Determination of Mean Rainfall from the Special Sensor Microwave/Imager (SSM/I) Using a Mixed Lognormal Distribution

Wesley Berg

Colorado Center for Astrodynamics Research, University of Colorado, Boulder

Robert Chase

The Analytic Sciences Corporation, Reading, Massachusetts

ASTRACT

Global estimates of monthly, seasonal, and annual oceanic rainfall are computed for a period of one year using data from SSM/I. Instantaneous rainfall estimates are derived from brightness temperature values obtained from the satellite data using the Hughes D-Matrix algorithm which was originally developed by Environmental Research and Technology Inc. (ERT). The instantaneous rainfall estimates are stored into one degree square bins over the global oceans for each month. A mixed probability distribution combining a lognormal distribution describing the positive rainfall values and a spike at zero describing the observations indicating no rainfall is used to compute mean values. The resulting data for the period of interest are fitted to a lognormal distribution by using a maximum likelihood method. Mean values are computed for the mixed distribution and qualitative comparisons with published historical results as well as quantitative comparisons with corresponding in situ rain gauge data are performed.