Translation et rotation des spectres des gouttes de pluie dans des averses de nuages stratiformes et de nuages convectifs

Lateral and rotary movement of raindrop spectra in showers from stratiform and convective clouds

Institut et Observatoire de Physique du Globe du Puy-de-Dôme Université de Clermond-Ferrand II 12, av. des Landais, 63000 Clermond-Ferrand

Abstract

Since the relatively recent appearance of apparatuses which give automatic and continuous information on distribution spectra for rain droplet diameters Di it has been possible to systematically study diameter variations in function of time. Basing himself on the fact that the majority of these spectra can be approximated by an exponential law n(Di) = No-λDi, Waldvogel (1974) proposed a both simple and quantitative parametrisation for representing the variations in time typical of the 2 parameters No and λ. During the 2nd and 3rd International PAP Campaigns (Projet d'Augmentation des Précipitations) the VIDIAZ spectral rain-gauge and a Joss-Waldvogel disdrometer were used to systematically measure droplet spectra of precipitation from stratiform and convective clouds. We have applied this parametrisation to analyse variations over time of No and λ, and have compared them with those of precipitation intensities R mm/h) obtained on two apparatuses. For these two types of rain the 2 devices used are first shown to be entirely suitable for analysing variations over time of these 3 parameters by virtue of their characteristics and for a sampling period set at one minute. These 2 types of rain are then shown to be closely correlated and the relationship linking them is determined ; this is found to be extremely similar to the relationship which can be established theoretically. By application of this theoretical relationship to the 2 situations analysed it is then found that a large increase in R is due to an almost constant increase of No and λ in the case of rain from stratiform clouds and to a diminution of No and λ, in the case of rain from convective clouds. Finally, in a more general way, the conclusion is drawn that any important variation in precipitation intensity is due to lateral movement of the droplet spectra in the case of rain from stratiform clouds and to a rotary movement of such spectra in the case of rain from convective clouds.