Aerosol radiative forcing and the accuracy of satellite aerosol optical depth retrieval

The minimum acceptable accuracy of the top of the atmosphere radiative flux of DeltaF = 0.5 W/m(2) leads to the required accuracy in satellite based aerosol optical depth (AOD) retrieval of about Deltatau = 0.015 over the land and Deltatau = 0.010 over the ocean. None of the current operational satellite based instruments for AOD retrieval has been able to achieve this accuracy. The RMSE (Root Mean Square Error) of the AVHRR (Advanced Very High Resolution Radiometer) is typically between 0.06 and 0.15, while the RMSE of the MODIS (Moderate resolution Imaging Spectroradiometer) over the land has been estimated to be Deltatau = 0.05 + 0.2tau, which varies between Deltatau = 0.07 and Deltatau = 0.21 within the limit of usual aerosol optical depth between tau = 0.1 and tau = 0.8. The Department of Energy research satellite instrument, the Multispectral Thermal Imager (MTI), is capable of retrieving aerosol optical depth with an accuracy of Deltatau = 0.03 using an off nadir view at medium scattering angles. Theoretical analysis suggests that the uncertainties in aerosol phase function (due to uncertainties in aerosol shape, size distribution and optical properties) are the major obstacles for accurate aerosol optical depth retrieval. These uncertainties lead to a much larger error in aerosol optical depth retrieval at large scattering angles (usually at close to nadir view) than at off nadir views at medium scattering angles. On the basis of our theoretical analysis and on MTI experience, we suggest that in order to achieve the required accuracy in AOD retrieval, future satellite instruments using a single or dual-view AOD retrieval algorithm should use off-nadir views at medium scattering angles (between 50degrees and 100degrees).