Grup de Meteorologia - UIB
MEDICANES: Mediterranean tropical-like storms
Lluís Fita     Romu Romero @




Medicane (Mediterranean+Hurricane) looks like a tropical-like storm over the Mediterranean Sea. The system is described as a cloudy rounded structures showing a free cloud area in the centre. Moreover, it is associated to strong cyclonic winds, heavy rainfalls and a warm core (Businger and Reed, 1989). Despite these systems do not reached the hurricane strength (33 ms-1 of wind speed at 10 m averaged over 10 minutes), they presented some dynamical and structural similarities with the tropical storms. These are rare cases, since have been detected (by the author from satellite images and bibliography) less than 20 cases during the last 25 years allover the Mediterranean basin. From the first one occurred on 1983 (Ernst and Matson, 1983) some studies have been done and Medicanes are also related to another rare arctic phenomenon like the Polar Lows (Rasmussen et al., 1987; Businger and Reed, 1988).

However some differences on the evolution of these systems in the Mediterranean basin in comparison to the tropical ones are observed. They show that a cold, upper low centre over the area was present in some cases. Meanwhile in the tropical cases there is any kind of upper level disturbance close to a hurricane evolution (see these two examples). In a medicane episode, the instability induced by the upper level low in addition to the low level stability seem to be the necessary conditions to develop these kind of tropical-like systems over the Mediterranean basin (Emanuel, 2005). It gives the idea that Medicanes are a result of mixing of two dynamical processes. A first phase dominated by the baroclinic development and the second one mainly derived by convective tropical-like activity and air-sea interaction. At the same time, a hurricane theory establishes that they can only be developed over Seas with a Surface Temperature above 26 °C (Emanuel, 1985). Medicanes have been developed above Sea Surface Temperatures about 15, 24 or 17°C (Fita et al., 2007).

Medicane seems to evolve like a Hurricane (Emanuel, 1985; Rotunno and Emanuel, 1986). A convective zone above the sea is organised around a weak vortex that reached the storm zone. The organisation of the storms around the vortex enhanced they activity with which the vortex gain vorticity. The enhancing of the vorticity of the structure enhance the capacity of extract moist and heat from the sea surface due to the increasing of the wind speed. The increasing of the moist and heat i the base of the structure contributes to enhance the convectivity. By this way a mutual reinforce dynamics is established between the convection, the vortex and the source of energy. By this way, the source of energy of a Medicane becomes the Sea and the main structure is derived from a extraordinary deep convective activity. According to this Latent Heat Flux from the Sea, Sea Surface Temperature and Latent Heat Release from convection are shown as key parameters in the Medicane formation (Lagouvardos et al., 1999; Reed et al., 2000; Pytharoulis et al., 2000 and Homar et al., 2002). They evolved above the Sea and they vanished when they reached the coast. As in the Hurricane, Medicanes developed a wall cloud. Satellite based analysis of the storms shown a first phase of formation (before eye detection) with a cold convective cloud tops. A second static phase in which the eye can be observed with an axis-symmetric cloud structure with the most intense activity. A third 'itinerant' phase where the structure travels in a specific direction with no heavy rains but with a gradual increase of the estimated wind speeds (Luque et al., 2007). A sailor witness of two episodes report about "white sea, and blindness conditions" that can be attributed to a kind of sea spray conditions.

Due to the densely populated coastal lines of the Mediterranean Sea, Medicanes have a strong potential of damage. A better knowledge of these storms is needed in order to prevent and diminish casualties and strong impact of the systems on the Mediterranean societies.

Climate change scenarios gives a warmer Mediterranean Sea. According to assumed dynamics of the Medicanes, a higher Sea surface temperature might derive to stronger medicanes or tropical cyclones (Gaertner et al., 2007). Numerical simulations with the MM5 nonhydrostatic primitive equation model of the 950116 case are performed to verify this hypothesis (assuming similar vertical structure of the atmosphere). A control simulation of the episode is compared to a simulation in which the Sea surface temperature has been 5 °C increased (see animation). A stronger medicane is mantained for a longer period of time is simulated with a warmer sea. Another study applies an empirical genesis index of tropical cyclones in the Mediterranean basin under future regional scenarios (Romero and Emanuel, 2006). Despite the increasing of the Sea surface temperature, results do not give a clear increase of medicane activity. Probably related to a decrease of intrusions of upper level cold disturbances in the Mediterranean basin on future scenarios.