Download The hurricane-like Mediterranean cyclone of January 1995

Meteorol. Appl. 7, 261–279 (2000)
The hurricane-like Mediterranean cyclone of
January 1995
Ioannis Pytharoulis1, George C Craig1, Susan P Ballard2
1
Department of Meteorology, University of Reading, Earley Gate, PO BOX 243, Reading
RG6 6BB, UK
2 Meteorological Office Unit, Joint Centre for Mesoscale Meteorology, University of Reading,
UK (now at NWP Division, Met. Office, London Road, Bracknell RG12 2SZ, UK)
The development of a hurricane-like vortex over the Mediterranean Sea was studied using (mainly) the
UK Met. Office Unified Model. The Mediterranean cyclone formed in the morning of 15 January 1995
over the sea between Greece and Sicily. Strong convection was observed prior to its genesis. During the
longest part of the cyclone’s lifetime, strong surface fluxes and, as a result, deep convection existed in its
vicinity. Its track was influenced by the surface fluxes and the flow in the wider region. The forecast of
the mesoscale and limited-area models reproduced the general characteristics of the actual system as
they appeared at the surface and upper-air charts and at the satellite imagery. The investigation of the
cyclone’s characteristics gave strong evidence (including an ‘eye’ and a warm core) to support the initial
assertion that it was similar to tropical cyclones and some polar lows. Baroclinic instability does not seem
particularly important, although the cyclone formed at the edge of a baroclinic zone. A numerical
experiment showed the vortex did not develop in the absence of surface heat and moisture fluxes.
Another experiment showed that sensible and latent heat fluxes were equally important in its
development.
1. Introduction
In mid-January 1995 a small hurricane-like cyclone was
detected by the Meteosat and the polar-orbiting
NOAA satellites over the central Mediterranean Sea.
Generally, most of the cyclogenesis occurring in the
Mediterranean is caused by the lee effect of the surrounding mountains (e.g. Buzzi & Tipaldi, 1978;
Flocas, 1994). In addition, Prezerakos & Flocas (1996)
have shown the importance of dynamically unstable
upper-tropospheric ridges in inducing surface cyclogenesis in the region and particularly over the Aegean Sea
(eastern Greece) where cyclogenesis is quite a rare phenomenon (Flocas & Karacostas, 1996). However,
sometimes small cyclones with intense convection and
looking remarkably like tropical cyclones develop over
the Mediterranean Sea. At least nine times in the past
forty years similar cyclones have been documented
(Winstanley, 1970; Mayengon, 1983; Ernst & Matson,
1983; Mayengon, 1984; Rasmussen & Zick, 1987;
Ziakopoulos & Marinaki, 1996; Malguzzi et al., 1998;
Reale, 1998; Reale & Atlas, 1998). Some observational
and numerical studies have already been carried out on
the cyclone of January 1995 (Pytharoulis, 1995;
Lagouvardos et al., 1996; Ziakopoulos & Marinaki,
1996; Blier & Ma, 1997). Vortices of much smaller size
are observed more frequently; for example, see Alpert
et al. (1994). However, their small size and short lifetime (a few hours) distinguish them from the class of
cyclones discussed here.
In this paper we undertake a close investigation of the
characteristics of the Mediterranean cyclone of January
1995 and try to understand the factors that contributed
to its genesis and development. Particular emphasis is
given to the question of the extent to which the superficial resemblance of this system to a hurricane represents an underlying dynamical similarity.
Our analysis is based on conventional and satellite
data, and additionally makes use of simulations
using the high-resolution mesoscale version of the
UK Met. Office (UKMO) Unified Model. While
conventional data are sparse over the sea, the availability of a successful numerical simulation makes an
examination of the mesoscale structure of the system
possible.
Before presenting the results of our investigation, we
consider first the evidence for the existence of such
hurricane-like vortices from previous studies, and second the possible mechanisms that might contribute to
their formation.
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1.1. Review of similar cyclones in the
Mediterranean Sea
Winstanley (1970) describes a similar cyclone that
developed over the central Mediterranean to the southeast of Malta on 23 September 1969 as the result of a
Saharan depression and a mid-tropospheric cold cutoff cyclone. It dissipated over north-eastern Algeria
after moving across north-western Libya and southern
Tunisia. The system reached the intensity of a cyclonic
storm and gale-force winds were recorded in its
vicinity. The existence of a cloud-free area in the centre
of the storm, the very strong surface winds and the
heavy, widespread and prolonged rainfalls indicate
that the system possessed the features of a tropical
cyclone.
Another vortex with tropical cyclone characteristics
formed in the Mediterranean Sea in January 1982. This
system appears to have had its genesis as an atypical
Atlas Mountains lee depression on 23 January 1982
(Ernst & Matson, 1983). The disturbance was first
detected over the sea north of Libya and dissipated
over the far eastern Mediterranean, after passing near to
Malta, Italy and Greece (Mayengon, 1983, 1984).
Although the available observations in the vicinity of
the cyclone were inadequate, they are consistent with
the interpretation that the system was a small, intense,
extra-tropical cyclone possessing some of the features
observed in tropical cyclones. These were the spiral
shape, the ‘eye’, the eyewall with the convectively driven cumulonimbus towers, the strong surface winds
and the fact that the strongest winds corresponded to
the eyewall surrounding the ‘eye’.
Another similar cyclone was observed in the autumn of
1983 (Rasmussen & Zick, 1987; Mayengon, 1984). It
was first detected on 27 September 1983 over the sea
between Tunisia and Sicily and, after traversing a large
circular path around Sardinia and Corsica, it finally
decayed following landfall near Tunis in the morning
of 2 October. During its lifetime the system made landfall twice: once on the south coast of Sardinia near
Cagliari on 28 September and once in western Corsica,
south of Ajaccio on 30 September. However, in each
case the old low-pressure centre dissolved and a new
one formed almost immediately over the sea. Baroclinic
instability did not appear to have been important in the
development of this cyclone. However, convection
because of the high sea-surface temperature was decisive in its formation and evolution (Rasmussen & Zick,
1987).
As in the previous cases, this cyclone exhibited tropical
cyclone features, including the ‘eye’, the strong sustained winds near the centre of the storm, the strong
convection with deep cumulonimbus towers in the eyewall and the existence of a warm core. The diameter of
the cyclone (measured using the closed 1010 hPa isobar
around the vortex center) was found to be between 200
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and 300 km during most of its lifecycle, except after the
second landfall (near Ajaccio) when it was reduced to
about 100 km (Rasmussen & Zick, 1987). Furthermore,
Rasmussen & Zick (1987) deduced that when the
cyclone was better organised, regions of upper-level
divergence and low-level convergence were almost
above the location of the surface low, indicating that
the vortex had had a vertical axis.
