing in situ data
of the various weather phenomena that are particular
to this region, such as tip jets, barrier winds, mesocyclones and lee cyclogenesis. These data, in turn, will be
used to improve the understanding of these phenomena,
including, for example, 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 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 set-up 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.
2.
Model and Experimental Set-Up
Seven experiments, using the NWP model HIRLAM
(High Resolution Limited-Area Model), version 7.0, were
performed over an 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 conditions, whereas 6hourly forecasts from ECMWF were used at the lateral
boundaries. The model grid mesh consisted of 306 × 306
horizontal grid points at 22 km grid spacing and 40 levels.
The HIRLAM analyses are based on three-dimensional
variational assimilation (3D-Var: Gustafsson et al., 2001;
Lindskog et al., 2001). Conventional observational data
c 2009 Royal Meteorological Society
Copyright 1987
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 parametrizations 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
cloud water scheme, and the Interaction Soil-BiosphereAtmosphere (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 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 observed by a dropsonde
near the southern tip of Greenland on 21 February (Renfrew et al., 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 days of February and the
first three days of March (26 February–3 March 2007),
as the blocking high over Greenland was first displaced
southwards, and then disappeared. This flow transition
occurred in connection with a major cold air outbreak
from the northwest, entering northwest Greenland on 27
February and then progressing southeastwards and later
south-southwest (Figure 1(a)–(c)). As shown by McInnes
et al. (2009), this cold air outbreak was associated with a
dramatic southward movement of an upper-level potential vorticity (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
southeast Greenland, as studied in idealized conditions
by Petersen et al. (2003, 2005) and for real cases by
Kristjánsson and 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 (Figure 2(a)–(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
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(a)
(b)
(c)
Figure 1. HIRLAM analyses of 700 hPa temperature (K) at: (a) 1200 UTC 28 February; (b) 0000 UTC 2 March; (c) 1200 UTC 3 March 2007.
activity to the south of Greenland and Iceland: A slowmoving 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 southeast Greenland (Figure 2(a)–(c)), while at the
same time the cold air outbreak from the north continued,
with the isotherms becoming increasingly aligned with
the shape of Greenland (Figure 2(a)–(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 for a major cyclone
development off southeast Greenland. Indeed, here a
rapid cyclogenesis took place on 2 March, reaching peak
intensity around 1200 UTC 3 March near 60◦ N, 39◦ W
(Figure 2(d)–(f)).
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, 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 1200
UTC 3 March, from four 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
c 2009 Royal Meteorological Society
Copyright 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).
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, for example, 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 southeast
Greenland.
4.
Simulations starting at 0000 UTC 2 March 2007
We first present simulations starting at 0000 UTC 2
March 2007, i.e. 36 hours before the time 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
2–7 days the global models are the models of choice.
The standard HIRLAM simulation from this starting
time will be referred to as CONTROL from now on,
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PHASE-LOCKING OF AN EXTRATROPICAL CYCLONE BY GREENLAND
(a)
(b)
(c)
(d)
(e)
(f)
1989
Figure 2. HIRLAM analyses of sea-level pressure (hPa, isolines) and 700 hPa temperature (K, shaded) at: (a) 0000 UTC 1 March; (b) 1200
UTC 1 March; (c) 0000 UTC 2 March; (d) 1200 UTC 2 March; (e) 0000 UTC 3 March; (f) 1200 UTC 3 March 2007.
while a corresponding simulation carried out with Greenland’s orography reduced to 0 m above sea level will be
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 Greenland, while the removal of orography is
done before the initialization. Conceivably, therefore, the
model might encounter spurious noise arising from the
sudden removal of the mountains, and such noise might
c 2009 Royal Meteorological Society
Copyright then considerably affect the results. However, in agreement with the earlier studies of Kristjánsson and McInnes
(1999) using the Norwegian Limited Area Model (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
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(a)
(b)
(c)
(d)
Figure 3. Operational HIRLAM forecasts of 925 hPa geopotential height (in tens of m, drawn every 20 m) at validation time 1200 UTC 3 March
2007: (a) 48 h forecast from 1200 UTC 1 March; (b) 36 h forecast from 0000 UTC 2 March; (c) 24 h forecast from 1200 UTC 2 March; (d) 12 h
forecast from 0000 UTC 3 March 2007. This figure is available in colour online at www.interscience.wiley.com/journal/qj
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
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 4(a)–(b)),
although at +36 h at 1200 UTC 3 March the low pressure
centre is displaced northeast by 300 km compared to
the analysis, and the central pressure is 3 hPa too high
(Figures 4(b) and 2(f)). We note an upper-level PV
anomaly that has moved southward over Greenland, and
is now wrapping around the low centre (Figure 4(b)).
