Relationship between clear‐sky atmospheric greenhouse effect and deep convection during the Central Equatorial Pacific Experiment: Model calculations and satellite observations

Abstract

This study investigates the relation between tropical convection and the atmospheric greenhouse effect using data collected during the Central Equatorial Pacific Experiment (CEPEX). We present calculations of total clear‐sky greenhouse effect and its partitioning between the lower, middle, and upper troposphere using high‐quality balloon soundings of temperature and humidity as input to a radiative transfer model. The soundings were taken from a ship every 6 hours from March 7 to 20, 1993, in the central Pacific. We examine the influence on atmospheric greenhouse effect due to changes in humidity and lapse rate and investigate the relationship between atmospheric greenhouse effect, water vapor, and deep convection. Our observations indicate that periods of active or suppressed convection with timescales of the order of days can produce large spatial gradients in clear‐sky atmospheric greenhouse trapping in warm, climatologically convective regions. While the sea surface temperature (SST) decreased by 2 K from west to east, temperatures above 850 mbar showed considerably less variation. Accordingly, lapse rate changes occurred primarily in the boundary layer and such changes account for 80% of the gradient in boundary layer greenhouse effect. The column‐integrated water vapor content from the surface to 850 mbar was nearly constant in each of the regimes. Conversely, large variations in column‐integrated water vapor above 850 mbar and particularly above 500 mbar account for nearly all the gradient in the greenhouse effect in the middle and upper troposphere. Coincident outgoing longwave radiation (OLR) analyses derived from satellite observations show active deep convection in areas with high clear‐sky greenhouse trapping and upper level moisture and generally clear, suppressed conditions elsewhere. In addition, the surface net flux and outgoing flux emitted to space decreased with increased SST. The reduced cooling of the ocean‐atmosphere system is consistent with a supergreenhouse effect operating in regions of deep convection.