Diagnostic analyses of dry intrusion and nonuniformly saturated instability during a rainfall event

Abstract

Using the one by one degree National Centers for Environmental Prediction final data (details of data set can be found in http://dss.ucar.edu/datasets/ ds083.2/) and numerical mesoscale results simulated by the WRF model, the characteristics of dry intrusion during a heavy rainfall event in northern China are studied. Since dry intrusion is a Lagrangian feature and should be identified with the aid of trajectory calculations, in this paper this process is simulated by the Lagrangian Flexpart particle dispersion model, and the trajectories of moist and dry air parcels are depicted and traced using a Lagrangian method. It is found that the cold, dry air with high potential vorticity shows up prior to the start of precipitation and persists and extends downward to lower levels during the raining periods. As the dry, cold air from upper levels intrudes into the warm, moist sector in the lower troposphere, the two air masses with different characteristics mix causing nonuniform saturation to occur in the atmosphere. On the basis of this nonuniform saturation concept, a new Brunt-Väisälä frequency for nonuniformly saturated moist atmospheres is derived and applied to the diagnostic study of precipitation cases. It is found to be more appropriate for depicting the instability of rainy regions than those stability indexes used in dry and moist saturated atmosphere. Copyright 2010 by the American Geophysical Union.