Investigation of Regenerated CCN Effects on Daytime Marine Stratocumulus Cloud using WRF-LES with Spectral Bin Microphysics Scheme

Weather Research and Forecasting (WRF) model with the spectral bin microphysics (SBM) scheme explicitly prognoses the size (number) distributions of cloud drops and CCN. However, the SBM scheme does not include CCN regeneration process that replenishes CCN after drop evaporation. This process is essential since the remnant (remaining) aerosols after evaporation of cloud drops could be activated as new droplets and affect subsequent development of the cloud. In this study, we implement five different CCN regeneration methods into the SBM scheme and simulate daytime marine stratocumulus cloud (MSC). The aircraft measurement data from the Variability of the American Monsoon System Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) campaign is used to derive the initial conditions of the simulations, and the simulation results are compared with the observation data. It is demonstrated that cloud drop evaporation and CCN regeneration process greatly affect the development of MSC since the warm and dry air entrained from above the cloud top could efficiently evaporate cloud drops. Without the regeneration process, the number concentration of CCN and cloud drops are significantly reduced and therefore the simulated number concentration of cloud drops are much lower but they are much larger than the observed cloud drops. When the regeneration process is applied, the total (cloud drop and CCN) number concentration is well-maintained, and the microphysical and radiative properties of the clouds are similar to the measurement data. We recommend the regeneration methods that preferentially replenish large CCN because these methods perform best for simulating daytime MSC with the most reliable cloud drop number concentration, which is the most important variable that depends on the regeneration methods. More detailed comparisons of the regeneration methods will be discussed at the conference.