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<br />8 <br /> <br />significant rainfall over the plains, and snowpack over the mountains. The microphysical <br />observations during the main frontal passage (Dec., 8) show the presence of stratiform <br />cloud layers, with a weak seeder-feeder mechanism active over the mountains, at least in <br />the portion of the storm penetrated by the aircraft. The seed crystals were aggregates of <br />dendrites falling from an upper level cloud layer into a lower supercooled liquid water <br />layer. The laboratory results ofLew et. al. (1986) suggest that the riming of aggregates <br />can lead to rapid growth by accretion, and may be an important process depleting low- <br />level supercooled liquid water in these types of storms. In regions without a seeder-feeder <br />mechanism, supercooled liquid water regions were maintained without significant <br />depletion for over 10 minutes. In the present case, these regions occurred over the lower <br />regions of the Atlas mountains. In convective clouds over the plains, spherical panicles <br />up to 1 mm in diameter were observed. Model calculations show that these panicles were <br />likely grown by direct coalescence with cloud droplets, and subsequent riming after <br />freezing. <br />During the post frontal period of the storm (Dec. 9), stratocumulus clouds were <br />penetrated by the UND aircraft. In the stratiform regions of the cloud (cloud top near -16 <br />C), aggregates of dendrites and stellars were observed at -10 C, suggesting an efficient <br />primary nucleation process near cloud top in these clouds. In the convective portions of <br />these clouds at -10 C. drops, frozen drops, and spherical graupel were observed, <br />suggesting an active coalescence process, followed by drop freezing and subsequent <br />riming. High liquid water contents close to adiabatic were observed for significant <br />periods of time in many of these clouds, suggesting that depletion of liquid water by <br />entrainment and growing ice panicles was not very efficient. Since the convection was <br />usually embedded within a stratiform cloud layer, the environmental air entrained into <br />the growing cloud laterally will consist to a large extent of cloudy air. and thus dilution <br />by entrainment will not be as significant as for an isolated cumulus cloud. Updrafts in <br />these convective clouds were generally moderate (4-6 ms-1 ), allowing the particles in the <br />