DIRECT ESTIMATE OF METHANE RADIATIVE FORCING BY USE OF NADIR SPECTRALRADIANCES

Direct determination of the radiative forcing of trace gases will be m ade possible by use of the next generation of nadir-looking spaceborne instruments that provide measurements of atmospheric radiances in the infrared spectral range with improved spectral and spatial resolution . An inversion statistical method has thus been developed and applied to the direct determination of the radiative forcing of methane, based on such instruments as the Fourier-transform Interferometric Monitor for Greenhouse Gases launched onboard the Japanese Advanced Earth Obse rving Satellite in 1996 and the Infrared Atmospheric Sounding Interfer ometer planned for the European polar platform Meteorological Operatio nal Satellite in 2000. The method is based on simple statistical laws that directly relate the measured radiances to the radiative forcing b y use elf an apriori selection of appropriate spectral intervals and g lobal modeling of methane spatial variations. This procedure avoids th e use of an indirect determination based on an inversion process that requires precise knowledge of the methane vertical profiles throughout the troposphere. The overall accuracy and precision of this new algor ithm are studied, and interfering gases and instrumental characteristi cs are taken into account. It is shown that radiative forcing can be d etermined at high horizontal spatial resolution with a precision bette r than 7% in cloud-free conditions and with well-known surface propert ies. (C) 1998 Optical Society of America.