Aerosol-cloud-radiation and surface flux interactions simulated in a large-eddy model

The effect of aerosol on cloud radiative forcing and the development of precipitation requires treatment of myriad atmospheric boundary layer processes. Here we focus on the formation of warm, continental convective clouds and discuss the influence of different aerosol loadings on cloud characteristics. For context we use a sounding from the Smoke Aerosols, Clouds, Rainfall and Climate (SMOCC) experiment (Brazil 2002). Our primary tool is a new large eddy model that comprises coupled dynamics, aerosol and cloud microphysics, radiation, and surface soil and vegetation. It is ideally suited to study the subtle interplay between aerosol loadings, net surface radiation, sensible and latent heat fluxes, and cloud development.

The sounding at Fazenda, Brazil during SMOCC was chosen because it generates warm, convective cumulus clouds, and allows for testing of the effects of varying amounts of aerosol on these type of clouds. Two sets of three-dimensional simulations were performed. Each set consists of four simulations with a range of aerosol concentrations. Set 1 treats the aerosol as cloud condensation nuclei CCN, but the aerosol and radiation modules are not directly coupled and therefore there is no feedback of aerosol to surface latent and sensible heat fluxes. Set 1 therefore simulates the effect of aerosol on precipitation (“second aerosol indirect effect”). Set 2 also includes the coupling of aerosol heating with the dynamical model and therefore the response of sensible and latent heat fluxes to aerosol is simulated. S2 thus includes both the second indirect effect and the influence of aerosol radiative properties on cloud formation (“the semi-direct effect”).

The results of these simulations suggest that in continental regions aerosol radiation and surface processes must be included in calculations of the response of cloud optical properties and precipitation to increases in aerosol. Neglect of these processes may result in an overestimate of the second aerosol indirect effect. Sensitivity of the warm convective cumulus clouds to surface soil and vegetation types is further explored and new results will be shown at the meeting.