Numerical analysis of a Mediterranean 'hurricane' over south-eastern Italy: sensitivity experiments to sea surface temperature

An unusually intense subsynoptic-scale cyclone has been documented in southeastern Italy on 26 September 2006. Numerical simulations are performedwith the Weather Research and Forecasting model (WRF). Model results show that the model is able to realistically capture the timing and intensity of the cyclone, identifying in this small-scale cyclone the typical characteristics of a Mediterranean tropical-like cyclone. Sensitivity experiments to sea surface temperature (SST) of the simulated cyclone are discussed in the present paper. In these experiments the SST was changed by a constant few degrees uniformly throughout the Mediterranean Sea. The simulated-cyclone sensitivity to SST is studied in the framework of an air-sea interaction theory, in particular during the two phases that are fundamental for the development of an environment favorable to severe convection and thus for the cyclone to be self-sustained. The modifications of the properties of the simulated cyclones are then analyzed using an objective evaluation, based on a phase diagram used to study the tropical-extratropical transition of cyclones. The present simulations show that the features typical of a tropical cyclone are progressively lost as the SST is reduced. In the colder SST experiments, the lower intensity of the sea-surface fluxes has the effect of delaying and reducing the development of convection and the intensification of the cyclone, so that the cyclone loses the characteristics of a tropical cyclone when SST is reduced by more than 4 °C. On the other hand, the cyclone shows a pressure minimum deeper than the control run, with a persistent symmetric warm core, for SST increased by more than 2 °C. Finally, when SST is changed by less than 2 °C, the cyclone depth and trajectory are not significantly affected by the change in SST and, like the control run, showtropical-like features only for a limited period.