You said “Medicane” ?

The numerous sailors who crisscross the “Big Blue” have known for ages that, if navigation is relatively easy, meteorology represents a particularly difficult constraint to understand. Indeed, if the coasts are mostly steep and the dangers very localized, if the tides are almost negligible, if currents are only to be taken into account in well-defined areas, the major difficulty comes from capricious weather, difficult to predict, can represent a real danger due to its changing moods, sudden, even brutal.

Introduction

The Mediterranean is indeed a sea apart. Completely surrounded by an alternation of mountainous reliefs and desert plains, steep capes and long beaches, dotted with numerous islands of vast dimensions and most often steep coastlines, it is subject to the coastal effects of strong thermal breezes, katabatic winds, and various local aerological phenomena combining with general meteorological conditions linked to the movements and conflicts of the air masses which sweep across it. A real headache for meteorologists, and even more for navigators since Ulysses and the Phoenician merchants !

Fortunately, meteorological science has progressed considerably, and high-resolution models allow excellent medium-term forecasts, that is to say at maturities of 3 up to 4 days. This is perfectly sufficient since we rarely spend more than 48 hours at sea to cross it from one coast to another. We can usefully use the model ARPEGE up to 4 days of Météo-France which covers the entire Mediterranean with a resolution of 6 nautical miles. We will complete for short distances with the models AROME up to 36-42 hours and very high resolution (1,5 MN), covering the western basin of France to southern Sardinia and the Balearic Islands.

For the central and eastern part, either from the Tyrrhenian Sea to Turkey, we can use the numerous WRF models offered by the website OpenSkiron (¹) with resolutions ranging from 12 km to 4 km. We will not forget to stay on standby and listen to local radio alerts concerning very localized phenomena..

Among these phenomena, it is one to which we must be particularly attentive, because extremely dangerous : charts portfolio medicane.

The doctor

The term “medicane” is the contraction of “ Mediterranean Hurricane ". Medicanes are subtropical depressions that can form when the sea surface is strongly warmed in summer, causing storm winds and heavy precipitation (²). When the water temperature occasionally exceeds 25-26°C, this can develop significant convective energy at the base of a warm and humid air mass, thus feeding a low pressure system with characteristics equivalent to tropical cyclones. An eye is sometimes visible and the weather phenomena they cause are particularly violent.

These are actually hybrid depressions, driven by “tropical” type processes such as the release of latent heat by convection, and “extra-tropical” type such as horizontal thermal contrasts. For this reason, we speak more accurately of “subtropical cyclones " because they are not hurricanes in the literal sense of the term.

Genesis of medicines

(³) The criteria and initial states most often necessary for the formation of a tropical system are as follows: :

• Underlying sea temperature above 25-26°C over a depth of at least 50 meters.
• Unstable air mass = high CAPE (⁵)
• High humidity in the lower and middle layers of the atmosphere
• Start of prior disturbance in the low layers at sea level
• Low shear (change in force or direction) wind with altitude
• Overlying cold air mass in upper layers

These initial conditions are quite often born by the arrival at sea of ​​an air mass heated by high temperatures on land., in certain favorable places around the Mediterranean, and who, thrust at sea by a combination of factors – orientation of pressure fields, synoptic wind, Coriolis force, for example – takes on moisture in superheated water with strong evaporation. This is often seen between the plains of southeastern Spain and the Balearic Islands, between the Maghreb and Sardinia, or between Libya and Sicily or Greece. Risks are increased from mid-summer to early winter, period when the sea becomes warmest and stays that way for a long time because of its great thermal inertia.

More and more common

Medicanes are not new phenomena. We have observed them episodically since satellite measurements existed.. Until 2015, there was one every 2 up to 5 years on average. but in recent years we have observed an increase in the frequency of medicanes from mid-summer to the end of autumn, which confirms the increasing probability that this phenomenon will become recurrent with global warming, like the Mediterranean rainy episodes (Cévennes episodes).

France has already experienced this phenomenon in the past. The last one dates from 6 au 9 November 2011. “Rolf” particularly affected the coast of Var and the Alpes-Maritimes. He fell 250 mm of rain in less than 48 hours to Bormes-les-Mimosas, dont 115 etc. one 12 hours only. The gusts of wind reached 85 knots at Saint-Raphaël and 83 knots in Hyères (⁴).

More recently, Greece was struck on the night of 18 au 19 th, 2020 by the strong winds and heavy rains of the medicane “Ianos”. These caused flooding in the southern part of the country, from the Peloponnese to the central-east. The strongest winds reached on average the 60 nodes with gusts locally greater than 85 nodes on the Ionian Islands and in particular the Kefalonia Islands, Lefkada and Zakynthos. This system also caused waves of more than 5 meters offshore. In addition, torrential rains accompanied this episode of bad weather with 100 up to 200 mm recorded over large areas and locally up to 400 mm on the foothills of the mountains of central Greece and the Peloponnese. The damage to boating was considerable.

Of 11 au 15 November 2021 the “Blas” medicane made a surprising trajectory in the south of the western basin, and an exceptional duration of more than 5 days, with intermittent increases in pressure as it travels. Unfortunately I did not capture the end of the phenomenon. But it went up along Sardinia to reach the Ligurian Sea on 15 November, with bursts of 60 up to 70 nodes between Corsica and Côte d’Azur.

How to predict and anticipate

With weather models in GRIB format, it is imperative to display the CAPE (⁵) to measure the level of instability of the air mass. It is also necessary to display the isobars with a reduced spacing (2 hPa) with a model like AROME or a WRF to 4 km. We can thus better see the appearance of a low pressure minimum during digging and follow the prediction of its evolution.. Attention, a model like the GFS, even with a resolution of 0.25°, is not sufficiently relevant to see this type of phenomenon. However, the ICON EU model of the German DWD was revealed, for the medicane Ianos, much more precise than the ARPEGE model, anticipating well before him the power and real path of the storm. It is therefore appropriate, as the most prudent boaters have done, to anticipate according to the most pessimistic model to take shelter.

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(¹) Open Skiron
(²) We also talk about T.M.S. (Tropical-like Mediterranean Storm).
(³) Source : Lionel Peyraud 8/11/2011
(⁴) Source : Keraunos
(⁵) CAPE : lire Predict storms with Weather4D
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