A model for estimation of rain rate on tropical land from TRMM microwave imager radiometer observations

Abstract

Over the tropical land regions scatter plots of the rain rate (RPR), deduced from the TRMM Precipitation Radar (PR) versus the observed 85 GHz brightness temperature (T85v) made by the TRMM Microwave Imager (TMI) radiometer, for a period of a season over a given geographic region of 3°×5° (lat × Ion), indicate that there are two maxima in rain rate. One strong maximum occurs when T85v has a value of about 220 K, and the other weaker one when T85v is much colder ∼150 K. Also these two maxima are vividly revealed in plots of RPR vs. (T19v-T37v). The strong maximum occurs when (T19v-T37v) is ∼20 K and the weaker maximum when it is greater than ∼40 K. Together with the help of a) earlier investigations based on airborne Doppler Radar observations and b) radiative transfer theoretical simulations, we infer the strong maximum is a result of relatively weak scattering due to super cooled rain drops and water coated ice hydrometeors associated with a developing thunderstorm (Cb) that has a strong updraft. The other maximum is associated with strong scattering due to ice particles that are formed when the updraft collapses and the rain from the Cb is transitioning from convective type to stratiform type. Incorporating these ideas with a view to improve the estimation of rain rate from existing operational method applicable to the tropical land areas, we have developed a rain retrieval model. This model utilizes two parameters, that have a horizontal scale of ∼20 km, deduced from the TMI measurements at 10, 19, 21 and 37 GHz(T10v, T19v, T21v, T37v). The third parameter in the model, namely the mean horizontal gradient <dT85v/dr> (K km-1) within the 20 km scale, is deduced from TMI measurements at 85 GHz that have the scale of ∼5 km. Utilizing these parameters our retrieval model is formulated to yield instantaneous rain rate on a scale of 20 km. This retrieval model is initially tuned with the help of a limited amount of PR rain rate. After initial tuning, the model is applied to widely different tropical land areas, and for different seasons. Our estimates of instantaneous rain rate, on a scale of 20 km, and seasonal averages on a scale of 3°×5° (lat × Ion) agree better with PR than that given by the operational TMI rain retrievals. © 2005, Meteorological Society of Japan.