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 26September 2006. Numerical simulations are performedwith the Weather Research and Forecastingmodel (WRF). Model results show that the model is able to realistically capture the timing andintensity of the cyclone, identifying in this small-scale cyclone the typical characteristics of aMediterranean tropical-like cyclone.Sensitivity experiments to sea surface temperature (SST) of the simulated cyclone are discussedin the present paper. In these experiments the SST was changed by a constant few degreesuniformly throughout the Mediterranean Sea. The simulated-cyclone sensitivity to SST isstudied in the framework of an air-sea interaction theory, in particular during the two phasesthat are fundamental for the development of an environment favorable to severe convectionand thus for the cyclone to be self-sustained. The modifications of the properties of thesimulated cyclones are then analyzed using an objective evaluation, based on a phase diagramused to study the tropical-extratropical transition of cyclones.The present simulations show that the features typical of a tropical cyclone are progressively lostas the SST is reduced. In the colder SST experiments, the lower intensity of the sea-surface fluxeshas the effect of delaying and reducing the development of convection and the intensification ofthe cyclone, so that the cyclone loses the characteristics of a tropical cyclone when SST is reducedby more than 4 °C. On the other hand, the cyclone shows a pressure minimum deeper than thecontrol 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 significantlyaffected by the change in SST and, like the control run, showtropical-like features only for a limitedperiod.