An explicit cloud microphysics/LES model designed to simulate the Twomey effect

A three dimensional model for simulating the effect of enhanced cloud condensation nucleus (CCN) concentrations on stratocumulus clouds is presented. The model is a largeeddy simulation version of the Regional Atmospheric Modeling System (RAMS) with explicit representation of the CCN spectrum and cloud droplet spectrum. Results of two dimensional tests of the model are presented. A new droplet activation scheme based on a bi-modal size-distribution of CCN is presented. Microphysical sensitivity to a five-fold increase in initial CCN concentrations is examined. Results show that droplet concentrations increase about three-fold, effective radii decrease by 40%, and liquid water contents show less significant variations. These results are consistent with both observations and expectations. We have identified numerically-produced anomalies in the cloud microstructure at the sharp cloud-top boundaries which we believe are associated with the advection scheme. Because the focus of the research is on examining the effects of increasing CCN on cloud albedo, these numerical artifacts need to be addressed.