Differences between East and West Pacific Rainfall Systems

Wesley Berg, Christian Kummerow, and Carlos A. Morales

Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado


A comparison of the structure of precipitation systems between selected east and west Pacific regions along the intertropical convergence zone (ITCZ) is made using a combination of satellite observations including vertical profile retrievals from the Tropical Rainfall Measuring Mission's (TRMM's) Precipitation Radar. The comparison focuses on the period from December 1999 to February 2000, which was chosen due to large discrepancies in satellite infrared and passive microwave rainfall retrievals. Storm systems over the east Pacific exhibit a number of significant differences from those over the west Pacific warm pool including shallower clouds with warmer cloud tops, a larger proportion of stratiform rain, less ice for similar amounts of rainwater, and a radar bright band or melting layer significantly farther below the freezing level.

These regional differences in the structure of precipitation systems between the east and west Pacific also exhibit seasonal and interannual variability. During the intense 1997/98 El Niño, warmer sea surface temperatures (SSTs) in the east Pacific led to precipitation systems with a very similar structure to those observed over the west. These differences in east versus west Pacific rainfall and changes associated with the El Niño-Southern Oscillation (ENSO) result in time-dependent regional biases in available long-term satellite precipitation datasets. Although all of the currently available infrared and passive microwave-based satellite retrievals exhibit similar spatial patterns and capture variability associated with ENSO, both the amplitude and sign of subtle climate signals, such as the response of tropical-mean rainfall to ENSO, depend on the retrieval algorithm used.