Analyses eulériennes et lagrangiennes des systèmes convectifs quasi-stationnaires sur les Alpes

摘要

This thesis is related to the scientific objective "Orographic precipitation mechanisms" of MAP (Mesoscale Alpine Programme) and investigates how the precipitation is triggered and enhanced over mountain slopes exposed to quasi-stationary humid air-flows. The problem of the numerical forecast of heavy rain at the scale of few kilometers is also tackled. Most heavy-precipitation events in autumn over the Alps are characterized by a quasi-stationary southerly or south-westerly flow advecting maritime air from the Mediterranean towards the Alpine ridge. The case of MAP IOP (Intensive Observation Period) 2B is modelled with the numerical simulation system MesoNH. An original Lagrangian analysis method enables to identify a dry air-mass at the scale meso-beta over the Mediterranean and to study its evolution and influence on the rain distribution over the Alps. The heavy precipitation over the southern slopes of the Alps is studied with idealised numerical simulations, where the southerly or south-westerly upstream flow is prescribed as uniform and stationary. The obtained rain distributions are in agreement with the climatological analysis. This indicates that the maximum areas correspond to regions of enhanced orographic lifting of the incident flow. Further simulations at high-resolution and with a purely resolved cloud physics allow to characterize triggering and development mechanisms which are specific for the south side of the Alps. Some were observed during MAP. They appear as very complex, even in the simulations despite the simplification of the upstream flow.