Correlating Stratocumulus Organizational Characteristics and Aerosol-Cloud Interactions in LES and SCM

The cell size of marine stratocumulus mesoscale cellular convection (MCC), ranging from ~10 km to ~100 km, has a nontrivial impact on the aerosol indirect effect. Our recent study has shown that MCC cell size is critical for regulating aerosol-induced cloud brightness. Large-scale MCCs exhibit almost three times stronger response of cloud brightness to aerosol perturbation compared to small-scale MCCs, primarily due to weaker entrainment efficiency in the case of the former. The magnitude of the aerosol indirect effect is also affected by the frequency of large and small MCCs, but it is currently unknown what environmental conditions favor their development and cell broadening, which hinders the prediction of their frequencies of occurrence in the current and future climate. Moreover, MCCs remain unresolved in climate models, making it impossible to evaluate their role. To better assess the impact of cloud scale on aerosol indirect forcing, it is necessary to establish a connection between cell size and a more general variable that is accessible to climate models and convenient for a broad range of researchers.

In this study, we establish a quantitative link between cloud scale and cloud water variance for MCCs, from 315 large-eddy simulations of diurnal stratocumulus (24 hours), varying in their initial conditions of temperature, moisture, boundary-layer height, and aerosol concentration. Using cloud water variance as a proxy for cloud scale, we examine its temporal evolution during the simulations and identify its relationship to environmental conditions. We quantify the aerosol indirect effect for clouds of different cloud water variance. We perform similarly initialized and forced runs of the NASA GISS ModelE3 Earth system model in single column model mode with the same forcings to evaluate the influence of cloud water variance on aerosol indirect effect across its small physics ensemble.