What Has Been Learned About The Influence Of Clouds And Other Feedbacks On Climate?


New Discrepancy Noted Between Predicted And Observed Absorption Of Solar Radiation By Clouds

The prediction and assessment of the effects of changes in atmospheric composition on the Earth's radiation balance currently rely on theoretical models of atmospheric radiation to predict how much of the incoming short wave solar energy is absorbed by clouds. Initial results of combined satellite, aircraft, and surface measurements of solar radiation at geographically diverse locations suggest that 25-40 watts per square meter more of this radiation is absorbed in cloudy sky regions than is currently predicted by available radiation models. This anomalous or enhanced absorption is thought to be due to clouds; however, there is no known physical mechanism that can explain the absorption, and there are concerns about the representativeness of the measurements. If confirmed, this unexplained discrepancy between predicted and observed absorption of solar radiation reflects a potentially important gap in understanding of the Earth's radiation balance. If the discrepancy is due to atmospheric absorption, changes will be needed in the radiation algorithms used in the atmospheric models. It is not yet clear the extent to which such changes would affect the global response to increasing greenhouse gases. However, the changes would likely result in a redistribution of energy from the surface to the atmosphere, which might resolve some long-standing problems of how present GCM results compare to observations. As part of the Atmospheric Radiation Program (ARM), an experiment involving the use of unmanned aerospace vehicles and ground based measurements is planned for 1995 to determine the validity and extent of the anomalous absorption. This experiment should also contribute mechanistic information needed to modify existing theoretical models so that they more accurately simulate the absorption of solar radiation.
References: (1) Absorption of Solar Radiation by Clouds: Observations Versus Models, Cess, R. D., M. H. Zhang, P. Minnis, L. Corsetti, E. G. Dutton, B. W. Forgan, D. P. Garber, W. L. Gates, J. J. Hack, E. F. Harrison, X. Jing, J. T. Kiehl, C. N. Long, J.-J. Morcrette, G. L. Potter, V. Ramanathan, B. Subasilar, C. H. Whitlock, D. F. Young, and Y. Zhou, Science, Vol. 267, pp. 496-498, 1995; (2) Warm Pool Heat Budget and Shortwave Cloud Forcing: A Missing Physics?, V. Ramanathan et al., Science, Vol. 267, pp. 499-503, 1995; (3) Direct Observations of Excess Solar Absorption by Clouds, Pilewskie, P., and F. P. J. Valero, Science, Vol. 267, pp. 1626-1629, 1995.

High Clouds Found To Be Important To The Stabilization Of Sea Surface Temperatures

Observations collected during the Central Equatorial Pacific Experiment (CEPEX) and from TOGA COARE have provided new insights into the processes that regulate the sea surface temperatures of the large "warm pool" of the tropical Pacific Ocean. These results suggest that optically thick, high-level clouds that form as a result of deep convection play an important role in stabilizing sea surface temperatures and limiting them to a maximum climatological value of about 31°C, as observed in both paleo and contemporary records. It appears that sea surface evaporation, which was previously thought to be the critical stabilizing factor, does play a role, but one that diminishes as the sea surface temperatures increase beyond a threshold value.
Reference: Warm Pool Heat Budget and Shortwave Cloud Forcing: A Missing Physics?, Ramanathan, V., B. Subasilar, G. J. Zhang, W. Conant, R. D. Cess, J. T. Kiehl, H. Grassl, and L. Shi, Science, Vol. 267, pp. 499-502, 1995.

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Last updated 04/10/96