Two other such cyclones were observed between 4 and
6 October 1996, and 7 and 9 October 1996 (Malguzzi
et al., 1998; Reale & Atlas, 1998). The first formed
over the sea between Sicily and Tunisia on 4 October
1996 and dissipated after landfall over Sicily and
southern Italy. Floods were induced in Sicily and
Calabria (southern Italy), and gusts up to 30 m s–1
were recorded in Calabria (Reale & Atlas, 1998). The
second cyclone formed over the sea to the west of
Sardinia on 7 October 1996. It weakened temporarily
after landfall over Sardinia in the early morning of 8
October but regained its strength some hours later
when it moved again over the sea, finally dissipating
after landfall over Calabria. Damage caused by the
strong winds associated with this cyclone was reported
from the Aeolian islands (north of Sicily), and high precipitation amounts were recorded in both the Aeolian
islands and Calabria (Reale & Atlas, 1998). Both systems exhibited the features of a tropical cyclone, such
as a spiral cloud structure, strong convection, strong
surface winds, and heavy precipitation. Additionally,
an ‘eye’ appeared in the satellite images of the second
system, and warm temperature anomalies were found
in its core. Convection was important for the maintenance of both systems (Malguzzi et al., 1998; Reale &
Atlas, 1998).
Four more similar cyclones have been documented in
the literature (but not in as much detail as those noted
above). These cyclones appeared on 26 March 1983
(Ziakopoulos & Marinaki, 1996), between 29 and 30
December 1984 (Ziakopoulos & Marinaki, 1996),
between 30 and 31 October 1997 (Reale, 1998) and
between 5 and 8 December 1997 (Reale, 1998). All of
them exhibited hurricane-like characteristics in the
satellite imagery.
1.2. Mechanisms responsible for development
Hurricanes intensify in response to release of latent
heat in cumulus convection, which in turn is forced by
surface fluxes of latent and sensible heat in a synoptic
environment of low pressure, without or with weak
baroclinicity. A number of theoretical descriptions of
this process have been formulated, notably Conditional
Instability of the Second Kind (CISK), introduced by
Charney & Eliassen (1964), and Ooyama (1964), and
Wind Induced Surface Heat Exchange instability
(WISHE, but formerly referred to as Air–Sea
Interaction Instability), described by Emanuel (1986).
Mediterranean cyclone
According to the Charney and Eliassen formulation of
CISK, the latent heat release by the cumulus convection in a warm core vortex is proportional to the moisture convergence in the boundary layer of the storm.
Therefore, a strengthening of the vortex will increase
the low-level moisture convergence, which in turn will
result in increased latent heat release and further
strengthening of the system. Ooyama (1982) pointed
out the importance of the sea surface fluxes for the
intensification of the tropical cyclones, and Emanuel
(1986) proposed (in the WISHE theory) that the latent
heat release is governed by the wind dependent surface
fluxes of heat and moisture from the underlying ocean.
Recent evidence suggests that WISHE may be a more
accurate theoretical model, but the structure of the
weather systems they describe is similar (Craig & Gray,
1996).
In mid-latitudes, however, most cyclones form as a
result of baroclinic instability. Unless this mechanism
can be ruled out, it cannot be established that the development of a Mediterranean cyclone is hurricane-like. A
similar difficulty in establishing the mechanism of
intensification has been noted for polar lows – smallscale cyclones that form during cold air outbreaks over
the northern oceans (e.g. Rasmussen et al., 1992). It is
generally believed that a spectrum of such systems can
occur. At one extreme are small baroclinic disturbances
that owe their small scale to low vertical stability and
shallow depth. At the other extreme are hurricane-like
vortices driven by surface fluxes, although sensible heat
fluxes are as important as latent heat fluxes for polar
lows (Rasmussen, 1979). In the case of Rasmussen et al.
(1992), the cyclone showed both characteristics at different times in its development.
In this study the data will be examined for the distinctive characteristics of both baroclinic instability and
intensification driven by surface fluxes. In particular,
baroclinic instability is the cooperative intensification
of an upper-level potential vorticity anomaly, and an
equivalent anomaly that develops on a low-level potential temperature gradient (Hoskins et al., 1985). Each of
these features will be present in an intensifying baroclinic wave, typically with a westward tilt with height.
On the other hand, strong surface fluxes will result in
intense convection and a warm core vortex that shows
little vertical tilt. The crucial dependence of a system on
surface fluxes is difficult to establish from data but will
be tested in numerical experiments.
Description of the Mediterranean cyclone and of the
synoptic situation in its vicinity using satellite images as
well as surface and upper-air charts is the focus of
Section 2. Section 3 gives an overview of the numerical
simulation, while Section 4 presents a detailed analysis
of the mesoscale structure of the cyclone. An analysis
of the mechanisms contributing to the intensification of
the disturbance is given in Section 5.
2. Overview of evolution of the Mediterranean
cyclone
2.1. Synoptic environment prior to formation
The cyclone that is the focus of this study formed in the
morning of 15 January 1995 over the open sea west of
Greece, close to the centre of a low of larger dimensions which had moved over Greece the previous day
(Figure 1(a), (b)). This larger low had formed in the
morning of 13 January over the central Mediterranean
between Libya and Italy, and in the following days
moved north-eastwards towards western Greece.
Strong winds were associated with this low prior to the
formation of the Mediterranean cyclone. The German
research vessel Meteor reported sustained winds of 73
knots (37.6 m s–1) close to the centre of the low at 1400
UTC on 14 January (Blier & Ma, 1997). After the formation of the Mediterranean cyclone, the centre of the
large-scale low continued moving eastwards along the
coastline of Turkey towards Cyprus, decaying as it
went (Figure 1(c), (d)).
Throughout this period a trough was present in the
mid-troposphere, extending southwards to northern
Africa. On 13 January the trough axis lay over southern Italy and southern Tunisia (Figure 2(a)), whereas
the next day the trough had moved eastwards and was
aligned towards central Libya. There were two lowpressure regions in the mid-troposphere associated
with this trough (Figure 2(b)): one over Ukraine (persisting there from the previous day) and the other,
which formed on 14 January, located over the central
Mediterranean. This latter low was probably connected
with the low-level cyclone over western Greece at 1200
UTC (Figure 1(a)) that had intensified over the previous two days. The fact that the positive vorticity area
(connecting the low-level cyclone and the mid-troposphere low) was almost vertical (as can be seen in
Figures 1(a) and 2(b)) is a sign that the surface cyclone
had reached the end of its intensifying phase. Indeed,
18 hours later the cyclone had filled 8 hPa and a new
low-pressure centre had formed near the old one. This
marked the appearance of the cyclone that is the subject
of this study.
