The radiative fluxes of LES-simulated trade-wind Cumulus with varying aerosol concentrations

Large-eddy-scale simulations of trade-wind Cumulus using different initial non-absorbing aerosol concentrations find that increased aerosol concentration are associated with smaller cloud fractions, smaller cloud horizontal dimensions, and more variable cloud top heights with a slighly higher cloud liquid water path (Xue and Feingold, 2006). These modeled associations are also evident in some observations (e.g., Matheson et al., 2005). A significant fraction of the LES-modeled clouds possess characteristic horizontal dimensions less than 1 km, which implies that horizontal photon transport is an important component of the radiation field. The impact of the cloud field changes on the domain-averaged radiation fluxes is not intuitively obvious, as the effect from the macrophysical changes acts to compensate that from the microphysical changes. We perform three-dimensional radiative transfer simulations using the I3RC community Monte Carlo model to assess the relative impacts. We find that the top-of-domain reflected shortwave flux is increased with higher aerosol loadings, i.e., the optical depth increases sufficiently that the macrophysical reorganization of the cloud field is a secondary influence. The relative influences of the macrophysical and microphysical changes will be assessed separately, as well as the difference between the Monte Carlo fluxes and those calculated using the independent pixel (or column) approximation.