An Investigation of CCN Regeneration Effects on Marine Stratocumulus Cloud Development in WRF-LES Simulations

Weather Research and Forecasting model (WRF) is widely used in atmospheric science community. The model has several options for calculation of microphysical processes and one of those is Spectral Bin Microphysics (SBM) scheme. The scheme predicts size (number) distributions of CCN, cloud drops and ice particles explicitly and therefore it provides more detailed description of cloud microphysical properties compared to bulk microphysics schemes. The scheme includes activation, diffusional growth, collision, coalescence and breakup of cloud drops, but it does not include CCN regeneration process that should occur after drop evaporation. Marine Stratocumulus Cloud (MSC) has wide extent and long lasting lifetime in the subtropical region. In these clouds drop evaporation and CCN regeneration processes have a great influence on the development of MSC. Because of the inversion layer covering cloud top, entrainment of warmer and drier air evaporates cloud drops strongly. In this study, five different regeneration methods are implemented in the SBM scheme of WRF-LES and to test their performance, the results are compared with the observational data obtained from the aircraft measurement of MSC during the Variability of the American Monsoon System Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). With all five CCN regeneration methods, the model simulates more reliable values of number concentration (N) and mean radius (r) of MSC than without CCN regeneration process. LWC and cloud top height are relatively higher with larger N for the methods that regenerate relatively larger CCN. It appears that N depends strongly on the regeneration methods, more so than r. However, the simulations with the regeneration process produce clouds that have larger radiation effect than observation.