Radiative Responses of Marine Stratocumulus Clouds to Cloud Drop Number Concentration Perturbation in the E3SM

Marine Cloud Brightening (MCB) is one of the leading approaches for potential solar climate intervention. A key expectation for MCB is that as the cloud drop number concentration (Nd) of marine stratocumulus clouds increases, the clouds reflect more incoming solar radiation and offset more warming caused by greenhouse gas increase. Many previous studies attempting to understand aerosol-cloud-radiation interactions in marine stratocumulus clouds have employed large eddy simulation (LES) models over small domains. On the other hand, more studies focusing on stratocumulus cloud radiative forcing induced by Nd perturbations at larger scales are needed to quantify the possible global impacts of MCB. In this study, we assess changes in cloud radiative effect (CRE) induced by Nd increase over marine stratocumulus cloud regions in the Energy Exascale Earth System Model (E3SM). We use E3SM version 2, for which it is known that the cloud reproduction over stratocumulus regions has been improved compared to the previous version. The model configuration for 10-year simulations uses climatology of monthly estimates of years near 2010 to represent the present-day atmosphere. Nd is specified over the stratocumulus cloud region in increased-Nd simulation, and we focus on the averaged CRE changes in shortwave radiation over the Nd perturbed region between simulations. An analytic approach, developed to diagnose and partition CRE changes in LES models, is applied to partition CRE change into contributions from the cloud albedo effect (known as Twomey effect) and cloud adjustments in cloud fraction and liquid water path. Our analysis shows that as Nd increases, in the E3SM, the model calculated CRE change is about 25 % less effective at reflecting solar insolation than in the analytic calculation. The same calculation is applied to the Community Earth System Model (CESM) version 2 simulation to understand this result, and the CESM-calculated CRE change is similar to the analytic calculation. The model-derived CRE change in the CESM is nearly double compared to the E3SM. From both model results, analytically calculated cloud albedo effects are similar. The distinct difference between the models is that the E3SM shows negligible cloud adjustment effects in analytic calculation, but the adjustment effects contribute to 37% of analytic calculation in CESM. Therefore, the less sensitive CRE change in the E3SM seems primarily due to weaker cloud adjustment in the E3SM in response to the Nd increase. Further studies will be conducted to understand the discrepancy in CRE changes between the analytic approach and model results in the E3SM.