Aerosol indirect effects and radiative-dynamical feedbacks in thin stratocumulus

Static radiative transfer modeling and large eddy simulation are used to explore feedbacks between the first aerosol indirect effect, radiative heating, and cloud dynamics in low liquid water path (LWP) stratocumulus (defined as LWP < 20 gm^-2). For stratiform cloud systems, longwave radiative cooling has a weak dependence on cloud microstructure and water path when these water paths are > 20 gm^-2. For clouds with lower water paths, however, longwave radiative cooling is sensitive to changes in cloud microstructure and water path. Our modeling studies of nocturnal low LWP stratocumulus suggest a strong feedback between changes in aerosol concentration, cloud top radiative cooling, cloud dynamics, and hence cloud lifetime.

For our nocturnal simulations, increasing droplet concentration (N_d) leads to decreases in LWP for these low LWP clouds. Owing to the evaporation-entrainment feedback, initial entrainment drying leads to more cloud breaks and lower LWPs when N_d is high. These cloud breaks result in reduced integrated radiative cooling and weakened cloud circulations when N_d is high. Entrainment drying then suppresses cloud growth through radiative cooling when high N_d (N_d = 1000 cm^-3). In contrast, entrainment drying does not counteract cloud growth when the droplet concentration is low (N_d = 50 cm^-3).

Our simulations also suggest that daytime low LWP stratocumulus are susceptible to changes in N_d. Shortwave warming generally results in decreases in LWP and cloud fraction. In this case, because LWP is already low, longwave cooling decreases concurrently with increased shortwave warming, weakening circulations and further decreasing LWP. Taken together these processes result in a negative feedback loop. For drop concentrations between 200 and 1000 cm^-3, longwave cooling is reduced enough during the morning hours such that the cloud layer cannot be maintained against the net warming of the layer by solar heating. The cloud layer dissipates as a consequence. In contrast, lower drop concentrations result in a cloud layer that is maintained against solar heating.