ATMOSPHERIC EFFECTS ON SMMR AND SSM/I 37 GHZ POLARIZATION DIFFERENCE OVER THE SAHEL

The difference of vertically and horizontally polarized brightness temperatures (referred to here as the polarization difference, DELTAT) observed at 37 GHz frequency of the scanning multi-channel microwave radiometer (SMMR) on board the Nimbus-7 satellite and special sensor microwave imager (SSM/I) on board the DMSP-F8 satellite could provide useful information about land surface change within the span of these global observations, November 1978 to August 1987 for SMMR and July 1987 to present for SSM/I. The atmospheric effects on the DELTAT are studied over two 2.5-degrees by 2.5-degrees regions within the Sahel and Sudan zones of Africa from January 1985 to December 1986 through radiative transfer analysis using surface temperature, atmospheric water vapour and cloud optical thickness developed under the International Satellite Cloud Climatology Project (ISCCP). The atmospheric effects are also studied using surface observations of air temperature and vapour pressure at Niamey (13.5-degrees-N, 2.2-degrees-E) for the period January 1979 to December 1990. It is found that atmospheric effects alone cannot explain the observed temporal variation of DELTAT, although the atmosphere introduces important modulations on the observed seasonal variations of DELTAT due to rather significant seasonal variation of precipitable water vapour. Therefore, these DELTAT data should be corrected for atmospheric effects before any quantitative analysis of land surface change over the Sahel and Sudan zones. The entire global data set from December 1978 to December 1990 has been archived for unrestricted distribution and use.