On reflectance ratios and aerosol optical depth retrieval in the presence of cumulus clouds

The traditional conversion of satellite-observed reflectances to the aerosol optical depth (AOD) is highly conjectural in the vicinity of clouds due to the 3D cloud-induces enhancement of the apparent reflectance. This study uses 3D Monte Carlo radiative transfer calculations and simulated cloud and aerosol fields to illustrate that for clear pixels the reflectance ratios for two pairs of wavelengths (660, 470 nm and 870, 470 nm) are less sensitive to the 3D cloud effects than the reflectances themselves. We develop a new algorithm for converting these two ratios to three spectral values of AOD and show that it is accurate to within 10% for the majority of clear pixels in our model-inverse problem. This preliminary study suggests that the proposed approach can be used to significantly improve the accuracy of the AOD retrieved from spectral satellite observations under partly cloudy conditions.