Genesis and maintenance of "Mediterranean hurricanes"

Abstract

Cyclonic storms that closely resemble tropical cyclones in satellite images occasionally form over the Mediterranean Sea. Synoptic and mesoscale analyses of such storms show small, warm-core structure and surface winds sometimes exceeding 25ms^-1 over small areas. These analyses, together with numerical simulations, reveal that in their mature stages, such storms intensify and are maintained by a feedback between surface enthalpy fluxes and wind, and as such are isomorphic with tropical cyclones. In this paper, I demonstrate that a cold, upper low over the Mediterranean can produce strong cyclogenesis in an axisymmetric model, thereby showing that baroclinic instability is not necessary during the mature stages of Mediterranean hurricanes.

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Advances in Geosciences, 2, 217–220, 2005
SRef-ID: 1680-7359/adgeo/2005-2-217
European Geosciences Union
© 2005 Author(s). This work is licensed
under a Creative Commons License.
Advances in
Geosciences
Genesis and maintenance of “Mediterranean hurricanes”
K. Emanuel
Program in Atmospheres, Oceans, and Climate, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
Received: 24 October 2004 – Revised: 21 May 2005 – Accepted: 6 June 2005 – Published: 27 June 2005
Abstract. Cyclonic storms that closely resemble tropi-
cal cyclones in satellite images occasionally form over the
Mediterranean Sea. Synoptic and mesoscale analyses of such
storms show small, warm-core structure and surface winds
sometimes exceeding 25ms
−1
over small areas. These anal-
yses, together with numerical simulations, reveal that in their
mature stages, such storms intensify and are maintained by a
feedback between surface enthalpy fluxes and wind, and as
such are isomorphic with tropical cyclones. In this paper, I
demonstrate that a cold, upper low over the Mediterranean
can produce strong cyclogenesis in an axisymmetric model,
thereby showing that baroclinic instability is not necessary
during the mature stages of Mediterranean hurricanes.
1 Introduction
The Mediterranean Sea is a favored location for the develop-
ment of cyclonic storms (Petterssen, 1956). Most ofthese are
of synoptic scale and baroclinic in origin, sometimes aided
by the peculiar nature of flow around the Alps and Pyre-
nees (Buzzi and Tibaldi, 1978). But once in awhile, intense
mesoscale vortices develop whose structure, as revealed by
satellite images, closely resembles tropical cyclones, having
a clear, circular eye surrounded by an eyewall and a roughly
axisymmetric cloud pattern (Mayengon, 1984). Experiments
with full-physics, three-dimensional models (Pytharoulis et
al., 2000) demonstrate that, at least in their mature phase,
such storms are maintained the same way as tropical cy-
clones, by inducing large enthalpy fluxes from the sea.
In this paper, I review one well-observed Mediterranean
hurricane, and model its development using a nonhydro-
static, axisymmetric, convection-resolving model, to show
that standard baroclinic processes need not be called on to
explain the final development, though they are probably nec-
Correspondence to: K. Emanuel
(emanuel@texmex.mit.edu)
essary in the early stages. I conclude with a brief theoretical
treatment of the problem.
2 An example
A well-studied example of a Mediterranean hurricane oc-
curred in mid January, 1995. As shown in Fig. 1, this storm
looks very much like a small hurricane in satellite images. It
developed directly underneath an unusually deep, cut-off low
at upper levels, and on the west side of a larger-scale surface
cyclone (Fig. 2). Mediterranean hurricanes usually, and per-
haps always, develop under deep upper tropospheric troughs,
in regions of small baroclinicity but large air-sea thermody-
namic disequilibrium owing to the unusually deep, cold air
associated with the trough. (This is also true of “polar lows”;
Rasmussen et al., 1992.) Soundings near the surface cyclone
reveal very large instability to ascent of surface air saturated
at sea surface temperature (Pytharoulis et al., 2000), a mea-
sure of potential intensity (Bister and Emanuel, 2002). At the
same time, the air is nearly saturated through a deep layer,
owing to its having been lifted (and thereby cooled) on the
approach of the upper low, as dictated by inversion of the
upper potential vorticity anomaly (Hoskins and Robertson,
1985). As we shall see in the next section, the atmosphere
over the relatively warm Mediterranean and under a deep,
cutoff cold low is an ideal embryo in which to gestate a hur-
ricane.
3 A numerical simulation
To demonstrate that upper cold lows over warm seas are con-
ducive to tropical cyclone development, I use a modified ver-
sion of the axisymmetric, nonhydrostatic, cloud-resolving
model of Rotunno and Emanuel (1987). The model has
been modified to ensure global energy conservation, includ-
ing dissipative heating. Any development in such a model
must occur owing to Wind-Induced Surface Heat Exchange

