Comparison of Drop Size Distribution Parameter (D0) and Rain Rate from S-Band Dual-Polarized Ground Radar, TRMM-Precipitation Radar (PR) and Combined PR/TMI: Two events from Kwajalein Atoll

The estimation of the DSD parameter (median volume diameter or D0) and rain rate from the TRMM-Precipitation Radar (or PR) as well from combined PR-TMI (TRMM microwave imager) algorithms are examined in this study for two TRMM satellite overpasses near Kwajalein Atoll. An operational dual-polarized S-band radar (KPOL) located in Kwajalein is central as the only TRMM ground validation site for measurement of precipitation over the “open” ocean. It is well-known that the accuracy of the TRMM-PR in retrieving D0 and R is better for precipitation over ocean than over land based on more stable surface reference technique for estimating the path integrated attenuation. Also, combined PR-TMI methods can only usefully be applied for precipitation events over the “open” ocean due to better knowledge of the surface emissivity. In this paper, we retrieve D0 and R from KPOL radar using Zh and Zdr data for the two TRMM overpass events over Kwajalein (08 September and 26 October 2008). Our main objective is to see if the D0 retrieved from PR or the combined PR-TMI algorithms are in agreement with KPOL-derived values (based on Zdr only). We also compare the variation of D0 versus R for convective rain “pixels” from KPOL, PR and PR-TMI. It is shown that the combined PR-TMI optimal estimation scheme does indeed adjust the D0 in the “correct” direction on average from the a priori state for the two events, if the KPOL data are considered to be the ground “truth”. This “correct” adjustment may be considered as indirect evidence of the value added by the TMI brightness temperatures in the combined PR-TMI variational scheme, at least for the 2 overpass event considered here.