Application of specific differential phase as indicator for severe rainfall produced by shallow convection

The specific differential phase K_dp is defined as the slope of range profiles of the differential propagation phase shift Φ_dp between horizontal and vertical polarization states observed by polarimetric radar. The K_dp is an important parameter for meteorological applications because it is proportional to precipitation particle concentrations and size and closely related to rain intensity. Past studies showed that the high K_dp value above the environmental 0 °C level potentially is an early indicator of heavy rain produced by summertime deep convection.

In autumn and winter, the stratiform precipitation system is the primary source of rainfall in north Taiwan. Additionally, embedded convective cells could lead to intense rain rates. But these cells’ top is not always developed higher than 0 °C level. This study uses a C-band polarimetric radar located in north Taiwan to discuss the evolution of K_dp and related rainfall of several heavy rain events in autumn. The application of K_dp to quantitative rainfall estimation is also illustrated.

The result shows that the value of K_dp > 2° km^-1 is closely related to the movement and intensity of the severe rainfall area (> 60 mm h^-1). K_dp > 2.0° km^-1 occurs for more than 30 minutes, which is related to the location of rainfall of 100mm in 3 hours. The development height of K_dp >1.5° km^-1 reaches the melting level, or there is a core area with K_dp >3.0° km^-1 below the melting level, which will cause local heavy rainfall on the ground in the next 10 to 20 minutes (>10 mm in 10 minutes). K_dp > 3.0° km^-1 occurs for more than 1 hour, which is related to the rainfall of up to 200mm in 3 hours.