Analyses of Atmospheric Radiation Measurement (ARM) program's Enhanced Shortwave Experiment (ARESE) multiple data sets for studying cloud absorption

Abstract

Following the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE), some studies reported a cloud absorption anomaly (CAA) of unprecedented magnitude. The largest discrepancy was found on a heavy overcast day (October 30, 1995) when cloud absorptance inferred from aircraft observations was 37% of the incoming solar irradiance, almost twice that of model calculations. The essential measurements supporting the finding were made with an airborne total solar broadband radiometer (TSBR). A thorough analysis is performed here, employing a variety of observations from more sources including aircraft, spacecraft, and ground-based instruments. It is found that albedos measured with the TSBR are systematically less than those inferred from other instruments. The difference in mean albedo between TSBR and that inferred from the scanning spectral polarimeter (SSP) on board the same aircraft amounts to 0.15, which is comparable to the reported CAA. SSP data were validated by (1) comparing them to data from the total direct diffuse radiometer (TDDR) spectral radiometer, (2) comparing the SSP's albedo-transmittance slope with that derived from ScaRaB satellite data, and (3) comparing SSP-derived albedos with those inferred from cloud optical parameters estimated from ground-based passive and active observations. All these comparisons show that SSP data are consistent with other measurements within the data uncertainties whose accumulated upper limit is <0.06. A reasonable doubt is thus cast on the claim of a very strong cloud absorption anomaly found using TSBR data on October 30. Copyright 1999 by the American Geophysical Union.