A ground based measurement of the anomalous cloud absorption effect

Abstract

Measurements of the solar shortwave flux using equipment based on solar cell sensors and pyranometers have yielded new information concerning the anomalous cloud absorption effect. A comparison of the solar fluxes for clear and overcast skies in January and February, 1995 suggests that the near‐infrared portion of the solar spectrum beyond 1.1 microns is absorbed preferentially by clouds. In addition, by comparing the observed and modelled column transmittances, it appears that the solar radiation beyond 1.1 microns is absorbed at higher transmittances than expected. On the average, the column absorption under heavy clouds was about 50 W/m², up to a maximum of about 75 W/m². An approximate estimate of the cloud radiative forcing (CRF) ratio was made for overcast days, and it was found to vary from 1.4 to 2.0, in general agreement with the CRF ratio reported by Cess et al. [1995]. It has been assumed previously in climate models that clouds scatter about 30% of the incident solar radiation while absorbing only about 4%. However, the maximum anomalous absorption of about 50 W/m² that we have measured is large in comparison to the radiative forcing of 3.3 W/m² that has been attributed to the presence of greenhouse gases since the pre‐industrial period, and may possibly imply that the climate predictions from general circulation models will have to be recomputed.