Validation of west and east Pacific rainfall estimates from the TRMM PR using a radiative transfer model

The differences between PR2A25 [Iguchi et al., 2000; Iguchi, 2007] and TMI2A12 [Kummerow et al., 2001; Olson et al., 2006] standard rainfall products over the east Pacific are well known [Berg et al., 2002, 2006]. The TMI2A12 is larger than the PR2A25 over the east Pacific, while they are in good agreement over the west Pacific, resulting in reverse west-east gradients of rainfall. The Global Satellite Mapping of Precipitation (GSMaP) algorithm for TMI [Kubota et al., 2007], which is a PR- consistent advanced microwave radiometer algorithm based on the study in Aonashi and Liu [2000], is also larger than the PR2A25 over the east Pacific, while they are in good agreement over the west Pacific. Here, we have shown that discrepancies between TMI-observed TBs at 19 GHz channel and those simulated from the PR rainfall estimates over the east Pacific are larger than those over the west Pacific, especially for lower-to-medium range (small attenuation). It is hypothesized that the drop size distributions (DSDs) of rainfall over the east Pacific have stronger maritimity (i.e., more small to medium size raindrops) than the initial DSD model of PR2A25, representative of the “data-rich” west Pacific, leading to underestimate of rain. The hypothesis is consistent with the convective epsilon from alpha-adjustment procedure in the PR2A25 algorithm.

There are large differences in the vertical structure of precipitation between the west and east Pacific. Larger population of warm rain and development of weak ice precipitation are found in the east Pacific compared to the west Pacific. The differences support the notion that maritimity of precipitation is stronger for the east Pacific than the west Pacific. Rosenfeld and Ulbrich [2003] indicated that orographic lifting of maritime air can supply a large amount of condensates, which create a larger number of small raindrops compared to maritime rain. Hence, rainfall rate for a given reflectivity is larger for maritime orographic rain than for maritime rain. Low-level upward motion is much stronger in the east Pacific than in the west Pacific. Akin to orographic lifting, the low-level upward motion in the east Pacific presumably have active cloud drop coalescence and produce a large number of small raindrops and, hence, leading to stronger maritimity of precipitation for the east Pacific than the west Pacific. Boundary layer convergence associated with meridional SST gradients may drive the low-level upward motion, a mechanism proposed by Lindzen and Nigam [1987], and, hence, produce “orographic rainfall” over the east Pacific, leading to the PR underestimate of rainfall.