A GROUND-BASED MEASUREMENT OF THE ANOMALOUS CLOUD ABSORPTION EFFECT

Measurements of the solar shortwave flux using equipment based on sola r cell sensors and pyranometers have yielded new information concernin g the anomalous cloud absorption effect. A comparison of the solar flu xes for clear and overcast skies in January and February, 1995 suggest s that the near-infrared portion of the solar spectrum beyond 1.1 micr ons is absorbed preferentially by clouds. In addition, by comparing th e observed and modelled column transmittances, it appears that the sol ar radiation beyond 1.1 microns is absorbed at higher transmittances t han expected. On the average the column absorption under heavy clouds was about 50 W/m(2), up to a maximum of about 75 W/m(2). An approximat e estimate of the cloud radiative forcing (CRF) ratio was made for ove rcast days, and it was found to vary from 1.4 to 2.0, in general agree ment with the CRF ratio reported by Cess et al. [1995]. It has been as sumed previously in climate models that clouds scatter about 30% of th e incident solar radiation while absorbing only about 4%. However, the maximum anomalous absorption of about 50 W/m(2) that we have measured is large in comparison to the radiative forcing of 3.3 W/m(2) that ha s been attributed to the presence of greenhouse gases since the pre-in dustrial period, and may possibly imply that the climate predictions f rom general circulation models will have to be recomputed.