218 K. Emanuel: Genesis and maintenance of “Mediterranean hurricanes”
7
Figure 1: Mediterranean Hurricane on 15 January 1995
Fig. 1. Mediterranean Hurricane on 15 January 1995.
(WISHE), since baroclinic instability is absent in axisym-
metric geometry. I run the model with a radial grid size of
3.75km and vertical level separation of 300m. The model is
initialized with an ambient sounding characteristic of radia-
tive equilibrium over a sea surface of temperature 24
◦
C, but
in this integration the sea surface temperature is reduced to
22
◦
C. (The sounding as produced by running thesame model
into a state of convective neutrality, as described in Rotunno
and Emanuel, 1987.) This has the effect of greatly reducing
the potential intensity, to reflect the low climatological po-
tential intensity over the Mediterranean in January. Into this
state I insert an upper cold low with maximum wind ampli-
tude at 10km altitude, decaying linearly tozero at the surface
and also decaying upward into the stratosphere. The maxi-
mum wind of 20ms
−1
of this initial cold low is at a radius
of 300km; thermal wind balance then gives a minimum tem-
perature perturbation of −4
◦
C at about 9 km altitude. Near
the center of this initial cold upper low, there is considerable
potential intensity. The initial wind and pressure fields are
in gradient and hydrostatic balance. Small perturbations are
introduced into the surface fluxes to initiate convection and
development.
Figure 3 shows the evolution of the azimuthal wind com-
ponent with time. After two days, a shallow, low-level,
warm-core cyclone has developed and by the third day has
maximum winds of about 24 ms
−1
at a radius of around
20km. If allowed to persist for several more days, this
storm extends upward and develops to hurricane strength
(∼35ms
−1
), and in large measure destroys the upper cold
low in the process. This development proceeds somewhat
faster than most tropical cyclones do, owing to its small scale
and the relatively large value of the Coriolis parameter.
8
8
Fig. 2. 500hPa geopotential height (dm, top) and sea level pressure
(hPa, bottom), at 00:00GMT on 15 January 1995. Analyses from
NCAR/NCEP 2.5 degree re-analysis data.
There is increasing evidence that real tropical cyclones
also develop inside cold core cyclones, though these are
mesoscale features that arise in association with mesoscale
convective complexes (Bister and Emanuel, 1997). The cy-
clonic vorticity together with the downdraft-suppressing hu-
midified air is highly conducive to tropical cyclogenesis.
The simulations described here show that the moist, cold air
found underneath deep upper cold lows over warm seas, such
as the Mediterranean, are likewise nearly ideal incubators for
hurricane-like developments.
4 Theory of cyclone developmentunder upper cold lows
Consider first a horizontally homogeneous atmosphere over-
lying an ocean surface of constant temperature in a state of
radiative-moist convective equilibrium. I will approximate
the temperature lapse rate in the troposphere as moist adia-
batic, having a constant value of the saturation moist static
energy, h
∗
, defined
h
∗
≡ c
p
T + gz + L
v
q
∗
, (1)

K. Emanuel: Genesis and maintenance of “Mediterranean hurricanes” 219
9
Figure 2: 500 hPa geopotential height (dm, top) and sea level pressure (hPa, bottom), at 00
GMT on 15 January 1995. Analyses from NCAR/NCEP 2.5 degree re-analysis data.
Fig. 3. Azimuthal velocity (ms
−1
) in the axisymmetric, nonhy-
drostatic model, after 1 day (top), 2 days (middle), and three days
(bottom) of integration.
where c
p
is the heat capacity at constant pressure, T is the
absolute temperature, g is the acceleration of gravity, z is the
altitude, L
v
is the latent heat of vaporization, and q
∗
is the
saturation specific humidity. Then the potential maximum
11
Figure 4: Potential maximum wind speed (ms
-1
) over the Mediterranean Sea at 12 GMT on 5
October 2004, from the NCEP analysis on a 1 degree grid.
Fig. 4. Potential maximum wind speed (ms
−1
) over the Mediter-
ranean Sea at 12:00GMT on 5 October 2004, from the NCEP anal-
ysis on a 1 degree grid.
wind speed squared is given by (Bister and Emanuel, 1998) :
V
2
p
=
C
k
C
D
T
s
− T
o
T
o

