Comparison of TRMM Rain-Rate Retrievals in Tropical Cyclones

Abstract

  Rain rates from four algorithms are examined in tropical cyclones (TCs). Old and new versions of the Remote Sensing Systems (RSS) rain estimates (RSS V03 and RSS V04) are compared with the standard version 6 TMI 2A12 and PR 2A25 algorithms, after averaging those down to the 0.25° scale used by RSS. RSS V03 produces more rain by a factor of two than the others, frequently assigning rain rates up to 25 mm h^-1 (which is an internal limit for that product). Among the three current algorithms, PR 2A25 produces the most rain when averaged over a 0 to 100 km radius in hurricanes. This results from PR 2A25 assigning much higher grid-scale rain rates (up to 100 mm h^-1) in the small fraction of grid boxes having heaviest rain. TMI 2A12 has the least rain, assigning moderate rain rates (5 mm h^-1) to more grid boxes than the other products. The differences between algorithms are greatest for the inner regions of Category 3 to 5 hurricanes. In weaker TCs, or further away from the TC center, the three current algorithms tend to agree on mean rain rate. However, they arrive at these areal means from completely different distributions of grid-scale rain rates. PR 2A25 gets a greater fraction of its rain from grid boxes having high rain rates, with little contribution from the light and moderate rain rates. RSS V04 gets much of its rain from grid boxes with 10 mm h^-1. TMI 2A12 gets less rain around 10 mm h^-1, but balances that with greater contributions both from the occasional higher (15 mm h^-1) and more common lower (5 mm h^-1) rain rates. At the 0.25° scale, the TMI-based products are better correlated with each other than with PR 2A25. The RSS products are better correlated with PR 2A25 than TMI 2A12 is. All the correlations increase when more zero-rain or light-rain grid boxes are included (i.e., the weaker TCs or greater distances from the center).