Remote sensing of surface solar irradiance with corrections for 3-D cloud effects

Surface solar irradiance (ISFC) can be inferred from satellite-observed radiance with retrieval algorithms based on the independent pixel approximation (IPA). As the spatial resolution of satellite sensors increases, the effects from spatially inhomogeneous cloud fields become more important. Clouds affect the distribution of radiation in a region larger than the resolution of an individual pixel and IPA is no longer justified. The objective of this study is to identify effects of clouds and find corrections for IPA to compensate for these effects. For this purpose, cloud fields are generated with the mesoscale model MM5 at high resolution. A shallow and a convective cloud field are then used as input for a 3-D Monte Carlo radiation model that computes the radiance distribution at the top of the atmosphere and at the surface. Surface irradiance is computed from the upwelling radiance with a method that simulates IPA retrieval. The retrieved irradiance is compared against the one computed with the 3-D model. Two effects of clouds are related to the solar and viewing geometry, noted as a shift of the apparent position of clouds and their shadows. Another cloud effect is the diffusion of radiation: Scattering removes part of the radiation from an atmospheric column and distributes it to neighboring columns. Corrections for these effects are suggested, which can improve the IPA-based retrieval of ISFC. All corrections depend upon the cloud altitude, which may be difficult to obtain in operational applications. A sensitivity analysis suggests that a coarse estimate of the cloud top altitude is sufficient for the corrections.