Improving the numerical prediction of a cyclone in the Mediterranean by local potential vorticity modifications

This study explores the benefit that can be drawn from incorporating local potential vorticity (PV) modifications into a numerical forecast. The case under study is the severe cyclogenesis that occurred in the Western Mediterranean from 9 to 12 November 2001. This case was shown to be extremely sensitive to the upper‐level flow, which governed to a great extent the deepening of the depression and the location and intensity of its associated precipitation. Corrections of PV were guided by METEOSAT‐7 water vapour observations and restricted to the upper troposphere, mainly altering the topography of the dynamical tropopause. Using both the French operational global model ARPEGE and the non‐hydrostatic mesoscale model Meso‐NH, it is shown that carefully designed PV corrections lead to a substantial improvement in the simulation of the storm, both in terms of surface pressure, cloud cover and precipitation forecasts. Furthermore, the impact of the modifications is shown to be a maximum when they are introduced at the time corresponding to the incipient stage of cyclogenesis.