A two-frequency logarithmic differential technique for retrieving precipitable water from the satellite microwave radiometer (SAMIR-II) on board BHASKARA II

The Satellite Microwave Radiometer (SAMIR-II) on board the second Indian remote sensing satellite Bhaskara-II launched on November 20, 1981 measured microwave radiation at signals of 19.35, 22.235, and 31.4 GHz. The observations are primarily affected by water vapor (WV), cloud liquid water, and wind speed. In general regression approach, a linear combination of all the three channels or its subset could be used for parameter retrieval. An improved algorithm using a logarithmic function of the differential signal of 22.235 and 19.35 GHz as predictor for retrieving water vapor has been developed. The effects of cloud liquid water and wind speed cancel out in the present algorithm because of very small differences in cloud liquid transmittances and emissivities at 1935 and 22.235 GHz. The in situ comparisons of the satellite derived precipitable water with radiosonde measurements gave an rms difference of 0.31 gm/cm² for water vapor range of 2.9 to 6.4 gm/cm². The analysis showed an improvement of 44 percent over the general regression approach where brightness temperatures were used as predictors. However, the present differential approach gave an rms accuracy of 0.31 gm/cm², identical to the accuracy obtained when functions of the brightness temperatures were used as predictors in the regression. The WV distribution over the Arabian sea has been studied for the periods 25 May to 25 June 1982 and 23 November 1981 to 31 January 1982. The study reveals the presence of high moisture value (6 gm/cm²) over the north and northeast part of the Arabian sea with a relatively low (2.5 to 3.5 gm/cm²) moisture content over the east African coast. The winter season indicates the low value (3 to 4 gm/cm²) of WV over the Arabia sea with the persistance of high equatorial moisture value (5 gm/cm²). Both summer and winter seasons WV distribution show a strong latitudinal variation.