On the buffering of CCN impacts on wintertime orographic clouds: An idealized examination

This study examines the role that cloud base height (or cloud base temperature) plays in the response of orographic precipitation to enhanced potential cloud condensation nuclei (CCN). A series of 2-D simulations along a transect of the Sierra Nevada was run at cloud-resolving scale to adequately represent the microphysical processes that lead to orographic precipitation, varying CCN concentrations for different cloud base temperature regimes. Cloud-base heights were systematically varied by modifying low-level moisture. Sensitivity runs indicate that for lower cloud base heights, the potential for drizzle (warm rain) formation increases such that at some point increased CCN concentrations can suppress drizzle formation leading to greater amounts of supercooled liquid water contents and then riming is enhanced with increased CCN concentrations. As a result ice-phase precipitation can be enhanced for low cloud base height clouds. This suggests that the conclusion from previous studies that enhanced CCN reduce precipitation cannot be generalized to warmer-based winter orographic clouds.