Comparison of the down-valley flow for the MAP IOP8 and IOP3 with the numerical laboratory Mesonh model.

This study documents with the Mesonh model the down-valley flows within major Alpine river valleys for the MAP IOP8(20-21 October 1999) and MAP IOP3(25-26 September 1999). Persistent rainy periods characterize these two IOPs, a down-valley flow is observed and is simulated while the western Po valley is concerned by a northerly flow in opposition to the meso-scale southwesterly flow.

The fine scale MesoNH simulations at 2.5km horizontal mesh are validated with the surface observations, the RAIN product derived from the operational Monte-Lema radar and the reflectivity and wind 3D-fields derived from the MAP-research ground-based and airborne radars. The validation of the two IOPs simulations is good and allows a detailed analysis. We use the Mesonh model as a numerical laboratory by performing sensitivity experiments to quantify the respective role of each echanisms previously identified as a potential cause of this reverse flow: wet or ry drainage flow, blocking upstream the Alps.

The different sensitivity experiments show that these two IOPs obey to two different conceptual schemes: 1) in agreement with the conclusion of Steiner et al (Map special issue,2003) performed with radar and surface data, the reverse flow of IOP8 mainly originates from diabatic effects i.e. the subsidence caused by melting of precipitation particles over the orography. 2) as illustrated in Brig meeting (Asencio et al,2003), the IOP3 reverse flow mainly originates from dynamical effects i.e. the upstream flow intensity and the transition from blocked to unblocked regime over the Alps.