A study of the effect of regenerated CCN on marine stratocumulus cloud development using the WRF-LES model with spectral bin microphysics scheme

Impacts of regenerated cloud condensation nuclei (CCN) on daytime marine stratocumulus clouds (MSC) and stratocumulus-topped boundary layer (STBL) are investigated, by implementing five different CCN regeneration methods in the spectral bin microphysics (SBM) scheme of the Weather Research and Forecasting (WRF) large eddy simulation (LES) model. Without the CCN regeneration process, the model shows unrealistic reduction of total (cloud drops + CCN) particle concentrations but produces very large cloud drops, compared to observations. Such reduction of particle concentrations is remedied when CCN regeneration process is implemented in the model. Evaluation of the model performance with aircraft measurement data confirms that CCN regeneration is critical for realistic simulation of MSC development. However, depending on the selected CCN regeneration method, results vary significantly especially at a later hour of simulation time (5 h). Such variabilities of the microphysical properties and dynamical structures are caused mainly by the different ways of regenerating CCN particles after entrainment of warm and dry air and subsequent evaporation of cloud drops near cloud top, which, in turn, affects cloud radiative properties such as SW heating and LW cooling that drives STBL convection. We are confident that this study can provide a guidance for simulating more realistic three-dimensional MSC cloud development by including the crucial process of CCN regeneration in the SBM scheme. All in all, we recommend the methods, which primarily replenish the depleted CCN with large CCN.