Variability of water vapor, infrared radiative cooling, and atmospheric instability for deep convection in the equatorial western Pacific

In the troposphere of the equatorial western Pacific, the water vapor varia bility dominates the temperature variability in changing the clear-sky infr ared (IR) cooling rate. The large water vapor variability, especially its b imodal distribution at certain levels of the upper troposphere, leads to di stinct structures of the clear-sky IR radiative cooling rate. The IR coolin g rate, its maximum normally in the upper troposphere (similar to 300 hPa) and minimum in the lower troposphere (similar to 650 hPa), tends to become vertically uniform when the upper troposphere is abnormally dry. A local, m aximum IR cooling rate may occur in the boundary layer when the lower tropo sphere becomes extraordinarily dry. The changes in IF: cooling due to the w ater vapor variability affect the rate of generation of convective availabl e potential energy (CAPE) and the conditional instability for deep convecti on. Little or no mean rainfall over an area of roughly 3 X 10(5) km(2) is o bserved when either the rate of generation of CAFE suffers from a reduction (magnitude of 50 J kg(-1) day(-1)) or IR cooling decreases with height. Th e observed variability of water vapor results from both local vertical proc esses and the large-scale horizontal circulation. Horizontal advection acco unts for a large fraction of the drying that is responsible for the changes in the IR cooling profile and in the atmospheric instability for deep conv ection. These results suggest that interactions among water vapor, radiatio n, and deep convection must be assessed by fully taking the large-scale cir culation into consideration. This study is based on an analysis of upper-ai r soundings collected during the Tropical Ocean Global Atmosphere Coupled O cean-Atmosphere Response Experiment Intensive Observing Period and calculat ions of a radiative transfer model.