Abstract : P.1
Ensemble activities at ARPA-SIM: the COSMO-LEPS and COSMO-SREPS systems

Chiara Marsigli, Andrea Montani, Tiziana Paccagnella

This presentation will show the most recent results of the two main activities on limited-area ensemble systems carried out at ARPA-SIM, namely the COSMO-LEPS and COSMO-SREPS projects.COSMO-LEPS is the limited-area ensemble prediciton system based on the non-hydrostatic "COSMO-model" (formerly known as "LM"), developed within the COSMO consortium since 2002. This system aims at improving upon the early and medium-range predictability of extreme and localised weather events, especially when orographic and mesoscale-related processes play a crucial role. The present status of the COSMO-LEPS system, based on 16 integrations of the non-hydrostatic "COSMO-model" (10 km of horizontal resolution, 40 vertical levels) and running as a "time-critical application" at ECMWF, will be illustrated with the recent upgrades (including the increase of the ensemble size and the upgrade of the vertical resolution in the limited-area-model integrations). The attention is focussed on the probabilistic prediction in the early and medium-range of surface parameters (e.g. total precipitation, surface winds, positive and negative temperature anomalies...) so as to assess the possibility to issue severe weather alerts in the Alpine area on the basis of the COSMO-LEPS products. COSMO-SREPS proposes a new strategy to provide high-resolution meteorological ensemble forecasts for the short-range (up to three days), with the aim to give probabilistic assistance to the short-range forecast of surface parameters, especially in case of severe weather and for hydrological applications. Different sources of forecast errors are considered, trying to describe the uncertainty affecting the scales of interest in the high-resolution quantitative precipitation forecast at the considered time range. In particular, errors in the model formulation are considered, together with errors in the initial and boundary conditions. A Multi-Analysis Multi-Boundary approach is followed for the initial and boundary condition perturbations: four "COSMO-model" runs at 25 km horizontal resolution, nested on four different global models, have been provided by INM (Istituto Nacional de Meteorologia, Spain). Aiming at encompassing also the smaller scale uncertainty, limited-area model perturbations have been applied as well. Each of the 25-km "COSMO-model" runs has been used to drive 4 runs at 10-km horizontal resolution, the runs being differentiated by model perturbations: the values of the parameters included in the schemes for the parameterisation of the sub-grid processes have been randomly changed (within their range of variability) in the different runs.

Results are presented for some selected events, comparing the performance of the 16-member COSMO-SREPS ensemble with that of the COSMO-LEPS system. The spread-skill relation of the two systems is analysed, showing that the short-range spread is increased to values closer to the forecast error. A positive impact on the COSMO-LEPS spread is also found by adding the same model perturbations to the COSMO-LEPS runs.