An Atmospheric Dry Intrusion Parameter and Its Application

Dry intrusion plays an important role in the explosive development of cyclones and the evolution of cold fronts. Characteristics of dry intrusion during a rainfall event that occurred in northern China are analyzed in detail in this paper. The IM (ingredients-based methodology) developed by Doswell et al. in 1996 and Wetzel and Martin in 2001 is utilized. All the physical representations of dry intrusion defined in the past studies, such as low relative humidity, cold advection, and high potential vorticity (on either isobaric or isentropic surfaces), are combined into a simple and convenient physical parameter to characterize dry intrusion. This is a new attempt to extend the IM that was primarily applied to research on heavy rainfall to the study of dry intrusion. The new dry intrusion parameter is used to analyze the isentropic evolution of dry intrusion during the rainfall event. The results show that this parameter can better quantify the intensity of dry intrusion and diagnose its evolution shown in satellite infrared and water vapor imageries.It is found that dry intrusion maintains during the rainfall period. The intensity of precipitation increases with the increasing dry intrusion, which has pushed the rainy region southeastward. From the results on the isentropic surface and the corresponding isobaric surface,it is inferred that the analyses of dry intrusion on both surfaces are consistent with each other. The isentropic analysis of dry intrusion reveals that cold and dry air at the upper level overruns that in the lower troposphere where moist and warm air is located.Thus, potential instability is built up in the vertical direction, which favors the occurrence of precipitation.In practice,we may identify dry intrusion regions by tracking strong signals of the dry intrusion parameter,and further identify the instability near the dry intrusion regions.This will aid in improving the accuracy of precipitation forecast.