Orographic air mass transformation

The loss of moisture as an airstream passes over a high mountain ridge plays a role in the regional climate of several areas around the world: e.g. Central Asia, the High Plains of North America and the Patagonia of South America. GCMs and mesoscale models, with grid scales from 300 km to 5 km differ widely in their predictions of orographic precipitation and moisture flux divergence over mountains. To illustrate and diagnose these differences, data and model output from the Mesoscale Alpine program (MAP) are examined. Of particular interest are occurrences of meridional flow across the main Alpine range which include detailed aircraft surveys. Aircraft cross-sections allow upwind or downwind moisture fluxes to be computed and compared with the output from numerical models. Included in the study are three cases of southerly flow (Sept. 20, Oct. 21 and 24) and one case of northerly flow (Nov. 8) from MAP IOPs 2,8,10 and 15. The southerly flow cases show considerable inhomogeneity in the inflow condition, low level blocking and convection over the Po Valley. The northerly flow case is more uniform with stratiform precipitation (snow) over Bavaria. In all four cases, evidence of channeling through the two major gaps in the Alpine range (i.e. the St. Gotthard and Brenner Passes) is seen. Additional information comes from conventional soundings, the dense Alpine raingauge network, airborne Lidar and rapid-scan meteosat sequences of drifting and precipitating clouds. Attention is paid to closing the air-mass and water-mass budgets.