Precipitation on the lee side of the Vosges Mountains: Multi-instrumental study of one case from the COPS campaign

Abstract

In this article, we focus on one case study from the Convective and Orographically-induced Precipitation Study (COPS), which took place in north-eastern France and south-western Germany during the summer of 2007, in a low mountain area. We investigate lee side precipitation due to shallow and deep convection during one Intensive Observation Period (IOP) of COPS which have been well documented by all the instruments. For that aim, we use a set of observations from radars, radiosoundings, satellite, and a network of Global Positioning System (GPS) receivers, as well as meteorological analyses and dedicated model simulation results. The combination of these measurements with GPS tomography results suggests the role of low level water vapour accumulation and convergence as a precursor to the convective initiation. The origin of this moistening and wind convergence seems to be linked to a slight change in the wind direction in the north-west part of the COPS domain. Using a high resolution X band radar, we also describe how small scale orography affects the precipitation locations, and we show the role of hills near the mouths of the valleys in convective enhancement. This observation is confirmed by model simulation showing that convection is no longer enhanced when the hills are suppressed. The further intensification of one convective cell over the Rhine Valley, which is climatologically frequent, is also discussed.