Nowcasting thinking and method of flash heavy rain

This paper overview the main idea and methodology used in subjective nowcast of flash heavy rain. Flash heavy rain refers to therainfall events with one-hour accumulated precipitation over 20 mm or three-hour accumulated precipitation over 50 mm. The identificationof a flash heavy rain event relies on two factors: the rainfall rate and the duration of rainfall. The rainfall estimation is based on the empirical relation between radar reflectivity factor (Z) and rainfall intensity (R), i.e., Z-R relationship. The simplest approach to estimate precipitation is to divide the convections into two categories:“continental severe convection”and“tropical maritime convection”, and then apply different Z-R relationship for each type. The primary errors in the precipitation estimates are resulted from improper Z-R relationship, terrain blockage, hail“pollution”, attenuation by heavy rain and hail, inaccurate radar calibration, attenuation by wetted radome, etc. Another important  factor in flash heavy rain nowcast is to estimate the duration of heavy rain. Both the larger of the size of heavy rain area is along the echo’s moving direction and the slower the echo moves, the longer the severe precipitation lasts. The backward propagation type MCS is more likely to lead to flash heavy rain for its relatively slow moving speed. In the case of strong vertical wind shear, the MβCS embedded in mesocyclones or wider range vortices have a higher probability to produce flash heavy rain.