Tropical Upper-Tropospheric Extended Clouds: Inferences from Winter MONEX

Abstract

The most common cloud species observed during the Winter Monsoon Experiment (WMONEX) wasthick (optically black) middle and upper tropospheric extended cloud. Data from the GeostationaryMeteorological Satellite (GMS) showed the extended cloud to occupy half the near-equatorial SouthChina Sea and Indonesia on some days with tops in the vicinity of the 200 mb level. Detailed observations from the WMONEX composite observing array indicated that the clouds extended up to 750 kmfrom the convective source regions, possessed bases in the vicinity of the freezing level and lay above agenerally suppressed and subsident lower troposphere. The observation of widespread precipitation fromthe extended cloud and the encountering of ice particles during the cloud penetrations suggest that theextended clouds are active in a diabatic heating sense.Calculations using a radiative transfer model and cloud and atmospheric states derived from WMONEXdata indicate substantial net heating at the base of the cloud (-20 K day1) and cooling at the top(-5 to -15 K day1), resulting in a heating rate differential between the base and top of the cloud of upto 35 K day". Net heating or cooling occurs depending upon the diurnal cycle. It is conjectured thatthe effect of the radiative heating is to destabilize the cloud layer. As the magnitude of the radiativeheating at the base of the cloud is at least within a factor of 2 of estimates of the cooling at the cloudbase due to melting for moderate disturbances and relatively greater for weak disturbances or in locationswell removed from the convective source in any disturbance, it is argued that radiative effects cannotbe ignored in the calculation of the total diabatic heating fields in tropical cloud systems.