We have extended the interpretations made in two prior studies of the aircraft shortwave radiation measurements that were obtained as part of the Atmospheric Radiation Measurements (ARM) Enhanced Shortwave Experiment (ARESE). These extended interpretations use the 500 nm (10 nm bandwidth) measurements to minimize sampling errors in the broadband measurements. It is indicated that the clouds present during this experiment absorb more shortwave radiation than predicted by clear skies and thus by theoretical models, that at least some (≤20%) of this enhanced cloud absorption occurs at wavelengths <680 nm, and that the observed cloud absorption does not appear to be an artifact of sampling errors nor of instrument calibration errors.

The objectives of the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE) are to directly measure clear and cloudy sky shortwave atmospheric absorption and to quantify any absorption found in excess of model predictions. We undertake detailed model comparisons to near-infrared and total solar flux time series observed by surface and airborne radiometric instruments during the ARESE campaign. Model clear-sky absorption biases generally fall within the range of uncertainty generated by sample size, and assumptions of aerosol properties and surface albedo. Direct measurements by stacked aircraft on the overcast day of October 30, 1995, confirm the detection of enhanced cloud shortwave absorption during ARESE. The detection is substantiated by, and consistent with, three independent measures of cloudy sky absorption estimated in previous studies: cloud forcing ratio, insolation forcing ratio, and albedo/transmission slope. A significant portion of the enhanced absorption occurs at visible wavelengths. Collocated measurements of liquid water path (LWP) suggest the magnitude of the enhanced absorption increases with LWP.