Variations in the Particle Size Distribution of Shallow Warm Rain Determined Using Measurements from the Global Precipitation Measurement (GPM) Dual-frequency Precipitation Radar (DPR)

The size distribution of raindrops has important impacts, not only on the precipitation rate, but also on the latent heat release, as well as the radiative and remote sensing properties of the precipitation. It is therefore important to better understand the spatial and temporal variations of these properties through changing weather patterns and atmospheric compositions. We utilize a simple retrieval method to determine the relative magnitude of the mass-weighted drop diameter (d_m) of the precipitation particle size distribution (PSD) of warm rain coming from shallow cumulus clouds using 4 years (4/2014 - 3/2018) of measurements from the Global Precipitation Measurement (GPM) Dual-frequency Precipitation Radar (DPR) operating in both Ku (13.6 GHz) and Ka (35.6 GHz) band. Contiguous raining pixels observed by the DPR are grouped into precipitation features (PFs). Shallow isolated PFs are selected from this dataset as PFs with an area of 4 pixels or fewer and a maximum echo top height lower than the freezing level, determined by collocated ECMWF reanalysis data.

Among other remote sensing properties, d_m is related to the Ku radar reflectivity (Z_Ku) and the dual frequency ratio (DFR; Z_Ku - Z_Ka). A look-up table showing d_m as a function of Z_Ku and DFR is determined by calculating Z_Ku and DFR from one year of ground based DSD measurements taken during NOAA's Hydrometeorology Testbed-Southeast Pilot Study (HMT-SEPS) and interpolating these points to a 0.1 Z_Ku by 0.1 DFR grid. The look-up table is used to estimate the approximate d_m of the shallow PFs based on the DPR measurements. The spatial and temporal distributions of d_m are examined in an attempt to study the variations in the PSD. By investigating the relative changes in d_m in regions and seasons with different surface types and prevailing meteorological conditions, we can attempt to determine the qualitative effects of these conditions on the PSD.