Aerosol Absorption in cloudy scenes using passive satellite instruments

A method is presented to determine the energy absorbed by aerosols overlying clouds, using passive remote sensing by satellites. In passive remote sensing techniques, determination of the optical thickness of aerosols is severely hampered by the brightness of clouds in a scene. This makes the study of the various direct and indirect aerosol effects on clouds difficult and passive satellite observations of these effects are often limited to aerosols in the vicinity of clouds. Very valuable information has become available from active space-based sensors and some dedicated field campaigns on aerosol indirect effects have been performed in the past. This new method will exploit the wealth of information from the passive, space-based spectrometer SCIAMACHY that has been obtained over the last 9 years. Cloud scattering is large in the entire solar spectral range and cloud optical thickness is generally much larger than aerosol optical thickness. Therefore, scattering aerosols cannot, at present, be distinguished from scattering cloud droplets. Cloud absorption, on the other hand, is negligible below about 1 micron, whereas aerosol absorption is largest in the UV wavelength range, and increases with decreasing wavelength. A frequently used satellite tool to detect absorbing aerosols, which uses this aerosol UV-absorption, is the Absorbing Aerosol Index (AAI), which can indicate absorbing aerosols overlying clouds. The AAI is available from SCIAMACHY and used to find aerosol-polluted cloud scenes. A quantitative measure of absorption by aerosols, like e.g. aerosol optical thickness and aerosol scattering albedo, has been proved hard to derive from the AAI, which is a radiation property and not an aerosol property. Therefore, in the method presented here, the radiation, or reflectance as measured by SCIAMACHY, is used directly to derive the absorbed energy by the aerosols in the UV and visible spectral range, without making any assumptions on the aerosol microphysical properties. A LookUp Table (LUT) of cloud reflectances was created, simulating the spectral reflectances in the UV and visible range of aerosol-free cloud scenes. From an actual, measured aerosol-polluted cloud scene the cloud optical thickness and cloud droplet size are determined at wavelengths above 1 micron, to minimise any aerosol effect on the cloud properties. These cloud properties are then used with the predetermined LUT to get the simulated aerosol-free cloud reflectance in the UV and visible spectral range for the measured scene. The simulated aerosol-free and measured aerosol-polluted spectral reflectances can be compared directly to determine the absorbed energy by aerosols in the scene.