Estimation and Model Validation of Surface Solar Radiation and Cloud Radiative Forcing Using TOGA COARE Measurements

Abstract

The Tropical Ocean and Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (TOGA COARE) radiation measurements in the western Pacific warm pool are used to estimate surface solar radiation budgets and to validate radiation model calculations. Clear-sky fluxes are identified from the measurements of direct and diffuse fluxes at the ARM (Atmospheric Radiation Measurements) site of Kavieng by simultaneously imposing conditions that 1) the direct downward solar flux is a maximum, 2) the diffuse downward solar flux is a minimum, and 3) the total flux varies smoothly with time. Averaged over the four TOGA COARE months, the clear-sky downward solar flux at Kavieng is 308 W m?2, with a range of 302?317 W m?2. The estimated clear-sky solar flux, together with the temperature and humidity radiosondings, are then used to validate radiation model calculations. Using an inferred aerosol optical thickness of 0.12, results show that clear-sky surface solar fluxes can be reliably computed from a radiation model. The effect of clouds on surface solar radiation is found to be large. Averaged over the four TOGA COARE months and the seven radiation stations, the surface cloud radiative forcing is 99 W m?2 with a range of 79?112 W m?2. This result of the mean cloud radiative forcing is in agreement with other current studies.