Cloud optical thickness retrieval from AVHRR data

The relation between the reflectivity of the atmosphere-surface system and the optical thickness of a homogeneous cloud layer in the atmosphere is investigated at 0.63 micrometer, which is the central wavelength of NOAA-AVHRR (advanced very high resolution radiometer) channel 1. A detailed radiative transfer model is employed, in which the multiple scattering and absorption resulting from cloud particles, molecules, aerosols, ozone and surface are fully taken into account. To estimate the sensitivity of the relation between atmospheric reflectivity and cloud optical thickness, the influence of variation of solar zenith angle, surface albedo, cloud particle effective radius and viewing geometry are examined. The relation between reflectivity and cloud optical thickness is mainly sensitive to solar zenith angle, surface albedo and viewing geometry. Therefore, these parameters have to be known for a retrieval of cloud optical thickness from reflectivity measurements. For actual retrievals, a database is prepared with atmospheric reflectivities for many solar zenith angles, viewing zenith angles, azimuth angles, surface albedos and cloud optical thicknesses. The retrieval method is applied to an AVHRR image of Sept 11, 1994. First results show a promising correlation with cloud optical thickness estimations from ground based measurements of direct solar irradiance.