Identification of developing severe convective cells over the Alpine region by merging Satellite, Radar and NWP data

The main goal of the EUMETSAT Fellowship project COALITION is the onset of a heuristic object-oriented model aimed at improving the forecast and warning of severe thunderstorms in the Swiss Alpine area. This is achieved providing real-time probabilistic information about the evolution of convective cells detected in their early stage. This paper proposes an innovative methodology, where the best available information on convective processes provided by different sources (Meteosat Second Generation Rapid Scan, Radar Nowcasting Products and High Resolution Numerical Weather Prediction models) is combined in order to increase the lead time in forecasting severe convective events. The COALITION algorithm merges evolving thunderstorm attributes (predictants, e.g. Cloud Top Temperature, radar based Vertical Integrated Liquid content) with severe convection predictors (environmental parameters, e.g. Meteosat Second Generation Convection Initiation, terrain slopes, CAPE,...). The storm evolution is the result of the interactions between convective signatures (objects) and the surrounding environment, which are described by a simplified one-dimensional Hamiltonian model. In a complex terrain like the alpine area, the orographic forcing plays an important role, it has therefore been included in the system as a possible convection triggering factor. The paper presents and discusses the methodology and the results of the COALITION algorithm. Statistical assessment based on a set of eighty different thunderstorm events is also reported. Preliminary verification statistics confirm that COALITION is able to nowcast the intensity of developing convective cells with sufficient skill up to a lead time of about 15 minutes. This system will be used operationally at MeteoSwiss during summer 2013.