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Archive

The Alès DSD measurement experiment


Chapon (1) Benoit

The knowledge of the drop size distribution (DSD) is still a key factor for the improvement of quantitative precipitation estimation from radar reflectivity factor. In order to describe the DSD of Mediterranean rainfalls (notably in function of the rain types), a laser-optical disdrometer was set up in Alès with the collaboration of the Ecole des Mines d'Alès (EMA) during the autumn 2004.
The system is composed by a transmitter which generates a flat, horizontal beam of light, and a receiver which converts the light into electrical data. The signal is affected when a hydrometeor falls through the beam within the measurement area (48 cm2). The degree of dimming is related to the size of hydrometeor as well as the duration of the affected signal is related with a fall velocity. The rain drop size and fall speed distribution at ground level is provided in the form of raw data of 1mn drops spectrum, decomposed in 32x32 size/fall speed classes. In addition bulk parameters such as rain rate, water content and also a classification of precipitation type is given.
The poster presents the present experimental framework set up last autumn and shows some results of the Alès 2004 experiment.