Evaluating the stochastic approach to shortwave radiative transfer in the tropical western Pacific

摘要

In the last decade numerous advances have been made in parameterizing cloud-radiation interactions. One recent area of improvement involves statistical representations of the influence of three-dimensional cloud fields on the domain averaged radiation. An off-line comparison of a traditional plane-parallel shortwave model against a stochastic model and observations is performed using four years of data from the Atmospheric Radiation Measurement Program's Tropical Western Pacific Clouds and Radiation Testbed site at Nauru Island. Cloud properties are derived from observed cloud chord lengths and input into the stochastic model, removing the necessity of explicitly simulating individual cloud fields and allowing for extended model runs. It is found that for certain cloud field geometries, as observed from the Nauru surface station, the stochastic radiative transfer model simulates more accurate surface fluxes than the traditional plane-parallel model. Results suggest that the relationship between vertical extent of liquid cloud and liquid water path is one indicator of radiatively significant cloud field configurations as is the relationship between solar zenith angle and cloud fraction. The stochastic model performs best when the ratio between diffuse and total shortwave surface irradiance falls between 0.4 and 0.5. A simple parameterization based on criteria developed from these results is applied to the plane-parallel modeled downwelling flux to represent the impact of complex cloud field geometry on the domain-averaged radiation. The plane-parallel simulations with the parameterization applied produce a mean downwelling flux that is 16 Wm?2 closer to observations than the plane-parallel model results and 5 Wm?2 closer than the stochastic model results.