2.2. The formation of the Mediterranean cyclone
A close examination of Meteosat infra-red (IR) imagery
showed that the initial formation time of the
Mediterranean cyclone was at about 0330 UTC on 15
January 1995 (not shown), over the sea between western Greece and Sicily. Formation time is considered to
be the time that the cyclone first exhibited its hurricane-like features in the satellite imagery. About one
hour earlier there was a hook in the cloud pattern
marking the place where the vortex appeared in the
0330 UTC image. High clouds indicated that strong
convection was present before the initial formation of
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Figure 1. The mean sea-level pressure analysis for Europe at (a) 1200 UTC on 14 January, (b) 0000 UTC on 15 January,
(c) 0600 UTC on 15 January and (d) 1800 UTC on 15 January 1995. Isobars every 4 hPa. (UKMO Daily Weather Summary).
the cyclone. Water vapour imagery sometimes gives
early indications of the formation of a cyclone before it
is visible in the cloud pattern (Weldon & Holmes,
1991). However, an investigation of the water vapour
images for the day before the formation of the
Mediterranean cyclone showed no evidence of the
impending cyclogenesis. After its formation the vortex
was clearly visible on the satellite images. It exhibited
an axisymmetric pattern, a cloud-free vertical area in
the centre corresponding to the ‘eye’ and spiral-shaped
clouds around the ‘eye’. The first time that the cyclone
was drawn on the UKMO six-hourly mean sea-level
pressure subjective analysis charts was at 0600 UTC
when its central pressure was 1002 hPa (Figure 1(c)). In
the subjective three-hourly mesoscale mean sea-level
pressure analyses of the Hellenic National
Meteorological Centre (HNMC), the Mediterranean
cyclone appeared at 0000 UTC on 15 January
(Ziakopoulos & Marinaki, 1996). Comparisons
264
between the mean sea-level pressure analyses of the
UKMO and the HNMC during the lifetime of the
Mediterranean cyclone showed generally good agreement with regard to the location and central pressure of
the cyclone. The two analyses usually agreed within 2
hPa, except for a few times near the last hours of the
cyclone’s lifecycle.
During the following hours the central pressure of the
vortex increased and at 0000 UTC on 16 January it had
reached 1013 hPa (in the UKMO six-hourly mean sealevel pressure analysis). The fact that the central pressure of the vortex was increasing (Table 1) is usually a
sign that the surface winds are losing strength.
However, this rule cannot be applied here since the system was a small-scale feature embedded in a large-scale
environment of rising pressure. The only direct surface
observations were those of some ships travelling in the
vicinity of the cyclone. The reported winds near the
Mediterranean cyclone
Table 1. The actual central pressure of the
Mediterranean cyclone (UKMO mean sea-level
pressure subjective analysis) and its predicted values
from 0600 UTC on 15 January to 0000 UTC on
18 January 1995
Forecast period and time/date
T+6
T+12
T+18
T+24
T+30
T+36
T+42
T+48
—
—
—
—
Figure 2. 500 hPa geopotential heights (full line) and
1000–500 hPa thickness lines (dashed line) at: (a) 1200 UTC
on 13 January and (b) 1200 UTC on 14 January 1995. The
geopotential height contours and the thickness lines are in
dam. Contour interval: 12 dam. (UKMO operational global
analysis charts).
vortex were generally not strong, being between 15 and
25 knots. The maximum value was 30 knots (15.5 m
s–1), recorded by a ship located near 35° N, 20° E at
1200 UTC on 15 January. The threshold value of the
maximum sustained surface winds for a tropical
cyclone to be upgraded from tropical depression to
tropical storm is 34 knots (Foley, 1995). Showers and
thunderstorms were reported near the cyclone especially near the end of the day. Two ships in the vicinity
of the cyclone reported winds of 17.5 m s–1 at 0000
UTC on 16 January (Lagouvardos et al., 1996).
The satellite images of 15 January reveal a well-organised vortex with a clear ‘eye’ and cumulonimbi rotating
anticlockwise around the core (Figure 3(a)). The existence of convective clouds near the ‘eye’, in the area
corresponding to the eyewall, is a typical feature of
tropical cyclones. Moreover, the visible band images
show many low clouds to be present at a larger distance
from the ‘eye’ (Figure 3(b)). After 1800 UTC the
0600 UTC 15/01/95
1200
1800
0000 UTC 16/01/95
0600
1200
1800
0000 UTC 17/01/95
0600
1200
1800
0000 UTC 18/01/95
Central pressure (hPa)
Actual
Predicted
1002
1004
1006
1013
1012
1010
1012
1012
1018
1016
1022
1022
—
1004
1006
1011
1014
1016
1019
1020
—
—
—
—
images exhibit a very tight vortex of smaller diameter
than before (Figure 4), and which was becoming more
axisymmetric. All that day, the convection was very
intense, with the cloud-top temperatures of the highest
cumulonimbus (as shown by the IR images) being
below –50 °C (not shown), corresponding to very high
cloud tops. At 0000 UTC on 16 January (Figure 4) the
number of cumulonimbi had increased to four and
were visible around the ‘eye’ (in the area corresponding
to the eyewall), together with one additional big tower
very close to the core of the storm. The cloud-top temperatures of these cumulonimbi were still colder than
–50 °C (not shown), corresponding to deep convection.
However, at that time, the ‘eye’ of the cyclone may
have been tilting with height since the sea surface was
not visible from above.
2.3. The track of the cyclone
During the first day of its existence, the cyclone did not
move significantly. From the time of its formation until
the end of the first day (15 January) it remained positioned over open sea between Greece and Sicily but
closer to Greece (Figure 5). This occurred even though
the larger low-pressure area in which the cyclone was
embedded was moving slowly eastwards. Probably the
strong convection that was observed west of Greece,
and which had favoured the development of the
cyclone, prevented it from following the movement of
the wider low-pressure area.
Over the previous days a high pressure area had been
building over central Europe, principally associated
with a strong anticyclone that had moved across from
the Bay of Biscay (between France and Spain). The
interaction of the anticyclone with the low-pressure
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Figure 4. Infra-red satellite image from the NOAA polarorbiting satellites at 0000 UTC on 16 January 1995. The
brighter grey-scales correspond to higher clouds.
Figure 3. (a) Infra-red and (b) visible satellite images from
the NOAA polar-orbiting satellites. Both at 1500 UTC on 15
January 1995. The brighter grey-scales correspond to higher
clouds.
area created strong pressure gradients and therefore
strong northerly winds over the central Mediterranean.
On 15 January the axis of the tightest isobars was lying
north and east of the Mediterranean cyclone (Figure
1(c)) and the system experienced only a weak steering
flow. However, after 1800 UTC on that day (Figure
1(d)) there was a strong north-easterly flow in the
vicinity of the cyclone, forcing it to move south-westwards.
2.4. Landfall and decay
On 16 January the cyclone first moved south-west
towards Libya and then, after 1800 UTC when it was
close to the Gulf of Sidra (in northern Libya), turned
more southerly towards the middle of the gulf (Figure
266
Figure 5. The track of the Mediterranean cyclone between
0300 UTC on 15 January and 0600 UTC on 18 January 1995
derived from the satellite images.
5). At the end of the day, the cyclone was near the
entrance of the gulf and its translation speed was
reduced. That day showers and frequent thunderstorms were reported from northern Libya. At 1200
UTC on 16 January, a ship (9VYT) provided an observation from about 50 km north-northeast of the
cyclone centre of winds blowing approximately eastsoutheast at 50 knots (25.75 m s–1) (Blier & Ma, 1997).
At 0000 UTC on 17 January, the central pressure of the
cyclone had increased only slightly (by 2 hPa) from its
value 12 hours previously to 1012 hPa (Table 1). This
pressure was almost the same as 24 hours previously.
