Small-scale tropical cumulus cloud systems and the atmospheric boundary layer: An observational study.

摘要

Aircraft data collected in the atmospheric boundary layer over the tropical Pacific Ocean are used to determine the governing turbulent processes particular to this region. The data were taken during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) and the Central Equatorial Pacific Experiment (CEPEX). The study focuses on small-scale ({dollar}<{dollar}100 km and a few hours) processes pertaining to intense, precipitating convective systems. Both the influence of the larger-scale ({dollar}>{dollar}100 km and several days) atmospheric circulation and the response of the upper ocean ({dollar}<{dollar}100 m and several days) in relation to these events are studied. Very low wind conditions ({dollar}<{dollar}3 m s{dollar}sp{lcub}-1{rcub}),{dollar} commonly found in the western equatorial Pacific, are also examined. The motivation for this study is the current lack of knowledge of the effect of small-scale systems on tropical climate, despite their persistence throughout the region.; A wide range of local stability conditions were observed in the tropical atmospheric boundary layer, providing a picture of the dominant turbulent eddy scales as a function of the surface stability. Our results show the importance of buoyancy-induced turbulence in maintaining surface fluxes, especially for very low wind conditions. In the vicinity of cloud systems, increased eddy scales, associated with upper-level circulations, result in mechanically forced turbulence and higher net surface fluxes.; Our results demonstrate that small, short-lived but intense convective systems have a major effect on the upper ocean heat budget, with enhancements of boundary layer turbulent latent heat and momentum fluxes being on the order of 60% and 90%, respectively. A net cooling of the ocean is mainly the result of the enhanced latent cooling and the reduction of shortwave radiation at the surface.; Overall, we find that small-scale cumulus clouds in the tropical marine boundary layer couple the surface supply of heat and moisture tightly to the free atmosphere. These clouds exist during both the active and suppressed phases of tropical deep convection, with their impact probably being most important during the suppressed phase, when the boundary layer is otherwise isolated from the free atmosphere.