Contrasting sensitivities of Mediterranean cyclone forecasts to model resolution and physics across cyclone types

Abstract. Hazardous Mediterranean cyclones, including tropical-like medicanes, produce some of the most damaging wind and rainfall events in southern Europe, yet their prediction remains challenging because they are often small in scale and their evolution can be strongly controlled by diabatic processes. While previous research has assessed the role of diabatic processes in individual case studies and/or using resolutions that require parametrization of convection, it remains unclear whether the sensitivity of modelled Mediterranean cyclones to explicit representation of convection is robust across cyclone types. Using a consistent multi-case framework, we identify systematic rather than case-dependent sensitivities. Simulations of seven hazardous MedCys, with a range of characteristics, are performed using four limited-area configurations of the Met Office weather forecast model downscaled from ECMWF IFS analyses. The currently operational physics packages for the global (GAL9) and limited area (RAL3) versions of the model are used with 12-km and convection-permitting, 2.2-km grid spacing, respectively; an experimental package with the scale-aware convection scheme, CoMorph, is also used at both resolutions to cleanly identify resolution dependence.

Sensitivity of dynamical variables and hazards to model configuration depends strongly on cyclone type, with medicanes exhibiting substantially greater and more robust dependence than more extratropical systems. The 2.2 km CoMorph configuration consistently over-intensifies medicanes, producing much lower minimum mean-sea-level pressures, and stronger winds and gusts than other simulations, especially for medicanes Ianos and Apollo. In contrast, the 12-km GAL9 configuration generally produces the weakest medicanes and tracks that is displaced eastward or southward; these biases are reduced using the 12-km CoMorph configuration. An in-depth analysis of medicane Ianos, which deepens by at least 20 hPa more in the 2.2-km CoMorph simulation than in the other simulations is presented. This CoMorph simulation yields a Ianos with a stronger potential vorticity tower, ascent and latent heat fluxes, and an eyewall structure resembling a tropical cyclone. These results demonstrate that the forecast skill of hazardous Mediterranean cyclones depends critically on the interaction between model resolution and physical parameterisations, implying that improvements in kilometre-scale forecasting require advances in model physics as well as increased resolution.