Simulation of Aerosol Indirect Effects on Cloud Streets Over the Northwestern Pacific Ocean.

We simulated aerosol effects on cloud streets formed during continental cold‐air outbreaks over the Northwestern Pacific using an aerosol‐sensitive cloud microphysical scheme coupled with the Weather Research and Forecast model. The micro‐ and macro‐scale responses were examined under a wide range of aerosol concentrations. The cloud streets generally remain in a transient state, indicating that the aerosol particles may significantly affect the cloud system's development. The simulation results revealed a clear Twomey effect, but they were contrary to some of the Albrecht effect, especially under relatively clean conditions. Such reversed Albrecht effects stem mainly from dynamic response to changes in atmospheric stability associated with drizzle evaporation in the sub‐cloud layer. The dynamic response works against thermal forcing from the large air‐sea temperature gradient in a cold‐air outbreak. As the aerosol increases, the drizzle mechanisms weaken, and the boundary layer becomes more convective, leading to stronger dry‐air entrainment from the free troposphere. In individual clouds, the effect of entrainment on cloud water content may be compensated by more substantial vertical vapor flux. These mechanisms lead to a reduction in the cloud amount. Furthermore, the reduced moisture flux from drizzle evaporation in the sub‐cloud layer, as well as the reduced sedimentation speed of the smaller cloud drops, tend to lessen cloud thickness. The overall reduction in cloud dimensions may offset the Twomey effect by up to 20% in cloud albedo. Plain Language Summary: Cloud streets formed during continental cold‐air outbreaks over the Northwestern Pacific are susceptible to aerosol effects. Our simulations revealed a clear Twomey effect, but they were contrary to some of the Albrecht effect, especially under relatively clean conditions. The key mechanism is that the suppression of drizzle under more numerous aerosols may destabilize the boundary layer and alter the dynamics as well as energy and moisture fluxes. These influences lead to significant cloud cover changes and thus offset the Twomey effect by up to 20% in cloud albedo. Key Points: Twomey effect (cloud albedo increases with increasing aerosol concentration) is substantial in cloud streetsCloud thickness decreases in response to increasing aerosol, which is contrary to the Albrecht effectIncreasing aerosol concentration generally reduces cloud cover, but the trend reverses when substantial drizzle develops [ABSTRACT FROM AUTHOR]