Current state-of-the-art techniques for estimating rainfall from satellite observations provide contradictory information with regard to the response of tropical rainfall to interannual variability associated with the El Niņo/Southern Oscillation (ENSO). Recent results from passive microwave observations taken by the Tropical Rainfall Measuring Mission (TRMM) show significant interannual variability in tropical rainfall, however, corresponding retrievals from the TRMM Precipitation Radar (PR) indicate little or no ENSO related variability. In a recent intercomparison of climate models with MSU rainfall estimates over the global oceans, Soden [J. of Climate., vol. 13, pp. 538-549, 2000] concluded that ".. either the sensitivity of the tropical hydrological cycle to ENSO driven changes in SST is substantially underpredicted in existing climate models or that current satellite observations are inadequate to accurately monitor ENSO related changes in the tropical mean precipitation". Subtle differences in the relative increase/decrease between the west Pacific warm pool and the east Pacific ITCZ appears to be the main factor influencing the overall difference in the response of tropical rainfall to ENSO. Interestingly, retrievals from the two sensors show remarkably good agreement over land regions where passive microwave techniques rely solely on the scattering signature by ice particles aloft. The discrepancy is primarily over tropical ocean regions, and specifically over the Pacific, where passive microwave retrievals utilize a much more physically-based relationship between the observed brightness temperatures and the retrieved rainfall rates.

To understand the differences between these retrieval techniques and subsequently determine the true response of tropical rainfall to ENSO, we have examined the sensitivity of the retrieval algorithms to a number of different cloud properties. One result of particular interest to this issue is the discovery of a much larger difference between the height of the freezing level, or zero degree isotherm, and the radar bright band, or melting layer, in the east Pacific than in the west. This discrepancy, however, disappears during El Niņo. Since passive microwave retrieval techniques from SSM/I and TMI measure the total column liquid water, an accurate determination of the height of the rain column is critical to the determination of the surface rain rate. Other issues such as differences in the amount of ice aloft, variations in the drop size distribution, and other factors are also examined to determine what the true response of the total global tropical rainfall is to changes associated with ENSO.