Daytime Variation of Shortwave Direct Radiative Forcing of Biomass Burning Aerosols from GOES-8 Imager

Hourly GOES-8 imager data (1345UTC-1945 UTC) from July 20-August 31, 1998 were used to study the daytime variation of shortwave direct radiative forcing (SWARF) of smoke aerosols over biomass burning regions in South America (4-16 S, 51-65 W). Vicarious calibration procedures were used to adjust the GOES visible channel reflectance values for the degradation in signal response. Using Mie theory and Discrete Ordinate Radiative transfer (DISORT) calculations; smoke aerosol optical thickness (AOT) was estimated at 0.67 μm. The GOES retrieved AOT was then compared against ground-based AOT retrieved values. Using the retrieved GOES 8 AOT, a four-stream broadband radiative transfer model was used to compute shortwave fluxes for smoke aerosols at the top of atmosphere (TOA). The daytime variation of smoke AOT and shortwave aerosol radiative forcing (SWARF) was examined for the study area. For selected days, the Clouds and the Earth’s Radiant Energy System (CERES) TOA SW fluxes are compared against the model derived SW fluxes. Our results show that the GOES derived AOT is in excellent agreement with AERONET derived AOT values with linear correlation coefficient of 0.97. The TOA CERES estimated SW fluxes compare well with the model calculated SW fluxes with linear correlation coefficient of 0.93. The daytime diurnally averaged AOT and SWARF for the study area is 0.63±0.39 and –46.8±19.23Wm respectively. This is among the first studies to estimate the daytime diurnal variation of SWARF using satellite data. Diurnal Variation of Shortwave Radiative Forcing, Christopher and Zhang (2000). 3