h
∗
s
− h
∗

, (2)
where C
k
and C
D
are the surface exchange coefficients for
enthalpy and momentum, T
s
and T
o
are the sea surface and
entropy-weighted outflow temperatures, and h
∗
s
is the satura-
tion moist static energy of air at sea level at sea surface tem-
perature. Usually, over the Mediterranean, V
p
is too small
to support tropical cyclones, but it can be large in the fall
(Fig. 4). Moreover, the air over the Mediterranean is usually
far too dry to allow tropical cyclones to develop.
Now suppose a synoptic scale, upper cold low is advected
into this environment. As the low approaches, the air mass
is lifted, decreasing the temperature aloft and increasing con-
vective instability. I shall suppose that convection occurs and
holds the lapse rate at a moist adiabatic value. Hydrostatic
equilibrium gives

∂φ
∂p

0
= −α
0
= −

∂α
∂h
∗

0
h
∗0
= −

∂ lnT
∂p

h
∗
h
∗0
, (3)
where φ is the geopotential, α is the specific volume,
the primes represent departures from the background state
along surfaces of constant pressure, and I have used one of
Maxwell’s relations (Emanuel, 1986). I can now integrate
this through the depth of the troposphere, to relate the pertur-
bation of h
∗
at the center of the cold low to the perturbation
of φ associated with cold low at the tropopause:
h
∗0
=
φ
0
cl
− φ
0
s
ln

T
s
T
o

'
T
o

φ
0
cl
− φ
0
s

T
s
− T
o
, (4)
where “cl” stands for “cold low”, I have assumed that the
tropopause temperature is also T
o
, and I have expanded the
logarithm assuming that T
s
and T
o
are not too different. Sub-
stituting this perturbation of the saturation moist static energy

220 K. Emanuel: Genesis and maintenance of “Mediterranean hurricanes”
into (2), I obtain an expression for the modified potential in-
tensity at the core of the cold low aloft:
V
2
mod
= V
2
p
−
C
k
C
D

φ
0
cl
− φ
0
s

(5)
Since φ
0
will be more negative in a cold upper low than at the
surface underneath it, this will give an increase of potential
intensity. Note that in evaluating (3), the ambient potential
intensity, V
2
p
, may very well be negative if the atmosphere is
stable to the lifting of a parcel saturated at sea surface tem-
perature. Examining Fig. 2, I note that the center of the cut-
off low aloft, φ
0
cl
'−4000m
2
s
−2
, while in the synoptic-scale
surface low underneath, φ
0
s
'−2500m
2
s
−2
. Even if the am-
bient potential intensity (i.e. in theabsence of the cold low) is
zero, this gives a maximum wind speed of around 40ms
−1
,
assuming that the exchange coefficients are equal.
5 Conclusions
The climatological potential intensity over the Mediterranean
Sea is usually only marginal for tropical cyclone formation,
and the atmosphere is usually far too dry to permit develop-
ment. But when a deep, upper-level cutoff low moves over
the region, the air mass must ascend and cool to maintain
balance. Thus the air under such an upper low is unusually
cold and humid. Its low temperature, in combination with
the relative warmth of the underlying sea, and its high rel-
ative humidity provide an ideal incubator for hurricane-like
development. A simulation with an axisymmetric, nonhy-
drostatic, cloud-resolving model shows rapid development
under these circumstances. Calculations of potential inten-
sity over the Mediterranean using daily re-analysis data are
underway; these should enable an assessment of the overall
risk of hurricanes in the Mediterranean region.
Acknowledgements. The author is grateful for the support provided
by the National Science Foundation, under grant ATM-0349957.
Edited by: L. Ferraris
Reviewed by: anonymous referees
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