Shortwave flux from satellite-measured radiance: A theoretical study over marine boundary layer clouds

Earth radiation budget measurements, important to climate monitoring and to validating climate models, require that radiances measured by satellite in struments be converted to hemispherical flux. This paper examines that prob lem theoretically, using inhomogeneous cloud models constructed from Landsa t scenes of marine boundary layer clouds. The spherical harmonics discrete ordinates method (SHDOM) code is applied to the model scenes to compute ful l two-dimensional radiation fields, which then simulate measured radiances. Inversion to flux is performed by several different methods, including pla ne-parallel table lookup and empirical angular distribution models with thr ee different ways of determining scene identification, to examine error sou rces and relative magnitudes. Using a simple plane-parallel table lookup re sults in unacceptably large flux bias errors of 11%-60%, depending on the o rbital viewing geometry. This bias can be substantially reduced, to no more than 6%, by using empirical angular distribution models. Further improveme nt, to no more than 2% flux bias error, is obtained if known biases in opti cal-depth retrievals are taken into account when building the angular model s. Last, the bias can be further reduced to a fraction of a percent using s cene identification based on multiple views of the same area. There are lim its, however, to the reduction in the instantaneous error with this approac h. Trends in the flux error are also identified, in particular an equator-t o-pole trend in the flux bias. Given the importance of satellite measuremen ts for determining heat transport from equator to pole, this consistent bia s should be kept in mind, and efforts should be made to reduce it in the fu ture.