A TRMM microwave radiometer rain rate estimation method with convective and stratiform discrimination

Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) radiomete r brightness temperature data, in the 85 GHz channel (T85) reveal distinct local minima (T85(min)) in a regional map containing a Mesoscale Convective System (MCS). This is because of relatively small footprint size (similar to 5.5 km) and strong extinction properties in this channel of the TMI. A m ap of surface rain rate for that region, deduced from simultaneous measurem ents made by the Precipitation Radar (PR) on board the TRMM satellite, reve als that these T85(min), produced by scattering, correspond to local PR rai n maxima. Utilizing the PR rain rate map as a guide, we infer empirically f rom TMI data the presence of three different kinds of thunderstorms or Cbs. These Cbs are classified as young, mature, and decaying types, and are ass umed to have a scale of about 20 km on the average. Two parameters are used to classify these three kinds of Cbs based on the T85 data: a) the magnitu de of the scattering depression deduced from local T85(min), and b) the mea n horizontal gradient of T85 around such minima. Knowing the category of a given Cb, we can estimate the rain rate associated with it. Such estimation is done with the help of relationships linking T85(min) to rain rate in ea ch Cb type. Similarly, a weak background rain rate in all the areas where T 85 is less than 260 K is deduced with another relationship linking T85 to r ain rate. In our rain retrieval model, this background rain constitutes str atiform rain where the Cbs are absent. Initially, these relationships are o ptimized or tuned utilizing the PR and TMI data of a few MCS events. After such tuning, the model is applied to independent MCS cases. The areal distr ibution of light (1-10 mmhr(-1)), moderate (10-20 mmhr(-1)), and intense (g reater than or equal to 20 mmhr(-1)) rain rates are retrieved satisfactoril y. Accuracy in the estimates of the light, moderate, and intense rain areas and the mean rain rates associated with such areas in these independent MC S cases is on the average about 15 %. Taking advantage of this ability of o ur retrieval method, one could derive the latent heat input into the atmosp here over the 760 km wide swath of the TMI radiometer in the tropics.