Aerosol Indirect Effects on Shallow Maritime Cloud Morphology

Maritime stratiform clouds are of interest to climate scientists since these types of clouds can potentially cover large regions of the ocean surface, resulting in a significant impact on the radiation budget. Research has shown that a number of factors affect the formation and break up of stratiform cloud decks. Among these influences are boundary layer stability and aerosol indirect effects. These two factors can potentially create competing processes that determine cloud structure, thus making the task of understanding how these processes develop and interact to bring about such changes in cloud structure challenging. For example, preliminary results from this study indicate that when the boundary layer is strongly stable, the aerosol effects are greatly diminished compared to those when the boundary layer is weakly stable. Developing a strong understanding of the feedbacks operating between these factors is tantamount to the comprehension of how these processes influence cloud structure.

The goal of this study is to investigate the impacts that boundary layer stability and aerosols have on cloud structure in order to quantify how these two factors contribute to the overall cloud structure modification. The Regional Atmospheric Modeling System, a cloud-resolving model with a sophisticated microphysics scheme, has been utilized to perform sensitivity simulations where both the boundary layer stability and aerosol number concentrations have been systematically varied. Results will be presented that show processes on both the cloud microphysical scale, as well as the boundary layer turbulent scale, which reveal how cloud structure is impacted by aerosols in varying boundary layer environments. Preliminary results suggest that in general boundary layer stability imposes a stronger influence on cloud morphology than does aerosol number concentration. Not only is the relative strength of the impact from aerosols modulated by the boundary layer stability, but also the manner in which aerosol effects develop through microphysical routes. For example, as the boundary layer stability transitions from stable to neutral to unstable, the precipitation rate tends to increase which in turn controls how the aerosol number concentration effect on the drop size distribution impacts liquid water content in the clouds. In lightly precipitating environments, increasing the aerosol number concentration tends to enhance evaporation leading to a decrease in the liquid water content of the clouds. In heavier precipitating environments, the liquid water content of clouds is increased due to the suppression of warm rain by the increase of aerosol number concentration. It is felt that this research will assist in enhancing our understanding of the relative importance of aerosols and environmental controls on stratocumulus clouds and the breakup of stratocumulus decks.