Mediterranean cyclone
Later, the vortex continued moving slowly southwards
(Figure 5) and at 0600 UTC the edge of its cloud pattern was over land. Finally, the ‘eye’ made landfall at
about 1800 UTC on 17 January (Figure 5). By that
time, the central pressure was 1022 hPa, significantly
higher than it had been at midnight when the vortex
was over the sea. Six hours later, at 0000 UTC on 18
January, a land station on the northern coast of Libya
(inside the Gulf of Sidra) recorded winds of 30 knots.
The satellite images of 16 January reveal that very
strong convection was still present, at least at the
beginning of the day. In the IR image of 0300 UTC
(Figure 6), three cumulonimbi can be seen close to the
‘eye’, with cloud-top temperatures below –50 °C (not
shown). Later, the convection was still strong but the
cloud tops of the highest cumulonimbus were slightly
warmer (–40 °C to –50 °C), corresponding to lower
clouds than earlier. That day, the vortex appeared to be
still tight but low clouds were apparent extending
many miles to the north and north-west of the ‘eye’
and forming a solid layer (not shown). In the late
evening of 16 January, the diameter of the high cloud
region had increased and the vortex did not appear to
be as tight as before. The next morning, when the
cyclone was positioned well into the Gulf of Sidra, only
a few high clouds were visible. The cyclone was mainly
composed of low clouds, indicating reduced convection. Finally, some hours after landfall the hurricanelike cloud pattern began to disappear.
Between 0600 and 1200 UTC on 17 January, when the
cyclone reached the land, the pressure dropped (Table
1) as usually happens with hurricanes when they make
landfall (Tuleya & Kurihara, 1978). The increased surface friction during landfall results in a decrease of the
horizontal wind speed. This leads to an increase of the
cross-isobar angle towards the low pressure. This
enhanced inflow increases the mean mass convergence,
increasing convection and resulting in a deeper low,
perhaps accounting for the 30-knot winds observed
earlier. However, without evaporation from the sea
surface, the convection quickly dries out the boundary
layer and the increased upward motion allows adiabatic
cooling to dominate diabatic warming (Wakimoto &
Black, 1994). Therefore, after the short intensification
and deepening of the cyclone by 2 hPa during landfall,
the central pressure began to increase significantly
(Table 1). At 0600 UTC on 18 January, the cyclone had
already begun to dissolve and by 1530 UTC no vortex
could be identified in the Meteosat images.
In conclusion, the Mediterranean cyclone formed at the
centre of a synoptic scale depression. But as the larger
low drifted eastwards, the small-scale vortex remained
over water, eventually drifting southwards following
the environmental flow, until it reached land and
decayed. As with the examples cited in the introduction, the system showed a strong resemblance to a hurricane on satellite imagery.
Figure 6. Satellite infra-red image from the NOAA polar-orbiting satellites at 0300 UTC on 16 January 1995. The brighter
grey-scales correspond to higher clouds.
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3. Overview of numerical simulation
The discussion so far has raised a number of questions.
In Section 1.2, the theories for the development of similar cyclones were described. A major question, then, is
how this particular cyclone formed and what factors
influenced its evolution. A further question concerns
whether the cyclone had kinematic and thermodynamic structure akin to tropical cyclones. However,
before examining the mesoscale structure of the simulated cyclone, it is necessary to verify as much as possible the accuracy of the model forecast. Since surface or
upper air observations are not available on the scale of
the system, the verification of the model forecast will
be based on the comparison of analysed surface and
mid-tropospheric charts and satellite images with the
corresponding model results. A more detailed comparison of simulation and data is given by Pytharoulis
(1995).
3.1. The model
The model used in this study is the UK Met. Office
(UKMO) Unified Model. A detailed description of the
characteristics and configurations of the operational
model is to be found in Cullen (1993), while a shorter
description is given in the Appendix. In this study the
global (1.25° × 0.83° horizontal resolution), limited-area
(LAM) (0.4425° × 0.4425°) and mesoscale (0.15° × 0.15°)
versions of the model were used, but the results were
taken from the LAM and mesoscale forecasts, using as
initial conditions the operational LAM analysis of 15
January 1995 at 0000 UTC (hereafter, this time will be
referred to as T+0). At that time there was no indication
of the presence of the small-scale Mediterranean
cyclone. The global and LAM models use 19 levels in
the vertical, while the mesoscale model has 31 levels. All
have increasing resolution towards the surface. The sea
surface temperature in all models was taken from a
global (1.25° × 0.83°) resolution analysis. Also, a full set
of parametrisations is included in the model.
3.2. Surface pressure and precipitation pattern
In the previous section, the track and the central pressure of the Mediterranean cyclone were described.
Here, the track of the system, according to the
mesoscale model forecast, will be analysed and its central pressure and location at different times compared
with the actual ones. In this section, references to the
Mediterranean cyclone are to the system as it appeared
in the model results. When the actual low is referred to,
this will be stated explicitly.
In the mesoscale model mean sea-level pressure forecast charts, which are extracted for every six hours, the
vortex first appeared at 1200 UTC on 15 January
(Figure 7(a)). Six hours previously at T+6 there was no
268
apparent vortex but the pattern of the inner closed isobar (not shown) indicated that the formation of a lowpressure centre over the sea between Greece and Sicily
was probable.
From its initial appearance until six hours later the system did not move significantly and was located over the
sea west of Greece. However, at 1800 UTC on 15
January (T+18) the system began to move south-westwards. At 0600 UTC on 16 January (T+30, Figure 7(c)),
it was located near the entrance of the Gulf of Sidra,
and afterwards headed south towards the eastern side
of the Gulf. It finally made landfall between 1800 UTC
on 16 January (T+42, Figure 7(d)) and 0000 UTC on 17
January (T+48), after which it disappeared.
The central pressure of the cyclone increased throughout its lifetime (Table 1). At T+12, when it initially
appeared in the mesoscale model charts, its central
pressure was approximately 1004 hPa. Twelve hours
later at T+24 it had reached 1011 hPa, having increased
by 5 hPa over the previous six hours. Later, it continued to increase and at T+42, just before landfall its
pressure had risen to 1019 hPa. Over the next six hours,
as the vortex made landfall, its central pressure
increased by only 1 hPa. This reduction in the rate of
the pressure rise may correspond to the transient intensification of the observed system (during landfall)
described in Section 2.4.
From a comparison of the location of the simulated
vortex (Figure 7(a), (b)) with the location of the actual
system it can be concluded that the model did very well
for the first 24 hours. At 0000 UTC on 16 January
(T+24) the separation between the forecast and the
actual position of the centre of the vortex was less than
1 degree of latitude (about 111 km) (Figure 7(b)), and
the predicted location of the cyclone was ahead of the
actual one. Afterwards, the model moved the cyclone
faster than it actually moved, although the error in
position never became greater than 2 or 3 degrees of latitude (Figure 7(d)). A consequence of this error is that
the model results are not useful from T+48 onwards
since the cyclone had already made landfall (according
to the model) whereas the actual system was still
located over the sea and thus in a completely different
environment.