In NOGREEN, as in Kristjánsson and McInnes (1999),
the cyclone evolution is more 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 potential
to eddy kinetic energy through rising warm air and
c 2009 Royal Meteorological Society
Copyright sinking cold air. The difference is already very evident at
+12 h (not shown) and +18 h (Figure 5(a) and (c)). Note
also that the upper-level PV anomaly that is advancing
southwards is considerably stronger and located further
northeast in NOGREEN (Figure 4(c)) than in CONTROL
(Figure 4(a)). Consequently, at +18 h, the cyclone in
NOGREEN has already deepened to 973 hPa and is
located northwest 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 southwest of Iceland, centred at
61.5◦ N, 32◦ W, i.e. 500 km farther south-southwest. 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 0600 UTC 3 March.
One interpretation of these results is that they indicate
a combination of a baroclinic forcing tending to create
a low northwest of Iceland and an orographic forcing
tending to create a low several hundred km further southwest. 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: their Fig. 13b). As Figure 5(a)
indicates, the upstream wind direction at 700 hPa (height
of the highest mountains) was from the northwest on 2
March, supporting this notion. In order to evaluate the
interpretation provided above, we show in Figure 6 results
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PHASE-LOCKING OF AN EXTRATROPICAL CYCLONE BY GREENLAND
(a)
(b)
(c)
(d)
1991
Figure 4. HIRLAM simulations of sea-level pressure (every 4 hPa, isolines) and potential vorticity at 300–500 hPa (PVU, shaded) in simulations
starting at 0000 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.
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 1200 UTC 3 March the positive upperlevel 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 anomalies 12 hours earlier, at the time of maximum deepening
of the low. Also at this time it is seen that the southward
propagating upper-level PV anomaly induces a large negative height perturbation (of −230 m) just southeast of
Greenland. A qualitatively similar result is found 12 hours
earlier (not shown), although the amplitude of the negative height anomaly off southeast Greenland is weaker
at that time, i.e. −170 m at 900 hPa. Even though a corresponding PV 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-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.
c 2009 Royal Meteorological Society
Copyright In Figure 5(c)–(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 (Figure 5(a)–(b)), the
interaction with the cold air is much weaker, and consequently the low deepens considerably 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 the
difference between the CONTROL and NOGREEN flow
patterns, we show in Figure 7 cross-sections through the
troposphere over southern Greenland, aligned approximately parallel to the upstream wind direction. In the
cross-section from the CONTROL run (Figure 7(a)), the
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(a)
(b)
(c)
(d)
Figure 5. Simulated 700 hPa geopotential height (m, isolines) and 700 hPa temperature (K, shaded) in simulations starting at 0000 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) at 0000 UTC 3 March 2007, based on HIRLAM analyses. The height of the 900 hPa surface (every 40 m) is shown as
green dashed curves.
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PHASE-LOCKING OF AN EXTRATROPICAL CYCLONE BY GREENLAND
(a)
1993
(b)
Figure 7. Cross-sections of potential temperature (K, isolines) and vertical velocity (Pa s−1 , arrows) at 0000 UTC 3 March 2007: (a) at +24 h
in the CONTROL run; (b) at +24 h in the NOGREEN run.
(a)
(b)
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 0000 UTC 2 March 2007: (a) at +24 h from the CONTROL run; (b) at +24 h from the NOGREEN run.
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 discern a downward transport of high-θ air from
above, in the lee of the mountain. In NOGREEN (Figure 7(b)), by comparison, in much of the figure (especially
near 64◦ N, 36◦ W) a 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.
