Estimation and Analysis of Climate-Scale Rainfall over the Tropical Pacific

Wesley Berg

Colorado Center for Astrodynamics Research
Department of Aerospace Engineering Sciences
University of Colorado, Boulder, CO

May 1993


The relatively recent advent of spaceborne passive microwave technology, which has the ability to see beyond the cloud tops viewed by traditional visible and infrared sensors, has provided new prospects for global rainfall estimation. Using data from the Special Sensor Microwave/Imager (SSM/I) a database of monthly rainfall estimates has been produced over the global oceans for a three and one half year period from July 1987 through December 1990. Comparisons with in-situ observations from a group of Pacific atoll rain gauge sites were used to calibrate the monthly estimates. These estimates were then compared to monthly rainfall estimates made from visible and infrared satellite observations. Based on this comparison, the SSM/I estimates were found to be as good or better than the other techniques, although the sampling provided by the SSM/I was determined to be the major limiting factor. An quantitative assessment of the errors revealed the uncertainty in the monthly SSM/I rainfall estimates averaged over 2.5 degree latitude/longitude bins to be on the order of 20 to 25% of the monthly values. A subsequent analysis of the monthly SSM/I rainfall time series revealed both the major features of the precipitation distributions and the seasonal fluctuations to agree with expected values. In addition, evidence of a strong interannual signal corresponding to the 1986-87 El Nino and the ensuing 1988-89 La Nina was found. Finally, an EOF analysis of the resulting monthly rainfall time series was performed which showed strong seasonal and interannual signals as well as evidence of variability over semiannual and intraseasonal time scales.