The influence of Sardinia on Corsican rainfall in the western Mediterranean Sea: A numerical sensitivity study

The interaction of orographic effects and moisture availability is of high importance to the precipitation amount and distribution in the western Mediterranean and neighboring land surfaces. In particular, the forecast of heavy precipitation events is still a challenge for operational weather forecast models. In this study, the thermal and dynamical interactions between the two neighboring islands of Corsica and Sardinia in the western Mediterranean Sea are investigated using the COnsortium for Small-scale MOdeling (COSMO) model. Six cases with different synoptic conditions are analyzed and the dependance of the Corsican rainfall on the presence and terrain characteristics of Sardinia is investigated. Besides a reference run with standard model orography, sensitivity runs with removed and flat island of Sardinia are performed. The numerical results show that the daily precipitation amount over Corsica can increase by up to 220% of the amount from the reference run. Whereas most of the sensitivity runs show a decrease of the precipitation amount under strong synoptic forcing, there is no systematic relationship on days with weak synoptic forcing. The differences in the precipitation amount are induced by (i) missing deviation or missing blocking of the southerly flow by Sardinia and (ii) by the influence of cold pools generated by deep convection over Sardinia. These differences can be attributed to changes of low-level convergence and moisture/heat content and their effect on thermodynamic parameters, like convective available potential energy or convective inhibition. Furthermore, the position and translation speed of frontal systems over Corsica on days with strong synoptic forcing also depend on the Sardinian orography. These results demonstrate the high sensitivity of numerical weather prediction to the interaction of neighboring mountainous islands.