Validation of TRMM PR rain estimates using a radiative transfer model

The TRMM PR estimates have been commonly used as reference to validate rainfall estimates from passive microwave imagers. Although the PR can provide height information based upon the time delay of the precipitation-backscattered return power, the backscatter quantity measured by the PR is sensitive to variant properties of the drop size distribution (DSD), leading to errors. Thus, the PR rainfall estimates need to be verified. In the previous study, we investigated consistency between TMI-observed brightness temperatures (TBs) at 10 and 19 GHz channels and those simulated from the PR rainfall estimates for ITCZ rain systems during the warm phase of the 1997/1998 ENSO using a radiative transfer model (Shige et al. 2006, GRL). Simulated TBs at 10 GHz from PR V6 exhibit better agreement with observed ones than those from PR V5, implying the algorithm improvements. However, discrepancies at 19 GHz suggest that uncertainty in the assumed DSD still remains in PR V6.

Here, the previous work by Shige et al. is extended to examine regional biases that exist between the PR2A25 V6 and TMI2A12 V6 algorithms. Precipitation systems over western Pacific and eastern Pacific during October 2000 are considered in the current investigation. While differences between PR2A25 V6 and TMI2A12 V6 products are small over western Pacific, PR2A25 V6 estimates are lower than TMI2A12 V6 estimates. Simulated TBs at 19 GHz from PR V6 exhibit less agreement with observed ones over eastern Pacific than western Pacific. Large differences over eastern Pacific are found for light rainfall rates where the surface reference technique (SRT) perturbs the initial DSD model only slightly. It is also shown that large differences are associated with shallower rain systems in the east. It is inferred that the initial DSD models used in PR2A25 is not appropriate for precipitation systems over eastern Pacific that are shallower than those in the west Pacific.