Synoptic and Mesoscale Circulations and Precipitation Mechanisms in Shallow Upslope Storms over the Western High Plains

Abstract

It is generally believed that synoptically driven storms, with Offing induced or enhanced by upslope flow over the high plains, produce most of the winter precipitation in eastern Colorado. Two extremely different circulations, the fully developed extratropical cyclone and the shallow arctic anticyclone, bound the range of upslope circulations. Two cases involving shallow upslope circulations were studied in detail for this work. Aircraft, standard synoptic scale and selected mesoscale data were available for the case studies. The synoptic and (in one case) mesoscale circulations, characteristics of the consequent upslope and overlying midlevel stratiform clouds, and the microphysical processes that generated the precipitation from these events were examined. Dynamically and microphysically, these cases were among the simplest of the varied upslope storm systems. The arctic air masses were about 100 mb in thickness. The troposphere in and above the arctic air was potentially stable in both cases. The upslope clouds resulted from topography induced upward air motions associated with easterly flow. The easterly flow was caused by horizontal pressure gradients within the anticyclones. In one case, mesoscale analyses revealed that the local topography retarded and diverted the approaching arctic air until it became deep enough to flood the entire area. The observed upslope cloud layers formed within the cold air mass. The limited available moisture was derived from local sources and the arctic air itself. Water contents were generally ?0.1 g m?3 or less in all cloud layers. Some aircraft icing confirmed the presence of the liquid, and water saturation prevailed in the clouds. Heterogeneous nucleation (primary ice generation) was the most likely source for ice particles in both cases. Ice multiplication could be neither confirmed nor denied. Once nucleated, ice crystals grew predominantly by vapor deposition, to produce some crystals with diameters as large as 2 mm. However, aggregational growth was observed in the storm with the warmest cloud temperatures, and accretional growth was possible in the storm with the greatest water contents. Natural seeding of the upslope clouds by ice particles failing from the midlevel clouds occurred in both cases. Survival of the crystals in descent between the cloud layers was strongly regulated by the atmospheric ice saturation ratio. Crystal growth in the clear-air occurred in one case, whereas substantial sublimation occurred in the other.