The forecast of the central pressure exhibited the same
pattern as the separation. During the first hours (T+12
and T+18) there was no discrepancy between the actual
and the predicted central pressure of the system (Table
1), and in the next 12 hours the discrepancy that
appeared was no larger than 2 hPa. However, at later
times this difference became significant since the central pressure in the modelled cyclone increased while
the actual one remained almost the same.
The pattern of the instantaneous rate of total precipitation (i.e. convective and dynamic precipitation),
Mediterranean cyclone
Figure 7. The mean sea-level pressure forecast and the instantaneous rate of total precipitation expressed in mm/hour, at (a)
T+12, (b) T+24, (c) T+30 and (d) T+42 (from 0000 UTC on 15 January 1995). The isobars are drawn every 2 hPa. The scale
below the panels corresponds to the rate of precipitation and is expressed in mm/hr. The actual location of the Mediterranean
cyclone is indicated by the small cycle.
expressed in mm per hour, is also depicted in Figure 7.
Here, precipitation includes both rain and snow. The
model almost always predicted a precipitation-free area
at the centre of the cyclone while the heaviest precipitation was predicted to fall in the area of the cyclone
corresponding to the eyewall. In the area close to the
cyclone the greatest portion of the precipitation was
produced by the convection scheme. However, the
heaviest precipitation rates were sometimes due to the
dynamic or ‘grid-scale’ precipitation scheme indicating
that there is grid-scale saturation and/or that resolved
scale motions are dominating the production of precip-
itation in those areas. Unfortunately, there are no measurements of precipitation in the region of the vortex
since it evolved over the sea. However, the precipitation pattern, as illustrated in Figure 7, seems to be very
reasonable. With one exception, rainfall was always
reported from ships in the vicinity of the cyclone. No
rainfall was reported at 1200 UTC on 15 January by a
ship located south of the vortex. From Figure 7 it is
apparent that the model predicted no rainfall south of
the cyclone at T+12, in agreement with this observation.
269
I Pytharoulis, G C Craig and S P Ballard
3.3. Conditional instability
In Section 2 we mentioned that very deep convection
was present on 15 January as well as in the first few
hours of 16 January. In the following hours the IR
images revealed that the convection was still strong but
did not reach the same heights as previously. Here we
will show that the observed cloud-top temperatures are
consistent with the thermodynamic structure of the
atmosphere predicted by the numerical model. This is
achieved by constructing tephigrams based on a vertical cross-section of the temperature field taken in the
area of the vortex (Figure 8). A location corresponding
approximately to the area where deep convection
would be observed (i.e. in the eyewall) was chosen and
the temperature at each pressure level was established
(starting at 1000 hPa and continuing for every 50 hPa to
the pressure level of 200 hPa). Temperature profiles are
shown in Figure 9 for 16 January at 0000 UTC, and for
the same day at 1200 UTC when the convection was
observed to have diminished.
A prediction for maximum cloud-top height (and minimum cloud-top temperature) can be obtained by considering the pseudo-adiabatic ascent of an air parcel initially saturated at the sea surface temperature. The distribution of the sea surface temperature, which did not
change significantly during the lifetime of the
Mediterranean cyclone, is depicted in Figure 8. At the
earlier times the sea surface temperature (SST) in the
location of the vortex was determined to be about
16.4 °C whereas later it was approximately 17.5 °C.
Figure 9. The two ‘ascents’ drawn on a tephigram. Full line:
first ‘ascent’ (T+24 from 0000 UTC on 15 January). Dashed
line: second ‘ascent’ (T+36 from 0000 UTC on 15 January).
At 0000 UTC on 16 January the lapse rate was conditionally unstable up to the level of the tropopause at
350 hPa (Figure 9). Assuming a rising parcel of air to
follow the saturated adiabat which is equal to the SST
at this location, we find that the cloud top should be at
about 280 hPa corresponding to a cloud-top temperature of –49 °C. The second ‘ascent’ (1200 UTC on 16
January, Figure 9) revealed the existence of conditionally unstable air at low levels as well as in the mid-troposphere with the temperatures at each level close to
those of the previous ‘ascent’. However, the conditionally unstable region was bounded by a temperature
inversion at approximately 450 hPa. Making the same
assumption as before for a rising parcel of air, we find
that the cloud top should reach the pressure levels of
300–310 hPa, with a temperature of about –41 °C.
These results agree well with the IR images which
revealed that the cloud-top temperatures of the highest
cumulonimbus were below –50 °C, for the first case
and between –40 and –50 °C for the second case.
4. Mesoscale structure of the Mediterranean
cyclone
Figure 8. The area of the first, 1 (at 0000 UTC on 16 January
1995) and second, 2 (at 1200 UTC on 16 January 1995) crosssections, the predicted position of the vortex (small cycle), the
approximate locations of the ‘ascents’ (star ‘*’) and the distribution of the sea-surface temperature (in °C). Contour interval: 0.5 °C. The locations of the cross-sections in Figure 13 are
depicted here by the lines (a) and (b).
270
The satellite images revealed that the Mediterranean
cyclone had possessed features typical of tropical
cyclones and polar lows. These included the cloud-free
region in the centre corresponding to the ‘eye’, the
non-symmetrically distributed deep convective clouds
in the eyewall and the spiral bands of clouds around the
‘eye’. However, a detailed analysis of the structure of
Mediterranean cyclone
the simulated cyclone will help decide whether the
Mediterranean cyclone was really hurricane-like.
4.1. Surface fluxes
Two very important phenomena that occur in the
region of the tropical cyclones and polar lows are the
convection and the strong wind-induced surface fluxes
of heat and moisture (Craig, 1995; Emanuel, 1986;
Emanuel & Rotunno, 1989; Rasmussen, 1979;
Rasmussen & Zick, 1987; Rotunno & Emanuel, 1987;
and others). The hurricane-like distribution of convective precipitation has been noted previously. Strong
surface fluxes of heat and moisture occurred close to
the area of the vortex when it formed, and during its
lifetime. The surface fluxes of sensible heat are presented in Figure 10 (the pattern of the latent heat is similar; not shown). The strongest fluxes were associated
with the strongest winds on the right side of the direction of storm movement. In particular, just before landfall, at T+30 (Figure 10(b)) and T+36 the maximum
sensible heat fluxes were observed south-west of the
vortex (the vortex was moving southwards at these
times). This is in agreement with the observations of
hurricanes which show that the most strongly developed segment of the eyewall cloud and the strongest
winds are usually found in the quadrant of the hurricane centred about 45° to the right of the direction of
the storm movement in the Northern Hemisphere
(Wallace & Hobbs, 1977). Generally, in the Northern
Hemisphere, winds measured on the right side of a
tropical cyclone tend to be stronger than those on the
left because of the storm’s translation speed (Burpee,
1986).
