FINITE-CLOUD EFFECTS IN LONGWAVE RADIATIVE-TRANSFER

As numerical weather and climate prediction models demand more accurat e treatment of clouds, the role of finite-cloud effects in longwave ra diative transfer clearly warrants further study. In this research, fin ite-cloud effects are defined as the influence of cloud shape, size, a nd spatial arrangement on longwave radiative transfer. To show the mag nitude of these effects, radiometer data collected in 1992 during the Atlantic Stratocumulus Transition Experiment (ASTEX) were analyzed. Th e ASTEX data showed that radiative transfer calculations that ignored the vertical dimensions of the clouds underestimated the longwave clou d radiative surface forcing by 30%, on average. To study further these finite-cloud effects, a three-dimensional 11-mu m radiative transfer model was developed. Results from this model, which neglected scatteri ng, agreed with the measurements taken during ASTEX on 14 June 1992. T his model was also used to reiterate that, for optically thick clouds, knowledge of cloud macrophysical properties can be more crucial to th e modeling of the transfer of longwave radiation than the detailed des cription of cloud microphysical properties. Lastly, techniques for the inclusion of these finite-cloud effects in numerical models were expl ored. Accurate radiative heating rate profiles were achieved with a me thod that assumed a linear variation of the cloud fraction within the cloud layer. Parameterizations of the finite-cloud effects for the mar ine stratocumulus observed during ASTEX are presented.