Bangladesh region is interesting regions in a core region of the Asian monsoon. The Bangladesh region is a very flat
region facing to the Bay of Bengal. The precipitaion characteristics are studied using the long-term Tropical Rainfall
Measuring Mission (TRMM) data. Over the Bangladesh, the stability of the atmosphere seems to affect the precipitation
system in the vertical profiles. In the pre-monsoon season, rain rate increases with height in the lower part of the profile,
while in the mature monsoon season, rain rate is nearly constant in the lower part of the profile. The structure of
precipitation system is more persistent and homogenous in the mature monsoon season. The rain top is higher in premonsoon season than in mature monsoon season. The rain total is generally determined by rain frequency. The
horizontal size of the precipitation systems is larger for pre-monsoon season than for mature monsoon season. In other
words, the precipitation system is small but many in the mature monsoon season. This fact may be explained that the
atmosphere is sufficently humid to be easily triggered by small liftings. These characteristics are reflected in the rain
retrieval biases in precipitation radar and microwave radiometers in space.
We have been developing a data-set of global land surface microwave emissivity calculated from 9-channel Bright ness Temperatures (TBs) from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and atmospheric profile data from Japanese 25-year Reanalysis Project (JRA-25). The surface emissivity is derived using the non-scattering radiative transfer equation for regions identified as no-rain by TRMM Precipitation Radar (PR). An Empirical Orthogonal Function (EOF) analysis has been applied to this emissivity data-set. Emissivities at high frequencies, difficult to estimate due to high sensitivity to clouds and water vapor, are estimated from lower frequencies by using the principal components. Contributions from EOF1 to EOF4 are
dominant and with the others being less than 1 %. Therefore, 5 high-frequency emissivities can be estimated
from the other 4 emissivities at lower frequencies with 4 principal components. For example, when 37 GHz Horizontal emissivity on June 1998 is estimated from 4 channels of 10 and 19 GHz, correlation coefficient with the original estimate is 0.93 and the result of linear fitting shows an inclination of 0.97 and a cutoff of 0.02 for global data. This estimation method is applied for each area, each land surface condition (surface type and soil wetness) and so on, in search of optimal performance of the algorithm. The advantage of using the EOF analysis as described above is to minimize the cloud contamination at high frequency TB. A cloud-clearing method is also explored to improve the reliability of the EOFs.