4.2. Kinematic structure
The vertical distribution of the field of the meridional
component of the wind velocity (corresponding to the
tangential velocity) for a cross-section in the area of the
vortex is illustrated in Figure 11(a). The location of the
cross-sections that appear in Figures 11 and 12 are
marked in Figure 8, for the time T+36. The most apparent features of the azimuthal wind velocity are the
decrease of the wind speed with height and the existence of much stronger winds in the western side of the
vortex. The decrease of the wind speed with height
indicates that the vortex had had a warm core. The
change in the direction of the meridional wind from
northerly to southerly (from west to east) at low levels
in the area of the core, and thus a cyclonic circulation,
is apparent in Figure 11(a). However, at upper levels
(particularly near the 400 hPa level) anticyclonic circulation is apparent only through a weak horizontal shear
of the meridional wind velocity. Probably the strong
general cyclonic flow at upper levels did not allow the
upper circulation of the Mediterranean cyclone to
become clearly anticyclonic. Inspection of the diver-
Figure 10. Surface fluxes of sensible heat for (a) T+12 and (b)
T+30 (from 0000 UTC on 15 January 1995). Contour interval: 20 W m–2. Values larger than 200 W m–2 are not plotted.
The scale below the panels is in W m–2. The predicted location
of the cyclone is indicated by the letter H.
gence field revealed that the upper-air flow was divergent only close to the vortex and the values of the
divergence were usually near 5 × 10–5 sec–1. Tropical
cyclones often reveal strong anticyclonic flow at upper
levels; however, these are much more intense systems
than the Mediterranean cyclone. Figure 11(a) also
shows an increase of the absolute value of the meridional wind from the eastern side of the vortex to the
western by about 9 m s–1. This is consistent with the
stronger surface fluxes to the west of the cyclone, as
discussed in the previous paragraph.
271
I Pytharoulis, G C Craig and S P Ballard
Figure 11. Vertical cross-sections taken from the area marked in Figure 8 at T+36 (from 0000 UTC on 15 January 1995) for the
fields: (a) meridional component of wind velocity (contours in m s–1), (b) zonal component of wind velocity (contours in m s–1),
(c) vertical velocity, ω (=dp/dt) (contours in Pa/sec), and (d) wet bulb potential temperature, θw (contours in K). Values higher
than –0.4, 0.4, –0.03, 281.9 are shaded in panels (a), (b), (c) and (d), respectively. Contour interval: 1.3, 0.7, 0.08, 0.25, respectively. Left edge: west; right edge: east. The approximate area of the core is indicated below each panel.
The features of the tangential wind velocity shown in
Figure 11(a), for the time T+36, appeared throughout
the life of the cyclone. The wind speed decreased with
height, revealing that the system had always had a
warm core and the upper flow never became clearly
anticyclonic. The difference in wind speed between the
west and east sides of the system was generally about 10
m s–1, but early in its lifetime it was even larger.
The vertical distribution of the zonal component of the
wind velocity (corresponding to the radial velocity) is
depicted in Figure 11(b). The features that appear for
this component are also consistent with the observa-
272
tions of tropical cyclones. At low levels inflow towards
the core of the system is observed. Convergence of the
radial component of the wind would be expected under
the eyewall (since ascent is expected there) with divergence under the area of the core since the air that
descends (in the core) will be spread out horizontally at
low levels. Indeed, these features are generally encountered in Figure 11(b), although the convergence of the
air under the eastern side of the vortex is not clear. In
the upper levels the outflow is apparent. The same features were also observed at the other times, but when
the convection was more intense, the convergence of
the air under the eyewall was much more pronounced.
Mediterranean cyclone
The pattern of vertical motions (Figure 11(c)) is consistent with the preceding discussion. Ascent due to
strong convection was observed in the region around
the core, corresponding to the eyewall, and descent in
the area of the core. Moreover, the strongest convection and as a result the greatest vertical velocities were
apparent on the western side of the vortex, where the
strongest sea-surface fluxes were encountered. Though
the same characteristics of the vertical motion can be
seen at all times, the height that the vertical motions
reached is not always the same. At the early times,
when the convection was very strong, the upward
motion in the eyewall reached very high levels.
However, at later times this height was lower than
before. At T+42 ascent was confined to the mid and
lower troposphere. The descent of dry air in the core is
illustrated in Figure 12, with the help of the field of the
relative humidity. Low values of the relative humidity
can be seen in the core whereas much larger values are
found in the eyewall.
ally observed in tropical cyclones (e.g. Jorgensen,
1984).
Looking at north–south as well as west–east cross-sections of θw, we observed that early in its lifetime the
system was slightly tilted. At T+12 it was tilted northwestwards with height (Figure 13; see Figure 8 for the
location of the cross-sections) and at T+18 (not shown)
it had had a slight southwards tilt. A possible explanation is that the system was not very strong at the early
times and the ambient flow influenced its vertical struc-
4.3. Thermodynamic structure
A very useful field that can be used in order to examine
the region of the core and the potential for moist convection, is the wet bulb potential temperature, θw. The
pattern of the vertical distribution of θw is depicted in
Figure 11(d) for T+36 and is consistent with the observations that have been made in tropical cyclones for
equivalent potential temperature, θe. The almost constant value of θw with height (near the core) indicates a
lapse rate close to moist adiabatic. From the θw and ω
(=dp/dt) fields, it was not possible to determine if there
was an outward tilt of the eyewall with height as is usu-
Figure 12. Vertical cross-section of the relative humidity (contours in %) at T+36 (from 0000 UTC on 15 January 1995).
Values lower than 49% are shaded. Contour interval: 5%.
The area of the cross-section can be seen in Figure 8. The
approximate area of the core is indicated below the figure.
Figure 13. Vertical cross-sections of the wet bulb potential
temperature (contours in K) at T+12 (from 0000 UTC on 15
January 1995) for (a) orientation west–east and (b) orientation
south–north. (The locations of the cross-sections are indicated
by the lines (a) and (b) in Figure 8.) Values higher than 281.9
K in panel (a) and 282.1 K in panel (b) are shaded. Contour
interval: 0.35, 0.45, respectively. The approximate area of the
core is indicated below each panel.
273
I Pytharoulis, G C Craig and S P Ballard
ture. Afterwards, when the system was much better
organised, its core was almost vertical.
4.4. Summary
After the presentation of the characteristics of the
Mediterranean cyclone, it can be concluded that the
system had many of the characteristics of the tropical
cyclones. The warm core, the strong convection and
the strong surface fluxes are the most important features of the environment of the hurricanes and they
were encountered in the Mediterranean cyclone.
Moreover, other significant features of the cyclone,
such as the eyewall (with ascent into it), the fact that the
strongest winds were found near the surface, the inflow
at low levels and outflow at higher ones and finally the
descent of relatively dry air in the core, support the initial assertion that the system was hurricane-like.
However, some differences such as the weak upperlevel anticyclone, the small values of θw and the tilt of
the ‘eye’ (the latter observed at the earlier stages of its
lifetime) indicate that the Mediterranean cyclone was a
weaker system.
(Figure 14(a), (b)) shows very small values of θ for that
part of the tropopause which was over the central and
eastern Mediterranean. This is an indication of the
trough seen in Figures 2(a), (b) (during the previous
days) since the small values of θ correspond to lower
levels of the troposphere. However, no small-scale
anomaly of the size of the studied cyclone is observed
on the iso-PV maps to be embedded in this trough.
