Study and Tests of Improved Rain Estimates from the TRMM Precipitation Radar

Abstract

Rain rate R estimation from the 2A-25 profiling algorithm of the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) is analyzed in two ways. Standard results from the operating version-5 algorithm are compared with those from the previous version 4. Also, various adjustments of the involved rain relationships in version 4 are explored, which leads to the proposal of two alternatives to the standard rain rate (Rstd-V4). The first one, (RN0), is based on N*0-scaled relations exploiting the concept of normalized Γ-shaped drop size distributions; the second one, (RkR), relies on using constant R?k instead of constant R?Z relation as in the standard, where Z is reflectivity and k is attenuation coefficient. Error analysis points out a lower sensitivity of the alternative estimates to errors in radar calibration, or initial relations, than the standard. Results from a set of PR data, over ocean and land, show that the version-4 alternatives, and version-5 standard (Rstd-V5), produce more rain than the version-4 standard, which may correct for some reported underestimation. These approaches are tested via point-to-point comparisons of 3D PR-derived Z and R fields (versions 4 and 5) with ?reference? fields derived from airborne dual-beam radar on board a National Oceanic and Atmospheric Administration P3-42 aircraft in Hurricanes Bonnie and Brett, for good cases of TRMM overpasses over the ocean. In the comparison domains, Bonnie is dominated by stratiform rain, and Brett includes convective and stratiform rain. In stratiform rain, the mean difference in Z, accounting for different frequencies and scanning geometries of both radars, lies within the uncertainty margin of residual errors in the radar calibrations. Also, the PR mean rain-rate estimates, RkR and Rstd-V5, agree fairly well with the P3 estimate, RP3, whereas Rstd-V4 and RN0 respectively underestimate and overestimate RP3. In convective rain (Brett case), the PR estimates of Z and R largely exceed the P3 counterparts. It is suggested that this may result from a corruption of the surface-reference estimation of the total path attenuation caused by strong surface winds.