Modeling and observational studies of shallow cumulus convection over the Indian Ocean: Aerosol and meteorological effects on diurnal cycles.

摘要

Anthropogenic aerosols affect the water and energy budgets of the trade wind boundary layer and of the ocean surface through the direct radiative effect and through aerosol-cloud interactions. To improve the understanding of the processes through which aerosol effects occur, large-eddy simulations (LES) initialized using observations made during the Indian Ocean Experiment (INDOEX) were analyzed to study the diurnal responses of shallow cumuli to increased aerosol numbers and to additional solar heating induced by absorbing soot aerosols. Simulations were also used to identify causes of the diurnal variation of shallow convection and cloudiness and to quantitatively examine how the effects of anthropogenic aerosols varied in response to variations in meteorological conditions observed during INDOEX.; Without the impact of large-scale forcing, the simulated diurnal cycles of turbulence and cloudiness were characterized by significant daytime reductions and midday minima. Periodic pulsations with a dominant time period of between 0.8 and 3 h driven by turbulent eddies were superimposed on the diurnal cycles. Simulation results show that the principal cause of the daytime reductions was not the diurnally varying surface fluxes or the cloud radiative effects, but rather the gaseous absorption of solar radiation mainly due to water vapor and ozone. Depending on its direction and magnitude, large-scale vertical motion either enhanced or reduced cloudiness, producing a much different pattern of diurnal variations from the one driven by gaseous absorption.; Additional solar heating induced by soot aerosols affected the static stability in the boundary layer. The vertical profile of soot aerosols had a critical influence on how they impacted cloud properties and turbulence. Changes in environmental relative humidity and the presence of a dry and stable layer had critical impacts on cumulus cloud properties, turbulence and lateral detrainment rate, and on how anthropogenic aerosols affect these quantities. For the range of meteorological conditions observed during INDOEX the semi-direct effect always dominated indirect effects, producing a positive daytime mean net indirect forcing, varying between 0.2 and 4.5 W m-2. Further, the indirect forcing was larger when the environmental relative humidity was higher and in the absence of any dry and stable layers.