Book Description

Boundary layer clouds are fundamental components of the climate system because of their first order impacts on the global energy balance. While marine boundary layer clouds have received most of the attention in this field because of their connection to global climate sensitivity and radiative forcing, continental boundary layer clouds also play a crucial role in controlling the amount of radiation reaching the land surface. Sitting at the top of the atmospheric boundary layer, these clouds are fundamentally linked to underlying land surface processes. We present three studies that explore the connection between land-atmosphere interactions and shallow clouds with eye toward improving process-based understanding of this system. The first shows that marine boundary layer clouds are an important sink for aerosol particles over the southeast Atlantic Ocean, helping mitigate the impact of continental African biomass burning aerosols transported to the vicinity of these low clouds. Then, we look to ground our mechanistic understanding of the response of continental shallow cumulus clouds to two different land cover changes, boreal forest expansion and tropical deforestation, using an idealized modeling framework. This analysis reveals the importance of environmental context, especially soil moisture availability and background atmospheric humidity, in determining the response of the convective boundary layer and cloud cover to these land surface perturbations. The co-evolution of boundary layer and lifting condensation level heights also emerges as key and should motivate examination of results from models that cannot faithfully represent these parameters. Finally, using a combination of the same idealized modeling system and observations from two ARM sites, we explore the effects of variations in the surface energy budget, in response to both land cover change and natural variability, on boundary layer convective velocities, initial shallow cumulus updraft speeds and the number of aerosol particles activated into cloud droplets. While the simulations and observations show that surface heat fluxes are not likely the dominant driver of changes in shallow cumulus droplet number in most settings, environments with lower evaporative fractions do favor a stronger coupling. We connect each of these studies to larger questions in cloud physics and land-atmosphere interactions, and hope to motivate further work on the detailed properties of this system.