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Joey F. Boatman and Roger F. Reinking


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.

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Joey F. Boatman and August H. Auer Jr.


The entrainment process and its resultant effects on the microphysics and dynamics within cumuli are not yet clearly understood. This research was undertaken to discover the role which cloud top plays in the entrainment process and to determine whether observed downdraft magnitudes could be explained on the basis of evaporative cooling.

An instrumented King Air research aircraft was used to acquire thermodynamic, microphysical and dynamical data within and near cumulus clouds of the high plains and midwestern United States. Temperature and liquid water content measurements made within the clouds were used to discover the source for any entrained air via a thermodynamic treatment initially pursued by Dufour (1956), and later expanded by Paluch (1979) and Betts (1982). The entrainment source regions found were either at or above the aircraft sampling level in 78 of the 87 cases examined. It was found that the environment within 20 mb of cloud top was the source for entrained air in these 78 cases. Evaporative cooling could not effectively transport air from above to the sampling level in the nine cases where entrained air from below the aircraft sampling level was measured.

Vertical velocity measurements acquired in the observed clouds were compared with those predicted by evaporative cooling, as suggested by Squires (1958b). It was concluded that evaporative cooling could adequately explain the magnitude of the observed downdrafts in the 51 cases where a comparison between the predicted and observed downdraft magnitude was possible.

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