A Comparative Study of GPM/DPR DSD Parameters at the NASA Wallops Precipitation Research Facility

The mass weighted mean drop diameter (D_m) and normalized intercept parameter (N_w) within GPM algorithms describe the raindrop size distribution within an atmospheric column. The GPM Level 1 science requirements state that D_m should be estimated within ±0.5 mm with respect to GV observations with no restrictions set on Nw.

To investigate if D_m requirement is being met, the System for Integrating Multiplatform Data to Build the Atmospheric Column (SIMBA) was developed and utilized to unify space and ground platform retrievals into a defined gridded column for inter-comparisons. The study compares D_m and N_w retrievals from the GPM Dual-Precipitation Radar (DPR), NPOL and NEXRAD-88D KDOX radar, and multiple 2DVD and PARSIVEL disdrometers from numerous GPM overpasses over the NASA Wallops Precipitation Research Facility (PRF). The SIMBA columns (5 km x 5 km horizontal x 6 km vertical) were centered on an instrument test-pad at the PRF and have a 500 m horizontal and 250 m vertical resolution for footprint-to-footprint variability analysis. A series of Wallops GPM overpass events (from 2014 to present) are evaluated for any biases between GPM DPR Level-2 Version 5 estimated D_m and NPOL/KDOX retrieved D_m at each horizontally averaged level in the column and near the surface with 2DVDs and PARSIVELs. GV radar estimates of D_m and N_w rely on empirical models using dual-polarization variables (horizontal and differential reflectivity) developed with disdrometer data, while DPR requires Ka/Ku dual-frequency reflectivity measurements.

A cluster of GPM overpass SIMBA columns (mostly stratiform samples) suggests D_m differences are generally within ±0.50 mm throughout the column. Near the surface, the lowest clutter free bin/height DPR measurements are compared against surface disdrometers that also yield D_m differences within ±0.50 mm. Interestingly, Log (N_w) profiles suggest DPR underestimates relative to NPOL and KDOX measurements at each level in the column. With limited GPM overpass samples over the Wallops PRF and the associated columns thus far, D_m Level 1 science requirement is being met.