Impact of Mesoscale Data Assimilation on the Fine Scale Numerical Simulations of the Map Iop2a

The intensive observing period number 2a (IOP2a) of the Mesoscale Alpine Programme (MAP Bougeault et al., 2001) took place on 17 September 1999 over the Lago Maggiore Area in northern Italy. Between 12 UTC and 00 UTC a convective line developed over the first slopes of the Lago Maggiore mountains and propagated south-eastwards. During the early night, the system intensified and produced 70 mm of precipitation in about 6 hours. Previous studies highlighted the strong sensitivity of the mesoscale numerical simulations of IOP2a to the initial conditions (Richard et al., 2003; Lascaux et al., 2004), stressing out the low predictability of this event (also confirmed by Hohenegger, personal communication). In particular, a simulation initialized with the ECMWF operational analysis of 17 September, 12 UTC succeeded in reproducing the development and propagation of the convective system, whereas the same simulation based upon the MAP reanalysis data completely failed. Different reasons may explain such a negative impact of the reanalysis. First, Keil and Cardinali (2004), showed that the assimilation of the Lonate wind profiler in the ECMWF system caused an excessive drying of the low-level moisture field. Second, the low-level convergence located over the Po valley, well-marked in the analysis, is no longer present in the reanalysis. From these past experiments, it seems that any further improvement of the IOP2a simulation could not be achieved without refining the model initial state. In this study we try to explore the potential benefit of using as initial conditions of the mesoscale simulation, a high-resolution analysis as an alternative to the ECMWF analysis or reanalysis. For this purpose we made use of the 3D Var ALADIN assimilation system of Météo-France, which has been recently interfaced with the Meso-NH model. Different Meso-NH experiments, based upon an ALADIN analysis, carried out with and without additional MAP observations, have been conducted and are analyzed.