An algorithm for retrieval of monthly rainfall over the oceans from the TRMM microwave imager (TMI)

One of the principal goals of the Tropical Rainfall Measuring Mission (TRMM) is to measure rainfall in the tropical and subtropical regions. In this study and through collective efforts, a multichannel algorithm has been developed for retrieval of monthly rain totals for 5° by 5° grid boxes over the oceans from the TMI data. This multichannel algorithm is based on relationships between rain rates and brightness temperatures (R-T) generated by a physically-based microwave radiative transfer model (RTM) for the different TMI channels. This RTM is based on the Wilheit et al. (1977) model with an updated water vapor absorption procedure, a water vapor absorption correction factor and reduced non-precipitating cloud liquid water (NCLW) contents which are consistent with many field experimental results.; The combination of R-T relationships for 19 and 21 GHz are used to obtain freezing level information for each pixel. The freezing level information is then used to solve for rain rates corresponding to the 10, 19 and 37 GHz brightness temperatures, respectively. Those rain rates are adjusted according to their corresponding beamfilling corrections and then accumulated to form rain histograms for each channel over each 5° by 5° grid box.; The rain rate histogram for each channel is processed so as not to have a cut-off at zero rain rate. The most probable rain rate retrieved by this algorithm is very close to zero. Any remaining offset of rain rate is shifted to zero accordingly.; In order to utilize the rain rates resulting from those three different channels, the rain rates derived from 37 and/or 19 GHz channels are “smoothed” to the resolution of the 10 GHz channel so that the rain rates retrieved from different channels represent the same area. The algorithm chooses the highest frequency possible among the 37,19 and 10 GHz channels. This gives us the greatest usable sensitivity to rain.; Global rainfall mappings clearly show the tropical large-scale feature such as heavy rainfall over the Intertropical Convergence Zone (ITCZ). Quantitative comparisons with the Pacific atoll rain data indicate that the current algorithm performs very well at least in the tropical regions.