In-cloud activation and mixing in Large Eddy Simulations of trade-wind cumulus

Recent modeling studies (e.g., Chosson et al. 2004, 2007; Grabowski 2006, Slawinska et al. 2008) demonstrate that assumptions concerning the microphysical evolution of natural clouds---the homogeneity of the cloud-environment mixing in particular---significantly affect the simulated albedo of a field of shallow convective clouds. In addition, in-cloud activation of cloud droplets, that is, activation well above the cloud base, has been argued to play a significant role as well (e.g., Gerber et al. 2008, Lasher-Trapp et al. 2005). In-cloud activation and cloud-environment mixing interact with each other and their impact on cloud characteristics is difficult to separate in either observations or in model simulations. It follows that understanding of microphysical properties of such clouds has important implications for the cloud physics in general and for clouds-in-climate problem in particular.

This study extends that of Slawinska et al. (2008), where the impact of different mixing scenarios has been investigated in simulations of BOMEX shallow convection using a single-moment warm-rain scheme. Here,the recently-developed double-moment warm-rain microphysical scheme of Morrison and Grabowski (2007, 2008) is used. Important features of the new scheme include prediction of the supersaturation field inside a cloud (essential for the in-cloud activation) and a possibility of prescribing the homogeneity of the cloud-environment mixing. The background CCN concentration is prescribed either as pristine (100 1/mg) or polluted (1000 1/mg). Two contrasting scenarios for the cloud-environment mixing are assumed, either homogeneous or extremely inhomogeneous mixing.

We will report results concerning aerosol concentration, activation and the homogeneity of mixing, and their impact on microphysical and macrophysical properties of shallow convective clouds. The differences between current and previous results (Slawinska et al. 2008) will be emphasized, with the particular focus on the in-cloud activation that was excluded in the previous study. The radiative properties will be diagnosed and compared with Slawinska et al. (2008).