A Preliminary Study of the Tropical Water Cycle and its Sensitivity to Surface Warming

Abstract

This paper presents the preliminary findings of an investigation of the water budget of tropical cumulus convection using the Goddard Cumulus Ensemble Model (GCEM). Results of an experiment designed to obtain a ?fingerprint? in the tropical hydrologic cycle in response to surface warming are also presented. The ensemble mean water budget shows that the distribution of water vapor and cloud water in the tropical atmosphere is maintained as a result of a balance between moisture convergence (including cloud scale and large scale) and condensation and reevaporation by various microphysical species within the cumulus clusters. Under radiative convective equilibrium conditions, 66% of the precipitation reaching the ground comes from the convective region and 34% from the stratiform but fixed large-scale vertical velocity, tropical convection is enhanced with more abundant moisture sources. Water vapor is increased throughout the troposphere with the surplus largest near the surface and decreases monotonically up to 10 km. However, the percentage increase in water vapor is largest near 8 to 16 km. As a result of the warming, the freezing level in clouds is elevated resulting in a large increases (decrease) cloud water just above (below) 5 km. As with water vapor, the fractional increase in cloud water and cloudiness amount is largest at the upper troposphere. In spite of the detailed microphysics and cloud-scale dynamical processes included in the GCEM, the results on changes in temperature and water vapor induced by surface warming are in agreement with those from general circulation models that use crude cumulus parameterization. This is consistent with previous findings that equilibrium water vapor distribution is a strong function of temperature. In an open domain such as the tropical convective environment, with a specified climatological vertical velocity, the ratio of increased precipitation to increased surface evaporation due to a 2°C surface warming is approximately 5. The increases is mostly found for convective rain and is negligible for stratiform rain. The climate implication of these changes is also discussed.