The Variability of Tropical Precipitation Profiles and Its Impact on Microwave Brightness Temperatures as Inferred from TRMM Data

Abstract

Precipitation radar and microwave radiometer data collected by the Tropical Rainfall Measuring Mission (TRMM) satellite are used to study the variability of precipitation profiles and the relationship between precipitation profile and microwave brightness temperature. The variability has been examined using empirical orthogonal function (EOF) analysis and microwave emission and scattering signatures. Precipitation profiles are divided into three groups according to emission signatures at 19.4 GHz and three groups according to scattering signatures at 85.5 GHz. For stratiform rain, the differences of vertical precipitation profiles among these groups are small and are mainly seen in the slope of profiles below the freezing level. However, clear differences in vertical precipitation profiles can be found among the deep-convective rain groups. The maximum rainfall rate occurs at a considerably lower altitude when low liquid-emission or low ice-scattering signatures are observed. When emission or scattering signatures are high, precipitation profiles peak near the freezing level, a feature that is similar to the one in stratiform precipitation profiles. The three patterns of the vertical profiles derived from microwave signatures are very similar to the three patterns derived by EOF analysis. This similarity suggests that the three patterns derived by microwave signatures represent the most significant variability in vertical precipitation profiles. Results also show that, for the same near-surface rainfall rate, the pixel group with anomalously high microwave emission also shows anomalously high microwave scattering, and vice versa, suggesting that the liquid and ice water amounts in tropical rains are correlated over scales the size of a satellite pixel. It is also found that, for a given surface rainfall rate, the brightness-temperature differences among the pixel groups are large, highlighting the importance of vertical precipitation profile in determining upwelling microwave radiation and, therefore, the need to incorporate realistic precipitation profile information in rain retrieval algorithms.