The role of small-scale orographic features in the spatial distribution of precipitation

Radar images from the Cevennes experiment, which took place during the autumns of 1986-88 on the southeastern flank of the Massif Central in France, reveal a characteristic and repetitive structure of the precipitation distribution. In southerly Mediterranean flows, which are frequent there in autumn, precipitation is often organized into narrow bands or plumes, oriented south-north. The period 14 and 15 November 1986 has been selected as an example on which to perform numerical simulations and analyse the mechanism yielding this specific precipitation organization. The numerical model correctly reproduces the bands, especially at high horizontal resolution (I km). These rainfall bands are focused on the small-scale topography, oriented east-west, composed of a succession of ridges and penetrating valleys. The bands are generated upstream of these ridges and enhanced on the lee side by convergence created by deflection around the obstacle and penetration of the flow into the valleys. Idealized simulations and sensitivity experiments have been carried out to verify these hypotheses. These simulations suggest that small-scale orography plays a decisive role in forcing the precipitation into bands, while upstream moisture conditions mainly affect the precipitation intensity and its south-north extent.