OPTICALLY THIN CIRRUS CLOUDS - RADIATIVE IMPACT ON THE WARM POOL

Over the convectively active tropical ocean regions, measurements made from space in the infrared (i.r.) and visible have revealed the prese nce of optically thin cirrus clouds which are quite transparent in the visible and nearly opaque in the infrared. The Nimbus 4 Infrared Inte rferometer Spectrometer (IRIS), which has a field of view (FOV) of alm ost-equal-to 100 km, has been utilized to examine the i.r. optical cha racteristics of these cirrus clouds. From the IRIS data it has been ob served that these optically thin cirrus clouds prevail extensively ove r the ''warm pool'' region of the equatorial western Pacific surroundi ng Indonesia. It is found that the seasonal cloud cover produced by th ese thin cirrus exceeds 50% near the central regions of the ''warm poo l''. For most of these clouds, optical thickness in the infrared is < 2. It is deduced that dense cold anvil clouds associated with deep con vection spread extensively and are responsible for the formation of th e thin cirrus. This is supported by the observation that the coverage of the dense anvil clouds is an order of magnitude less than that of t he thin cirrus. Observations made by the Earth Radiation Budget Experi ment (ERBE) reveal that the ''warm pool'' region constitutes a local m aximum in net input of radiative energy. From these observations, toge ther with a simple radiative energy balance model, we have inferred th at the greenhouse effect produced by the optically thin cirrus clouds can be a significant factor in maintaining the ''warm pool''.