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

Abstract

Global estimates of monthly, seasonal, and annual oceanic rainfall are computed for a period of 1 year using data from the Special Sensor Microwave/Imager (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 in 1° 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 raingage data are performed.