This is true not only for the first hours of its lifetime
but also for later stages when the cyclone was more
intense. While the large-scale trough determines the
environment where the small-scale cyclone forms, it
would not lead directly to cyclogenesis on that scale. In
the case of subsynoptic cyclones, much smaller PV
anomalies are required.
Another way to investigate the upper-air flow in order
to see if there is any anomaly that may interact with a
low-level baroclinic zone is by looking for the existence of an upper-air jet. The existence of an upper-air
jet in the region of a system may be decisive for its track
5. Mechanisms of intensification
Section 1.2 presented the theories that have been formulated mainly to explain the development of the tropical cyclones and the polar lows but which might also
be applied in hurricane-like systems such as the
Mediterranean cyclone. Surface heat and moisture
fluxes are believed to be responsible for the growth of
tropical cyclones and to contribute strongly to the
intensification of polar lows. However, in the extratropics where baroclinicity is important, the baroclinic
theory may be also valid. Therefore, in the following
investigation of the factors that influenced the development of the Mediterranean cyclone, both the air–sea
interaction theory and the baroclinic theory must be
examined. Moreover, the importance of the location of
the subtropical jet will be discussed.
The baroclinic theory states that the disturbances
which arise due to baroclinic instability evolve through
a cooperative interaction of an upper-level vorticity or
potential vorticity anomaly with a low-level baroclinic
zone. Hence, if the Mediterranean cyclone was a baroclinic disturbance two features should be found in its
vicinity. The first is an upper-air anomaly over the
location that the cyclone formed and the other is strong
baroclinicity at low-levels.
A complete and easily assimilated view of the upper
tropospheric situation is provided by maps of potential
temperature (θ) on the PV=2 PVU surface (1 PVU –
potential vorticity unit – is equal to 10–6 m2s–1kg–1K)
which follows the tropopause (Hoskins & Berrisford,
1988). The θ distribution on the PV=2 surface map
274
Figure 14. θ-distribution on the PV=2 surface at (a) T+6 and
(b) T+12 (from 0000 UTC on 15 January 1995). Contour
interval: 10 °C. The range below the panels corresponds to the
potential temperature (in °C). The predicted location of the
cyclone is indicated by the letter H.
Mediterranean cyclone
as well as for its intensity, mainly because of the vertical motions that are associated with the jet. Ascent
occurs on the right of the jet entrance and on the left of
the jet exit, while descent occurs on the other sides of
the jet entrance and exit. Since such rising motions
must imply vorticity stretching in the column below,
cyclonic vorticity will tend to increase below the right
flank of the jet entrance and the left flank of the jet exit
(Uccelini & Johnson, 1977).
When the Mediterranean cyclone formed, the subtropical jet was lying along the northern coastline of Africa,
over northern Egypt and north-eastern Libya (Figure
15(a)). Afterwards, it moved slowly eastwards and its
maximum wind speeds were reduced (Figure 15(b)).
Since ascent is observed to the right of its entrance and
to the left of its exit, and descent on the other sides, the
presence of the jet-stream does not seem to have promoted the evolution of the Mediterranean cyclone.
However, it may have played a role in inducing the
eastward movement of the wider low-pressure area
where the Mediterranean cyclone was embedded, since
the low-pressure area was located below the left flank
of the jet exit.
In the first few hours of 15 January, although the parent low centre was decaying, it featured an occluded
surface front that extended westwards, ending near the
region where the vortex formed (Figure 1(b), (c)).
Indeed, the model forecast for 0300 UTC on 15
January (T+3, Figure 16; obtained from the LAM)
shows strong θ gradients at low levels over the northern Aegean Sea and Greece, extending north-east
towards the Black Sea. The baroclinic zone extends
with diminished intensity to the west of Greece.
However, the Mediterranean cyclone formed at the
southern edge of this zone and not inside it.
It can be deduced that the Mediterranean cyclone was
not primarily a baroclinic disturbance. The low-level
baroclinicity does not appear to have been important
since the cyclone formed at the edge of the weaker
zone. Moreover, there was not any upper level feature
capable of interacting cooperatively with the low-level
baroclinicity. As a result, another mechanism was
probably decisive in the development of the cyclone.
The existence of strong wind-induced surface fluxes of
heat and moisture is the most decisive part in the development of a cyclone according to the air–sea interaction theory. Even though we saw in Section 4 that the
surface fluxes were important for this cyclone, stronger
evidence is required in order to decide which one was
the driving mechanism for the storm. An experiment
was therefore carried out in which the surface latent
and sensible heat and moisture fluxes were turned off
and the evolution of the pressure and precipitation patterns with time was observed. If the cyclone developed
then the surface fluxes of heat and moisture would not
be decisive for the system.
The results of this experiment are depicted in Figure 17.
Although a low centre formed again (but at T+6) and
moved southwards as did the actual system, it was less
Figure 15. Isotachs of zonal wind (in m s–1) at the pressure
level of 250 hPa for (a) T+6 and (b) T+24 (from 0000 UTC on
15 January 1995). Contour interval: 10 m s–1.
Figure 16. θ-distribution (in °C) on the ‘η’ level eta = 0.8698
at T+3 (from 0000 UTC on 15 January 1995). Contour interval: 1 °C. The location of the actual cyclone is indicated by the
hurricane symbol.
275
I Pytharoulis, G C Craig and S P Ballard
intense than in the control experiment, and by T+36
(Figure 17(d)) no vortex could be identified. The precipitation was much weaker and at T+36 (1200 UTC on
16 January, Figure 17(d)) there was no precipitation at
all.
In a second experiment, the model was run with only
the latent heating turned off (at all levels). The result
(not shown) was that there was less development than
in the control run but more than in the no surface heat
and moisture fluxes run. This result points out the
importance of the sensible heat fluxes in the development of the Mediterranean cyclone. This is in contrast
to tropical cyclones, where latent heating is dominant,
but consistent with polar lows where latent and sensible heating are thought to be comparable (Rasmussen,
1979).
From the previous discussion and mainly from the ‘no
fluxes’ experiment, it is obvious that the surface heat
and moisture fluxes were extremely important for the
development of the Mediterranean cyclone. Although
the system formed initially when they were turned off,
the lack of fluxes did not allow it to intensify, since the
necessary energy did not exist. The expected structure
of a baroclinic instability was not found even though
Figure 17. The mean sea-level pressure forecast and the instantaneous rate of total precipitation expressed in mm/hour at (a)
T+12, (b) T+24, (c) T+30 and (d) T+36 (from 0000 UTC on 15 January 1995) from the model run with no surface heat and
moisture fluxes. The scale below the panels corresponds to the rate of precipitation and is expressed in mm/hr.
276
Mediterranean cyclone
some baroclinicity was present at low levels. An interesting point from both experiments and especially from
the ‘no fluxes’ run is that a vortex still formed initially.
The strong pre-existing convection or the orography
(Ziakopoulos & Marinaki, 1996) may be responsible
for the initial development of the vortex. The strong
north-easterly flow that existed over northern Greece
prior to formation may have interacted with the high
orography of western Greece in order to produce
small-scale low-level vorticity anomalies on the lee side
of the mountains (over the sea).
