Surface emissivity and hydrometeors derived from microwave satellite observations and model reanalyses

Satellite-observed Microwave Sounding Unit (MSU) channel 1 (Ch1) brightness temperature, and General Circulation Model (GCM) reanalyses over the globe, as well as radiative transfer simulations, have been used to investigate microwave surface emissivity, and low-tropospheric hydrometeors, during the period from January 1981 to December 1993. The average model Ch1 temperature has been derived from three kinds of GCM reanalyses, based on the MSU weighting function. Since the Ch1 temperature constructed from GCM-reanalysis air temperature neglects effects from surface emissivity differences and hydrometeors, it is highest in the summer hemisphere. On the other hand, MSU temperature over land is much higher than it is over ocean, due to depressed surface emissivity over water. Over the high latitude ocean the MSU Ch1 temperature is enhanced because of ice/snow emissivity, while it is reduced over the high latitude land. The difference values of Ch1 temperature between reanalysis and MSU decrease, in the regions of the ITCZ, SPCZ and sea ice mainly because of increased MSU temperature. The values decrease by about 4-6 K in the ITCZ and SPCZ regions, due to hydrometeors. This difference is found to decrease by about 10-30 K in sea ice regions, due to change in surface emissivity. To explain these results, we have estimated the contribution of surface emissivity and hydrometeors to the MSU Ch1 temperature, utilizing radiative transfer theory. The increase of 4-6 K in the temperature over the ITCZ and SPCZ is estimated to result from hydrometeors that give a precipitation rate of 1-1.5 mm/day under the horizontal homogeneity of rain within the MSU radiometer field of view (fov, similar to110 km), and 7-11 mm/day under inhomogeneous rain distribution within the fov, consistent with TRMM observations. The increase of 10-30 K over the high latitude ocean, arises from ice emissivity of 0.6-0.9. Seasonal movements of sea ice boundary also have been discussed. This study could be valuable by providing an independent technique of computing surface emissivity and hydrometeors over the globe, and for long time periods from microwave measurements, such as MSU and AMSU.