Microphysical observations in winter storms over the Atlas mountains in Morocco

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 typically 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 inefficient at 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 particles sizes.