Properties of Clouds and Precipitation Inferred from TRMM PR and TMI

Two years of data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and Microwave Imager (TMI) over ocean have been analyzed with a combined radar-radiometer retrieval algorithm. The retrieval framework is based upon optimal estimation theory and adjusts three parameters describing the raindrop size distribution (DSD), ice particle size distribution (PSD), and cloud water path (cLWP) in each radar profile, in addition to retrieving non-precipitation parameters (wind, water vapor, and cLWP) in profiles with no radar echo. The combined technique is able to retrieve two independent parameters of the DSD. Comparison with ground disdrometers at multiple locations reveals that the combined algorithm is able to reproduce observed climatological differences of median drop size in moderate and light rain (< 10 mm/hr), where radar-only methods such as 2A25 have relied upon default DSD assumptions. Analysis of the retrievals reveals that three correlated sets of variables representing storm dynamics, background environment, and cloud microphysics account for approximately 50% of the variability in the absolute and reflectivity-normalized median drop size. Regional patterns of DSD and the sources of variability identified herein are consistent with previous work documenting regional DSD properties in convective rain, but are also extended to lighter intensities. In particular, mid-latitude regions and tropical regions near land tend to have larger drops for a given reflectivity due to higher contributions from stratiform and deep convective rain, whereas the smallest drops are found in the eastern Pacific Intertropical Convergence Zone, where shallow convection is most prevalent.