Evaluation of S-Band Radar Rain Rate Retrieval Algorithms and Precipitation Variability Over a Dense Rain Gauge Network

The Global Precipitation Measurement (GPM) Precipitation Research Facility at NASA Wallops Flight Facility has recorded high temporal (50 second) and spatial (250 m) resolution PPI radar data over a dense rain gauge network using NASA's research-quality NPOL (S-band, dual-polarization) radar. The rain gauge network contains 20 tipping bucket gauges distributed through an approximate 25 km² grid located 30km from the NPOL site. Precipitation rates derived from three polarimetric retrieval algorithms (in polar space) will be interpolated to a 0.5 km horizontal and vertical resolution grid directly over the gauge network. Rain rate bias statistics from the polarimetric retrievals and the non-polarimetric Multi-Radar/Multi-Sensor System (MRMS Q3) gauge-adjusted Z-R retrieval will be determined via independent gauge comparison from six cases individually and collectively. The selected cases from June 2015 span the spectrum from light rain, strong convection, widespread stratiform, and a modified tropical storm. The analysis will investigate how the bias statistics from the polarimetric and MRMS retrievals vary from event-to-event and in total, and determine if there is a preferred retrieval that is most appropriate for a specific event type. In addition, the dimensions of the dense gauge network were intentionally set to be nearly identical to the GPM Dual-frequency Precipitation Radar (DPR) nadir footprint-scale of 25 km². A secondary analysis will assess the degree of rainfall accumulation variability (spatial and temporal) within the sub-grid scale footprint. Initial analyses of rain gauge data alone indicate that variability within the grid can exceed 300% depending on precipitation event type.