Diagnosis of a Moist Thermodynamic Advection Parameter in Heavy-Rainfall Events

A moist thermodynamic advection parameter, defined as an absolute value ofthe dot product of horizontal gradients of three-dimensional potentialtemperature advection and general potential temperature, is introduced todiagnose frontal heavy rainfall events in the north of China. It is shownthat the parameter is closely related to observed 6-h accumulative surfacerainfall and simulated cloud hydrometeors. Since the parameter is capable ofdescribing the typical vertical structural characteristics of dynamic,thermodynamic and water vapor fields above a strong precipitation regionnear the front surface, it may serve as a physical tracker to detectprecipitable weather systems near to a front.A tendency equation of the parameter was derived in Cartesian coordinatesand calculated with the simulation output data of a heavy rainfall event.Results revealed that the advection of the parameter by thethree-dimensional velocity vector, the covariance of potential temperatureadvection by local change of the velocity vector and general potentialtemperature, and the interaction between potential temperature advection andthe source or sink of general potential temperature, accounted for localchange in the parameter. This indicated that the parameter was determined bya combination of dynamic processes and cloud microphysical processes.