Atmospheric Research, 24 (1989) 103-122 103 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Microphysical Observations in Winter Storms over the Atlas Mountains in Morocco OMAR BADDOUR 1and ROY M. RASMUSSEN .2 1M~tdorologie Nationale, Division de Ddveloppement et de Recherche, Casablanca (Maroc) 2Bureau of Reclamation, Denver, Colorado (U.S.A.) (Received December 27, 1988; accepted after revision April 13, 1989 ) ABSTRACT Baddour, O. and Rasmussen, R.M., 1989. Microphysical observations in winter storms over the Atlas Mountains in Morocco, Atmos. Res., 24: 103-122. A case study is presented for a winter storm occurring on December 8 and 9 over Morocco in 1985. Using microphysical data collected by the University of North Dakota Citation aircraft, we show that two main precipitation mechanisms were dominant during this storm. The first was a seeder-feeder type mechanism in the stratiform regions of the cloud. The seed crystals were typ- ically aggregates of dendrites and stellars of relatively low concentration. The liquid-water content in the lower cloud layers was relatively high ( > 0.2 g m -3) for over 10 min duration, suggesting that the seed crystals were relatively inefficientat depleting the available supercooled liquid water. In convective regions of these clouds, a coalescence process, followed by freezing and subsequent riming into graupel was observed. Model calculations verified this behavior, and showed that the observed moderate updrafts and high liquid-water contents were adequate to produce the observed particle sizes. RESUME On prdsente un cas d'~tude d'un syst~me nuageux hivernal qui a int~ress~ le Maroc. En utilisant les donn~es microphysiques collect~es par l'avion Citation de l'universit~ de North Dakota, on montre que deux m~canismes principaux de prdcipitation dominent durant le passage de ce sys- t~me nuageux: le premier est le m~canisme "seeder-feeder" qui se produit dans la partie strati- forme du syst~me nuageux. Des agr~gats en faible concentration, form,s typiquement soit par des cristaux en forme de branches (dendrites), soit par des cristaux en forme d'dtoile, pr~cipitent dans les couches inf~rieures du syst~me nuageux. Dans ces couches, on a enregistr~ un contenu en eau liquide relativement important ( > 0,2 g m-3) et d'une durde supdrieure h 10 minutes. Ceci sugg~re que ces cristaux tombant des couches sup~rieures sont relativement inefficaces pour puiser l'eau liquide surfondue prdsente. Dans les rdgions convectives du syst~me, on observe le processus de coalescence, suivi par la *Current affiliation: Applied Research Corporation, 3005 Center Green Drive, Suite 225, Boulder, CO 80301, and Research Application Program, National Center for Atmospheric Research, Box 3000, Boulder, CO 80307, U.S.A. 0169-8095/89/$03.50 © 1989 Elsevier Science Publishers B.V.