FEASIBILITY OF SIMULTANEOUS SURFACE TEMPERATURE-EMISSIVITY RETRIEVAL USING SSM I MEASUREMENTS FROM HAPEX-SAHEL/

An algorithm to simultaneously retrieve land surface temperatures and spectral emissivities in the microwave spectrum for applications with SSM/I measurements is developed and tested over the HAPEX-Sahel region . A closed but overspecified system of radiative transfer equations, w ritten in terms of unknown surface temperatures and unpolarized spectr al emissivities at 19, 37, and 85 GHz, is solved with a non-linear lea st-squares optimization technique. Clear-sky multispectral SSM/I brigh tness temperature measurements at full resolution pixel scale, obtaine d at least twice during a diurnal cycle (corresponding to the ascendin g and descending nodes of a DMSP satellite orbit), are used as input o ver a target area. It is assumed that the spectral emissivities remain constant during the rime period under consideration. High-resolution temperature-moisture profiles obtained from the Meteo France-CNRM ball oon-launch station sited at Hamdallay are used to help account for atm ospheric effects in the radiative calculations. Retrieved surface temp eratures are consistent with ground-based radiometer skin temperature measurements and in situ soil temperature measurements obtained during the 1992 Intensive Observing Period (IOP) at the West-Central supersi te for the 8 October golden day case. Qualitative agreement is also fo und with split window surface temperature estimates retrieved from AVH RR satellite measurements, although this cannot be considered a rigoro us verification procedure because of uncertainties in the IR technique and dissimilar overpass times between DMSP and NOAA satellites. The m icrowave emissivity estimates are consistent with results reported in the published literature. The algorithm is applied over an extended No rth African region surrounding the HAPEX-Sahel study area for an ensem ble of golden days to illustrate its potential for large-scale applica tions. As an independent test of its utility, output from the algorith m is used over the study area in conjunction with a physically based p recipitation retrieval algorithm which requires representative estimat es of surface temperature and emissivity in the forward radiative tran sfer model calculations. This test is successful in that rain events a re correctly identified and their intensity is accurately retrieved in the course of the 1992 IOP, demonstrating the value of a simultaneous temperature-emissivity algorithm in aiding other types of terrestrial remote sensing schemes.