Model Assessment of the Ability of MODIS to Measure Top-of-Atmosphere Direct Radiative Forcing from Smoke Aerosols

Abstract

The new generation of satellite sensors such as the moderate resolution imaging spectroradiometer (MODIS) will be able to detect and characterize global aerosols with an unprecedented accuracy. The question remains whether this accuracy will be sufficient to narrow the uncertainties in estimates of aerosol radiative forcing at the top of the atmosphere. The discussion is narrowed to cloud-free direct forcing. Satellite remote sensing detects aerosol with the least amount of relative error when aerosol loading is high. Satellites are less effective when aerosol loading is low. The monthly mean results of two global aerosol transport models are used to simulate the spatial distribution of smoke aerosol in the Southern Hemisphere during the tropical biomass burning season. This spatial distribution allows us to determine that 87%?94% of the smoke aerosol forcing at the top of the atmosphere occurs in grid squares with sufficient signal-to-noise ratio to be detectable from space. The uncertainty of quantifying the smoke aerosol forcing in the Southern Hemisphere depends on the uncertainty introduced by errors in estimating the background aerosol, errors resulting from uncertainties in surface properties, and errors resulting from uncertainties in assumptions of aerosol properties. These three errors combine to give overall uncertainties of 1.2 to 2.2 W m?2 (16%?60%) in determining the Southern Hemisphere smoke aerosol forcing at the top of the atmosphere. Residual cloud contamination uncertainty is not included in these estimates. Strategies that use the satellite data to derive flux directly or use the data in conjunction with ground-based remote sensing and aerosol transport models can reduce these uncertainties.