Abstract : 2E.7
Simulation of heavy precipitation events with the COSMO model: results within the PREVIEW project

Silke Dierer
Federal Office of Meteorolgy and Climatology, MeteoSwiss

The improvement of flash flood forecast is one of the aims within the EU project PREVention, Information and Early Warning (PREVIEW). The term ‘flash floods’ describes fast rising floods due to heavy precipitation events with typical timescales of several hours and typical spatial scales between 100 and 1000 km2. Flash floods are dangerous, because they combine the destructive power of a flood with rapid rise of water and difficulties to forecast the events. An important reason for the latter is the limited quality of precipitation forecast on short timescales and small domains. Thus, the increased resolution of new generation weather forecast models promises a step forward.

The flash flood working group within the PREVIEW project investigates the benefit of increased resolution for the forecast of heavy precipitation events. Several flash flood episodes in Piemonte and Cevennes are simulated with the high-resolution atmospheric models COSMO (formerly known as LM), MM5 and Meso-NH using grid sizes between 2 and 3 km.

The results of the COSMO model with 2.2 km grid size in the configuration of the Swiss weather service are presented and compared to results received with 7 km grid size. Sensitivity studies are performed in order to investigate the effect of different initial conditions, different vertical coordinates (z-coordinates instead of terrain-following coordinates), increasing vertical resolution and latent heat nudging on the precipitation forecast. Additionally, the results of the COSMO model are compared to results of MM5 and Meso-NH. The aim is to determine the most suitable configuration with respect to forecasting heavy precipitation events.