Impacts of Mesoscale Cloud Organization on Aerosol-induced Brightness

While mesoscale cellular convection (MCC) of marine stratocumulus has been widely studied, its future projection remains unclear, in part due to a lack of knowledge of how MCCs of different cell sizes respond to anthropogenic aerosol perturbation. To understand cloud water adjustment to aerosol perturbation, recent studies use linear regression between pixel-level liquid water path (LWP) and cloud droplet number concentration (Nd) measurements from satellite snapshots, yet their results may be biased by MCC due to potential internal correlation between LWP and Nd within mesoscale cells. Here, we focus on quantifying the impact of MCC on the aerosol-induced brightness, using 7 years of MODIS 1-km cloud property retrievals over the North Atlantic region.

Applying a novel 2D wavelet analysis, we classify MCCs by their cell sizes into four groups of 2° x 2° scenes, at 8 km, 16 km, 32 km and 64 km scales respectively. We compute the correlation between local fluctuations of LWP and Nd filtered to the cell scale for each MCC scene and refer to that as the mesoscale correlation. Our results show that there is a clear mesoscale correlation between LWP and Nd within cells, and that the mesoscale correlation varies with cell size. For mature MCCs with relatively large sizes, larger cloud droplets and smaller Nd are found in thicker cell cores while smaller cloud droplets and greater Nd are found at cell edges. In contrast, for small cell sizes, thicker cell cores are found to contain higher Nd but much weaker mesoscale variability in droplet size. As a result, the bigger cloud droplets at the tops of more turbulent cloud cores in larger MCCs would hamper cloud-top evaporation, making cloud water less sensitive to aerosol perturbations compared to smaller MCCs. These results suggest an important role of MCC cell sizes in aerosol-induced brightness, which needs to be taken into consideration for accurately projecting future MCC and climate change.