Variations of Cloud Radiative Properties With Temperature: a Comparison Between Low and High Cloud Behavior

Cloud optical property changes can affect significantly the radiative balance of the planet and produce large feedbacks on climate warming. Changes in the optical thicknesses of low clouds (where the solar albedo effect dominates) would produce opposite feedbacks than similar changes in the optical thicknesses of high clouds (where the thermal greenhouse effect is more pronounced). A recent analysis of data from the lnternational Satellite Cloud Climatology Project (ISCCP), revealed an overall tendency (with some important exceptions) for low cloud optical thickness to decrease with temperature. Such a decrease, if applied to a greenhouse scenario would produce a strong positive feedback on climate warming.

Field measurements in high cirrus clouds show that, overall, the optical thickness of those clouds tends to increase with temperature, but the increase is faster at colder than at warmer temperatures. Preliminary analysis of ISCCP data, also shows a fast increase of cirrus cloud optical thickness with temperature in cold temperature ranges, but a much slower increase and even small decreases of cirrus cloud optical thickness with temperature in warm temperature ranges. Increases in cirrus cloud optical thickness with temperature also imply a positive feedback on climate warming. In this presentation, a comparison of the patterns of optical thickness variation with temperature between low and high clouds will be made, based on an analysis of several years of satellite data for both cloud types. Furthermore, the climatic implications of the patterns observed in the two cloud types will be discussed.