Impact of initial condition uncertainties on the predictability of heavy rainfall in the Mediterranean: a case study

This study explores the predictability of a heavy rainfall event that struck North Africa on 9 and 10 November 2001. This case is a paradigm of Mediterranean extreme events characterized by the presence of a deep upper-level trough associated with an intense cyclone which developed over the Western Mediterranean basin. Using the French non-hydrostatic mesoscale model MESO-NH, numerical experiments starting from various initial atmospheric states were conducted to assess the impact of initial condition uncertainties on the precipitation and cloud cover forecast. To generate a set of perturbed atmospheric states, a simple date-shifting initialization method was used. Two sets of simulations were run, using lateral boundaries and initial conditions derived from both the French operational global assimilation system Action de Recherche Petite Echelle Grande Echelle (ARPEGE) and the European Centre for Medium-Range Weather Forecasts (ECMWF) system. Initial perturbations applied to the upper-level trough propagated and intensified throughout the simulation, leading to some discrepancy in the forecast of the low-level cyclone. While it was found that the upper-level trough and the surface cyclone controlled the location of the overall precipitation pattern, the predictability of smaller-scale features such as localized heavy rainfall was directly related to specific mesoscale structures. The low-level jet associated with the Surface cyclone and the location and the intensity of the surface-low both impact upon the triggering and the sustainment of the convective cells. In consequence, small-scale perturbations of these mesoscale features led to large errors in the precipitation forecast, especially in the Algiers area. Copyright (C) 2008 Royal Meteorological Society