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
c 2009 Royal Meteorological Society
Copyright Farewell the fluxes are 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, northwest 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, except for
the area near Mt. Gunnbjørn (at 68.92◦ N, 29.88◦ W),
which at 3700 m is Greenland’s highest mountain. Being
located only some 500 km north-northwest of Iceland,
this mountain area undoubtedly has a strong influence
on barrier flow along the southern part of Greenland’s
east coast, and it conceivably also influences cyclone
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evolution in this area. This idea has gained support from
a recent modelling study of a polar-low development
in January 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, here a negligible influence was
found, indicating that the orographic influence on the
cyclone 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 0000 UTC 1 March 2007
or at 1200 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 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 0000 UTC 2 March was
already preconditioned 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 preconditioning phase is addressed in this section, in which we present
results from simulations starting 24–36 h earlier, i.e. at
0000 UTC 1 March or at 1200 UTC 28 February. We
start by looking at the results from 0000 UTC 1 March
2007. Once again, we present results from one simulation with a standard orography (GREEN 1March) and
another with all orographic heights over Greenland set to
0 m (NOGREEN 1March).
Already at 0000 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 (explained in more detail in the next section),
and there is only a weak tendency for the cold air to
be advected east (Figure 9(a)), behind a developing low
(not shown). In NOGREEN 1March, by comparison, the
cold advection behind the low near Jan Mayen is much
(a)
(b)
(c)
(d)
Figure 9. Simulated 850 hPa temperature (K, shaded) in simulations starting at 0000 UTC 1 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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PHASE-LOCKING OF AN EXTRATROPICAL CYCLONE BY GREENLAND
stronger, and the cold air curves cyclonically toward
Jan Mayen (Figure 9(c)). This large difference in the
progression of the cold air mass northwest 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, 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 9(b)) is
displaced some 100–200 km further northeast 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 9(d)). This
huge shift in cyclone position of some 750 km highlights
the strong control exerted by Greenland on the southward
1995
propagation of the upper-level PV anomaly, and hence on
the position of the developing cyclone. With such a long
integration time, possible contributions from the growth
of errors/perturbations in the initial state cannot be ruled
out. Only an ensemble approach, as suggested in the first
paragraph of section 4 would enable a fully objective
separation of such 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
GREEN 1March and NOGREEN 1March; we term these
runs GREEN 28Feb and NOGREEN 28Feb, respectively. While the two simulations with Greenland’s
orography intact, GREEN 28Feb and GREEN 1March,
show similar results (Figures 10(a)–(b) vs. 9(a)–(b) and
Figure 11(a) vs. 11(b)), there are large differences
between NOGREEN 28Feb and NOGREEN 1March. In
NOGREEN 28Feb (Figure 10(c)–(d)), the cold air outbreak behind the Jan Mayen low extends even further
east than in NOGREEN 1March (Figure 9(c)–(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
1200 UTC 3 March in NOGREEN 28Feb (Figure 11(c)),
whereas the corresponding low in NOGREEN 1March is
(a)
(b)
(c)
(d)
Figure 10. Simulated 850 hPa temperature (K, shaded) in simulations starting at 1200 UTC 28 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.
c 2009 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. 135: 1986–1998 (2009)
DOI: 10.1002/qj
1996
J. E. KRISTJÁNSSON ET AL.
(a)
(b)
(c)
(d)
Figure 11. HIRLAM simulations of sea-level pressure (hPa, isolines) and potential vorticity (PVU, shaded) at 300–500 hPa at 1200 UTC 3 March
2007 (as in Figure 4(b) and (d)): (a) in simulation GREEN 28Feb starting at 1200 UTC 28 February (+72 h); (b) in simulation GREEN 1March
starting at 0000 UTC 1 March (+60 h); (c) in simulation NOGREEN 28Feb starting at 1200 UTC 28 February (+72 h); (d) in simulation
NOGREEN 1March starting at 0000 UTC 1 March 2007 (+60 h).
located near 64◦ N, 20◦ W (Figure 11(d)), as compared to regime, e.g. according to the idealized studies by Petersen
61.5◦ N, 32◦ W in CONTROL (Figure 4(b)).
et al. (2003, 2005). The same result was obtained by
McInnes et al. (2009), who based their estimates of nondimensional height on soundings from Egedesminde on
the west coast of Greenland (68.7◦ N, 52.9◦ W) at 0000
6. Discussion
UTC 2 March, taking the variation with height into
In Schär’s (2002) regime diagram for the flow interac- account in their calculation of N , in accordance with Reitions with the major mountain ranges, three regimes were necke and Durran (2008). This large value of Nh/U sugidentified: ‘quasi-geostrophic solutions’ for small Rossby gests a strong blocking effect of Greenland in this case.
