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 presence of optically thin cirrus clouds which are quite transparent in the visible and nearly opaque in the infrared. The Nimbus 4 Infrared Interferometer Spectrometer (IRIS), which has a field of view (FOV) of almost-equal-to 100 km, has been utilized to examine the i.r. optical characteristics of these cirrus clouds. From the IRIS data it has been observed that these optically thin cirrus clouds prevail extensively over the ''warm pool'' region of the equatorial western Pacific surrounding Indonesia. It is found that the seasonal cloud cover produced by these thin cirrus exceeds 50% near the central regions of the ''warm pool''. For most of these clouds, optical thickness in the infrared is < 2. It is deduced that dense cold anvil clouds associated with deep convection spread extensively and are responsible for the formation of the 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 the thin cirrus. Observations made by the Earth Radiation Budget Experiment (ERBE) reveal that the ''warm pool'' region constitutes a local maximum in net input of radiative energy. From these observations, together with a simple radiative energy balance model, we have inferred that the greenhouse effect produced by the optically thin cirrus clouds can be a significant factor in maintaining the ''warm pool''.