THE METEOROLOGICAL "BOMB" IN THE MEDITERRANEAN.

Piervitali, E. (Istituto di Fisica dell'Atmosfera, CNR-Roma, Italy)

Colacino, M. (same affiliation)

Conte, M. (same affiliation)

T.BERGERON is reputed to have defined a "very rapidly deepening extratropical low" as a depression in which the central sea level pressure falls at a rate of 1hPa/hr or more for a period lasting at least 24 hours. As Bergeron's definition was referred to the latitude of 60N, ageostrophically equivalent rate can be obtained for a latitude j by multiplying this rate by sin j /sin 60=B0. The resulting critical rate, denoted as 1 bergeron by SANDERS and GYAKUM (1980), varies from 28 hPa/24hr at the Pole to about 9 hPa /24hr at 20 N, which is the deep southern limitat which the phenomenon has been observed in our hemisphere. In the Mediterranean, applying the geostrophic correction, the critical value of 1 bergeron is obtained with a deepening of 20 hPa/24hr at the extreme northern boundary and of 14 hPa/24hr at the southern limit of the basin; an average value of 17 hPa/24hr is the critical value for an average latitude of 38 N. SANDERS and GYAKUM(1980) defined this explosive extratropical cyclogenesis as meteorological "bomb". In this paper a synoptic climatology of this extreme meteorological system in the Mediterranean area is presented for the 31 year period 1965-1995 in which 101 events have been recorded. An accurate analysis of the 101 events points out that the most part (93%) of the "bombs" occurs, in broad outlines, following only two fundamental types of development.In the first type a lot of elements defined in the study case by KARACOSTAS and FLOCAS (1983) are recognisable; that is, broadly speaking,the "bomb" develops from an interaction between a baroclinic, open longwave and an unstable short wave. The second type of development follows the dynamics reported in the work of CAPALDO and CONTE et al. (1980); in this case the "bomb" originates from the interaction between a middle latitude depression at synoptic scale, deeply penetrating in the Mediterranean, and a depression of African origin, sometime at sub-synoptic scale. Three preferential regions of development of the bomb appear in the basin, that is: an area including parts of the Corsican-Sardinian sea and the central-south Tyrrhenian sea, a second area including the central-south Adriatic sea and the northern Ionian sea, a third area including the Aegean sea.

Intense wind, torrential precipitations and floods are associated with the"bomb" that produces severe damages to the territories influenced by the phenomenon, which is clearly an extreme event. The "bomb" is much more frequent in the cold season. During summer only three events were found in 31 years.The "bombs", which are very severe cyclonic systems of meteo-marine type, should be reduced in annual frequency of occurrences by the observed increase of the anticyclonic patterns in the Mediterranean. This conclusion appears to be supported by a negative and significant correlation between annual number of bombs and annual 500 hPa height; actually high values of 500 hPa height correspond to low annual numbers of "bomb" and viceversa.


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