Figure 12 schematically summarizes the proposed
numbers (Ro = U /(fL); U being the wind speed, f the
Coriolis parameter and L the length scale of the moun- interaction between Greenland’s orography and lee
tain), a ‘flow over’ regime for large Rossby numbers cyclone formation. With northwesterly flow impinging on
and small values of the non-dimensional height, Nh/U the mountains of southern Greenland, low-level vorticity
(N being the Brunt–Väisälä frequency, h the mountain is produced east of Cape Farewell. As the upper-level PV
height and U the wind speed), and thirdly, a ‘flow around’ anomaly enters from the north, a cyclonic development is
regime for large values of Nh/U . Greenland was placed at induced off the coast of southeast Greenland. These two
the boundary between ‘quasi-geostrophic solutions’ and systems are independent of each other, except for the fact
the ‘flow around’ regime. We have investigated the value that, due to topographic blocking, Greenland’s cold masof the non-dimensional height at the time preceding the sif distorts the PV anomaly, so that it becomes shaped
lee cyclone development studied here, i.e. at 0000 UTC 2 almost like Greenland itself (McInnes et al., 2009). The
March 2007. It turns out that the values in the lower tro- result of these three interacting factors: (1) the upper-level
posphere, in the unperturbed flow upstream of the west PV forcing; (2) the modification of the PV anomaly by
coast of Greenland, are typically of the order 5 or so the cold Greenland plateau; (3) the orographic production
(not shown), suggesting that conditions are favourable of cyclonic vorticity in the lee off southeast Greenland,
for flow splitting, corresponding to the ‘flow around’ is the evolution of an explosive cyclone between Cape
c 2009 Royal Meteorological Society
Copyright Q. J. R. Meteorol. Soc. 135: 1986–1998 (2009)
DOI: 10.1002/qj
PHASE-LOCKING OF AN EXTRATROPICAL CYCLONE BY GREENLAND
Figure 12. A schematic figure of the hypothesized relationship between
the orographic and baroclinic influence on cyclone formation off southeast 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.
Farewell and Iceland. This location is thus the result
of a phase-locking between free dynamic forcing and a
combination of orographic and thermal forcing due to
Greenland. It is no coincidence that this location, which
is very close to the climatological Icelandic Low, coincides with the favoured location for lee cyclone formation
in the idealized simulations of Petersen et al. (2003, 2005)
with a 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 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 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 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’ southwest 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 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
c 2009 Royal Meteorological Society
Copyright 1997
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.
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, for example, the investigation by Kristjánsson et al. (2009), indicate that model
forecasts of cyclogenesis in the region off east Greenland
are extremely sensitive to the orography of Greenland.
In the days preceding the event, a blocking high over
Greenland gave way to gradually increasing westerly
flow at upper levels, as a strong upper-level PV anomaly
propagated 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.
In the first set of simulations, starting at 0000 UTC 2
March, as the cyclone development commenced, a CONTROL simulation with the standard model settings gave
a fairly similar 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 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 northnortheast, 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 east 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
were found when Greenland’s orography was intact,
while the simulations without Greenland’s orography
Q. J. R. Meteorol. Soc. 135: 1986–1998 (2009)
DOI: 10.1002/qj
1998
J. E. KRISTJÁNSSON ET AL.
(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
preconditioning phase, which has not been dealt with in
earlier 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 Icelandic Low to the east. In combination with the findings of Petersen et al. (2003, 2005), our
results provide a possible mechanism for understanding
those earlier results.
Acknowledgements
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 future’ (grant no. 175 992). We thank
the anonymous reviewers for constructive and thorough
comments that led to significant improvements of the
manuscript. During the course of this work we have
also benefited from illuminating discussions with Haraldur Ólafsson, Ian Renfrew, Guðrún Nı́na 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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