Simulation of boundary layer clouds with double-moment microphysics and microphysics-oriented subgrid-scale modeling

Mixing of cloud with dry environmental air changes the cloud droplet spectrum and crucially affects optical properties of clouds. This effect is still poorly understood (Brenguier and Grabowski 1993, Burnet and Brenguier 2007) and it is a significant source of uncertainty in aerosol indirect effects. In a nutshell, the issue is whether the mixing results in the reduction of only the droplet size (as in the homogeneous mixing), only the droplet concentration (as in the extremely inhomogeneous mixing), or both the concentration and the size (as in the inhomogeneous mixing). On the theoretical grounds, homogeneity of mixing depends on the relative magnitude of the time scales for droplet evaporation and turbulent homogenization. Results from direct numerical simulations suggest that a simple relationship exists between the ratio of the time scales and the slope of the mixing line on the diagram representing the relative change of the droplet concentration versus the change of the droplet radius cubed (Andrejczuk et al. 2009). In that study, the time scale for droplet evaporation was calculated as a function of the droplet size and the subsaturation predicted by the model. The time scale for turbulent homogenization was derived as a function of the turbulent kinetic energy dissipation rate and the simulated mean width of cloudy filaments.

In a LES model applying a double-moment microphysics scheme (i.e., when the droplet concentration as well as the mixing ratio are predicted) the mixing scenarios is determined by a single parameter (see Eq. 11 in Morrison and Grabowski 2008). However, no approach is available to guide the selection of this parameter during model simulations. In other words, currently the parameter can only be assumed constant in space and time during the simulation. We will present a consistent approach to predict this parameter (and thus the homogeneity of mixing) during model simulations. This involves addition of two model variables to predict the evolution of small-scale turbulent stirring toward the microscale homogenization following an approach described in Grabowski (2007) and Jarecka et al. (2009) and based on ideas put forward by Broadwell and Breidenthal (1982). Using such a relatively sophisticated subgrid-scale model, the mixing scenario can be predicted locally at each model time step as a function of local conditions.

Double-moment LES model EULAG with the new subgrid scale mixing scheme will be used to simulate shallow convective clouds from BOMEX experiment and stratocumulus clouds observed during EUCAARI-IMPACT campaign. Results of these simulations will be compared with results from a traditional double-moment scheme. We expect better predictions of the microphysical properties and the cloud field albedo.