A case study of an orographic cyclone formation south of the Alps during the MAP-SOP

The cyclonic formation of IOP15 of the Mesoscale Alpine Programme (MAP) (6–7 November 1999) is examined with respect to mesoscale features in the geopotential, temperature, wind and precipitation 􏰰 elds. The effect of orography at different scales is evaluated using a mesoscale meteorological model (BOLAM). Model errors are determined by comparing standard and special MAP SOP observations with model output. An Optimal Interpolation reanalysis of the initial condition, aimed at assimilating mesoscale structures, is applied, resulting in an overall reduction of the forecast error, including position of local scale precipitation maximum in the Lago Maggiore target area south of the Alps. Different orography representations have an impact on the error patterns, since it is shown that the Alpine orography largely controls the cyclone growth over the Mediterranean. Particular attention is devoted to the model verification of precipitation and wind fields in the vicinity of the Alps, where special observations allow for fine-scale analysis.