Determination of radiative forcing of Saharan dust using combined TOMS andERBE data

We determine the direct radiative forcing of Saharan dust aerosols by combi ning aerosol information derived from Nimbus-7 TOMS with radiation measurem ents observed at the top of atmosphere (TOA) by NOAA-9 ERBE made during Feb ruary-July 1985. Cloud parameters and precipitable water derived from NOAA- 9 HIRS2 were used to aid in screening for clouds and water vapor in the ana lyses. Our results indicate that under "cloud-free" and "dry" conditions th ere is a good correlation between the ERBE TOA outgoing longwave fluxes and the TOMS aerosol index measurements over both land and ocean in areas unde r the influence of airborne Saharan dust. The ERBE TOA outgoing shortwave f luxes were also found to correlate well with the dust loading derived from TOMS over ocean. However, the calculated shortwave forcing of Saharan dust aerosols is very weak and noisy over land for the range of solar zenith ang le viewed by the NOAA-9 ERBE in 1985. Sensitivity factors of the TOA outgoi ng fluxes to changes in aerosol index were estimated using a linear regress ion fit to the ERBE and TOMS measurements. The ratio of the shortwave-to-lo ngwave response to changes in dust loading over the ocean is found to be ro ughly 2 to 3 but opposite in sign. The monthly averaged "clear-sky" TOA dir ect forcing of airborne Saharan dust was also calculated by multiplying the se sensitivity factors by the TOMS monthly averaged "clear-sky" aerosol ind ex. Both the observational and theoretical analyses indicate that the under lying surface properties, dust layer height, ambient moisture content, and the presence of cloud all play important roles in determining the TOA direc t radiative forcing due to mineral aerosols.