The Dynamical and Microphysical Structure of an Occluded Frontal System and its Modification by Orography

Abstract

An occluded front moving over Washington State was investigated with serial rawinsonde ascents, aircraft penetrations, raingage measurements and conventional observations. The rawinsonde data showed that the frontal system contained alternate mesoscale tongues of air with low and high values of wet?bulb potential temperature (??). These tongues included a pre?frontal surge of low??? air aloft, centered at about the 700 mb level, a low?evel high??? tongue along the front, and two post?frontal high??? tongues. The low? and high??? tongues were of the order of 50?100 km in width. The frontal precipitation was confined to a mesoscale band, 80 km in width, along the front. The cloud associated with this hand was characterized by a vertical circulation similar to, but much less vigorous than, an organized convective system. Cloud microphysical data indicated that a narrow cumulus?scale updraft zone was located near the leading edge of the frontal cloud. The concentrations of ice particles in the frontal cloud were probably on the order of 50 ?minus;1 but could have been as high as 500 ?minus;1. These values exceed the optimum for the efficient release of precipitation by the Bergeron?Findeisen process. However, cloud particles collected in flight revealed that both riming and aggregation played important roles in particle growth within the frontal cloud. As the frontal system passed over the Cascade Mountains, the amount of cloud and precipitation ahead of the front decreased while that behind the front increased. The decrease in cloudiness ahead of the front is attributed to the low?level moisture source being cut off by the mountains. The increase in clouds behind the front was apparently due to orographic lifting of the cold air. This study confirms that mesoscale processes play an important role in the production of frontal precipitation. It also indicates that the microphysical aspects of precipitation growth are more complex than classical models would suggest.