6. Conclusions
The development of a hurricane-like cyclone over the
Mediterranean Sea has been studied in this work. The
major tool was the Unified Model of the UK Met.
Office and particularly its limited-area and mesoscale
versions.
The Mediterranean cyclone formed on 15 January
1995, over the sea between Greece and Sicily but closer
to Greece. Before the formation of the cyclone, a synoptic scale low was situated over the area and strong
convection was observed. Afterwards, the larger low
continued to move eastwards and decayed, but the vortex remained in a wider low-pressure area.
Strong convective activity was associated with the
Mediterranean cyclone during almost all of its lifetime.
Its track was mainly influenced by the steering flow
due to the parent low to the east and an anticyclone
over central Europe. The system evolved for about
three days and finally it made landfall in northern
Libya at about 1800 UTC on 17 January 1995 and
decayed.
The satellite images and the model results gave strong
evidence to support the idea that the Mediterranean
cyclone was similar to tropical cyclones and the more
convective polar lows. The most important features
were the strong surface fluxes and, as a consequence,
the strong convective activity, the warm core, the weak
anticyclonic upper-level flow, the fact that the
strongest cyclonic winds were blowing at the surface
(consistent with the warm core structure of the
cyclone) and the cloud pattern which was composed of
spiral bands of clouds with an ‘eye’ in the centre.
Finally, the theories which are valid for the development of tropical cyclones and polar lows were examined in order to find which factors influenced its evolution. Baroclinic processes were not found to be responsible for the development of the system even though it
formed at the edge of a weak low-level baroclinic zone.
The surface fluxes probably played the most important
role in its development. The importance of the surface
heat and moisture fluxes was verified by an experiment
in which they were turned off and the system did not
develop as much as in the control run. In a second
experiment in which only the latent heating was
switched off (at all levels), there was less development
than in the control run but more than in the previous
experiment. This result showed that the Mediterranean
cyclone was similar to polar lows in which the sensible
heat fluxes are comparable to the latent heat fluxes, in
contrast to tropical cyclones, where latent heat fluxes
dominate. The examination of the role of the nearby
orography in the initial development of the cyclone is
proposed for future work.
The formation of hurricane-like systems in the
Mediterranean Sea is not an unprecedented event. At
least nine similar cyclones have been documented in the
last few decades. The strong winds, heavy rain, very
bad visibility and generally bad weather associated with
these cyclones, as well as the fact that they form and
evolve mainly over the sea, make them a great danger
for ships travelling in their vicinity and for the contiguous coastal regions. These reasons, together with the
fact that a large number of ships ply the Mediterranean
Sea, support the idea that these systems should be studied in detail when they form. In particular, the meteorological offices of Mediterranean countries must use
their fine mesh models to improve forecasts of the formation and the track of these cyclones. Indeed, such
small-scale and rapidly evolving systems provide a very
good opportunity for the meteorological offices of any
country in the world to check the performance of their
fine mesh models in difficult situations.
Appendix. Description of the model
The UK Met. Office Unified Model is a hydrostatic,
grid-point model using spherical polar coordinates,
which can be run in global or limited-area configurations on a regular latitude–longitude grid at any desired
horizontal or vertical resolution. Hybrid sigma/pressure co-ordinates are used in the vertical (Simmons &
Burridge, 1981). A conservative split-explicit integration scheme is used (Cullen & Davies, 1991) and cloud
water and ice are included as prognostic variables, making allowance for fractional cloud cover, after Smith
(1990). The co-ordinate pole can be rotated so that the
equator runs through the area of interest, thus allowing
more uniform resolution in limited-area configurations. The coordinates used for the pole of the LAM
and the mesoscale models (of this study) are 30°N,
160°E and 52.5°N, 202.5°E respectively.
The LAM forecast was a rerun of the operational forecast to provide extra diagnostics and to provide boundary condition data for a high resolution mesoscale forecast. The LAM uses a five-minute timestep, has resolution 0.4425° × 0.4425° (i.e. about 50 km × 50 km), has
its own data assimilation sequence and uses lateral
boundary conditions from a previous global forecast.
The mesoscale forecast was run for an area over the
277
I Pytharoulis, G C Craig and S P Ballard
central and eastern Mediterranean with 140 × 140 gridpoints and resolution of 0.15° × 0.15° using a timestep
of 1.5 minutes. The initial conditions for the mesoscale
forecast were taken from the LAM analysis interpolated onto the mesoscale grid. Both models were run to
produce 48-hour forecasts.
A full set of parametrisations is included in the model.
The boundary-layer scheme is formulated in terms of
conserved variables and so can represent both dry and
cloudy boundary layers. It also includes a representation of non-local mixing (Smith 1994). Mixing coefficients are stability-dependent, using a moist
Richardson number. Large-scale precipitation is calculated in terms of the liquid water or ice content of the
cloud, and cooling of the atmosphere due to evaporation of precipitation is included (Smith, 1990; Gregory,
1995). The annual cycle of radiation is imposed on the
model which employs an interactive radiation scheme.
The cloud optical properties depend on the predicted
cloud-water path and phase at both long wave (Senior
& Mitchell, 1993) and short wave (Slingo, 1989). A
mass-flux convection scheme with stability-dependent
closure (Gregory & Rowntree, 1990) is included which
incorporates a representation of deep convective down
draughts (Gregory & Allen, 1991). Surface hydrology
and a soil-temperature model are also included
(Dolman & Gregory, 1992). The form drag of unresolved orography is represented via an effective roughness length for momentum which is calculated from the
standard deviation and average slope of the unresolved
orography in all versions of the model and a parametrisation of gravity wave drag is included in the LAM and
global versions (Milton & Wilson, 1996). The LAM
forecast used the same orography and surface characteristics as used operationally. The mesoscale forecast
used values specially derived for its increased horizontal resolution. Soil and vegetation characteristics were
derived from the 1° Wilson and Henderson-Sellers
data, orography and its standard deviation from a 5′
global dataset, and surface roughness parameters from
a 100 m dataset where available and 5′ data elsewhere.
Acknowledgements
We would like to thank G. Hutchinson for his help in
extracting the results using the Hewlett-Packard workstation at the Joint Centre for Mesoscale Meteorology
(JCMM) at the University of Reading, Nigel Roberts
for his useful comments about the water-vapour satellite images, D. Ziakopoulos (head of forecasting at the
Hellenic National Meteorological Centre) for providing us with the three-hourly surface subjective
mesoscale analyses of the HNMC, O. Reale for
providing some useful observations, and E. Rasmussen,
R. Riddaway (editor of Meteorological Applications)
and an anomymous reviewer for their helpful comments. The UK Met. Office provided us with the
Unified Model and the Daily Weather Summary
278
charts. We are also grateful to the Graphics
Department of the Met. Office for their assistance in
the preparation of the figures. The main part of the
work was done in the JCMM using the HewlettPackard workstation system. An animation of
IR imagery of the studied Mediterranean cyclone can
be accessed at the following Internet site:
http://www.met.rdg.ac.uk/Data/Global/special.html
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