Warm-Rain Precipitation over Complex Terrain: a New Polarimetric Classification Scheme

Accurate quantitative precipitation estimation over mountainous basins is of great importance because of their susceptibility to natural hazards. It is generally hard to obtain reliable precipitation information over complex areas, due to the scarce coverage of ground sensors associated to partial beam blockage and high elevation of the study sites. Warm-rain processes are characterized by precipitation growth due to collision-coalescence of droplets in the liquid layer of a cloud. These processes lead to high precipitation rates and have been observed in several flash flood events in complex terrain. Their characteristics are often difficult to identify. X-band mobile dual-polarization radars located in complex terrain areas provide fundamental high-resolution information on raindrop sizes, shapes, and particle size distributions (PSDs). This study analyzes a dataset collected during the IPHEx field campaign (North Carolina, US) over a mountainous basin, in which the NOXP X-band dual-polarimetric radar was used along with rain gauges, Parsivel and 2DVD disdrometers. Polarimetric variables are used to generate the Cao-Zhang and Kumjian-Ryzhkov parameter spaces, which provide clues on the segregation of warm-rain precipitation from convective and stratiform processes. A new classification algorithm is proposed to identify warm-rain precipitation by merging the information coming from the polarimetric parameter spaces. The method is validated with disdrometer PSDs for the classified warm-rain events: they show a clear shift towards high concentration of small median volume diameter (D0) drops compared to the PSD climatology for the field campaign. Finally, the rainfall rate derived from radar data without the assumption of warm-rain is found to underestimate the precipitation at the ground collected by rain gauges, suggesting the necessity of a correct classification of precipitation and subsequent estimate of rainfall rates. The proposed next-generation polarimetric classification scheme can be used to identify warm-rain precipitation worldwide over any kind of topographies. Future development of this work will aim at identifying warm-rain precipitation worldwide making use of the Ka- and Ku-band channels of the Dual-frequency Polarization Radar on-board the GPM core observatory.