Numerical Analysis of a Mediterranean “Hurricane” over Southeastern Italy

Abstract

The presence of a subsynoptic-scale vortex over the Mediterranean Sea in southeastern Italy on 26 September 2006 has been recently documented by the authors. The transit of the cyclone over land allowed an accurate diagnosis of the structure of the vortex, based on radar and surface station data, showing that the cyclone had features similar to those observed in tropical cyclones. To investigate the cyclone in greater depth, numerical simulations have been performed using the Weather Research and Forecasting (WRF) model, set up with two domains, in a two-way-nested configuration. Model simulations are able to properly capture the timing and intensity of the small-scale cyclone. Moreover, the present simulated cyclone agrees with the observational analysis of this case, identifying in this small-scale depression the typical characteristics of a Mediterranean tropical-like cyclone. An analysis of the mechanisms responsible for the genesis, development, and maintenance of the cyclone has also been performed. Sensitivity experiments show that cyclogenesis on the lee side of the Atlas Mountains is responsible for the generation of the cyclone. Surface sensible and latent heat fluxes become important during the subsequent phase of development in which the lee-vortex shallow depression evolved as it moved toward the south of Sicily. During this phase, the latent heating, associated with convective motions triggered by a cold front entering the central Mediterranean area, was important for the intensification and contraction of the horizontal scale of the vortex. The small-scale cyclone subsequently deepened as it moved over the Ionian Sea and then maintained its intensity during its later transit over the Adriatic Sea; in this later stage, latent heat release continued to play a major role in amplifying and maintaining the vortex, while the importance of the surface fluxes diminished.