RADAR MEASUREMENT OF PRECIPITATION IN COLD MOUNTAINOUS AREAS - THE GARONNE BASIN

The object of this paper is to discuss, using the example of the upper basin of the Garonne river, the conditions of use of radar for the me asurement of precipitation in order to forecast floods in areas with s ubstantial relief and a cold season, which is the case for most French basins. The principle of radar measurement of precipitation fields is summarily expounded and the factors influencing the quality of these measurements are presented, notably those in relation to the relief an d climatology of precipitation and to the 0-degrees-C isotherm (precip itation melting level). The main difficulty is that, at mid-latitudes, the precipitation melting layer (i.e. the 0-degrees-C isotherm) is lo cated at low altitude, which implies that, except in summer, radar doe s not observe a homogeneous scattering environment viz. rain or snow. Most of the time, the radar beam reaches, at a short distance, the alt itude of the upper limit of the rain. This problem is made worse in mo untainous areas because, for the azimuthal sweeping of the radar beam, significant elevation angles need to be used in order to avoid ground echoes.