Behavior of Deep Convective Clouds in the Tropical Pacific Deduced from ISCCP Radiances

We analyze the properties of deep convection over the equatorial Pacific and its relationship to sea surface temperature (SST) and surface wind divergence using ISCCP radiance data for July 1983 and January 1984. Deep convective clouds (DCC) are diagnosed with both a combined visible-infrared threshold method and an infrared-only threshold method. The visible channel is important in diagnosing deep convection in different regions with different surface and atmospheric properties because of the existence of large-scale variations of DCC top altitudes. The ITCZ and SPCZ exhibit changes in these two months that are characteristic of both the El Niño and seasonal cycles. Deep convection is latitudinally confined to a much smaller spatial scale than that suggested by maps of outgoing longwave radiation. Diurnal variations of DCC cover and associated mesoscale cirrus/anvil cloud (CAC) cover are out of phase, with deep convection peaking in the early morning throughout the equatorial Pacific. The diurnal cycle is strongest in the west Pacific and Indonesia, where deep convection is most intense. DCC top temperatures are at a minimum several hours before the maximum in DCC cover and out of phase with CAC top temperatures. Two types of relationships between deep convection, SST, and surface wind convergence are suggested: 1) When a large area within a region is covered by SSTs greater than 28°C, deep convection is enhanced in areas where SST exceeds 28°C in the absence of strong surface divergence. Strong monthly mean surface convergence does not enhance deep convection in this case. 2) When the warmest SSTs in a region are less than about 28°C, deep convection is significantly enhanced by strong surface wind convergence near the local maximum in SST (26°–28°). An analysis of a slightly different version of the ISCCP data for the period July 1983-July 1985 suggests that the first type of relationship is the typical situation in the west Pacific, while the second relationship is most obvious in non-El Niño northern winters in the east Pacific. DCC top height generally increases as SST increases, but the visible reflectance of DCC does not vary strongly with SST.