Improved Algortithm to Estimate Convestive and Stratiform Rain From TRMM Microwave Radiometer Data

A regional map of 85 GHz brightness temperature (T85), observed by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) radiometer, reveal distinct local minima in a Mesoscale Convective System (MCS). This is because of relatively small footprint size (~ 5.5 km) and strong extinction properties in this channel of the TMI. These minima produced by scattering correspond to local maxima in a map of rain rate for that region deduced from simultaneous measurements made by the Precipitation Radar (PR) on board the TRMM satellite. On the basis of such similarities, we can infer from TMI data the presence of three different kinds of "thunderstorms" or Cbs: young, mature, and decaying. These Cbs have a scale of about 20 km on the average. Two parameters enable us to infer these three kinds of Cbs objectively: a) the magnitude of scattering depression deduced from local T85 minima and b) the mean horizontal gradient of T85 around such minima. Also, using these parameters, we can determine the rain rate from the Cbs. Utilizing a similar procedure, we can determine the areally extensive rain rate from the T85 map where there are no minima.

In this retrieval model, there is a weak background rain rate in the entire MCS area, where T85 is typically less than 260 K. When the local minima in T85 are absent, this background constitutes non-Cb stratiform rain. In the small areas of 20 km diameter surrounding each local T85 minimum, the Cb rain is super imposed on this background to give the total rain rate. Initially, this retrieval model is tuned with PR obsevations over several MCS cases. After such tuning, the model is applied to independent MCS cases. The areal distribution of intense (> 20 mm/hr), moderate (10-20 mm/hr), and light (1-10 mm/hr) rain rates are retrieved satisfactorally. The area where the rain rate exceeds 20 mmhr-1, and the mean rain rate in that area, is estimated to an accuracy better than 10 %, on the average, in these independent MCS cases. The light rain area and the corresponding mean rain rate in these cases is estimated on average with an accuracy of about 15 %. The area and the mean rain rate for the moderate rain region is retrieved with an accuracy that is also about 15 %.