Land-surface response to shallow cumulus

Cloud feedbacks in the climate system are a major source of uncertainty in model projections of global warming. Analysis of satellite data and climate models suggests that much of this uncertainty is associated with low clouds. Due to their relatively small size, radiative impact, and their tendency not to produce rain, shallow cumulus clouds have received much less attention than the other cloud forms (i.e., stratocumulus, deep precipitating convection, etc.). A significant portion of the research has focused on understanding and parameterizing the transport and mixing associated with shallow cumulus and their roots within the subcloud layer. However, even though the impact of cumulus convection on surface fluxes has been shown to significantly affect larger-scale circulations and the skill of medium-range weather forecasts, the coupling between shallow cumulus and the land surface has received very little attention.

Passing shallow cumulus clouds briefly and intermittently reduces the surface solar irradiance. The surface responds by adjusting the balance between sensible, latent and soil heat flux. Precisely how the surface partitions the available energy among these fluxes and how boundary layer motions respond to these brief periods of cloud shading remains unclear. To analyze these relationships, we use a cloud-permitting version of NCAR's Large Eddy Simulation code coupled with the NOAH Land Surface Model. Using initial conditions and forcing from a case during the Southern Great Plains 1997 study, we investigate the following questions surrounding the coupling between shallow cumulus and the land surface: To what magnitude do shallow cumulus clouds affect the surface energy balance on average? How does the surface react to sudden and local net radiation variations? What are the respective roles between the cloud-induced secondary circulations and the solar irradiance reduction on the surface energy balance? What is the atmospheric response to these cloud-induced surface fluxes heterogeneities? How do shallow clouds affect the entrainment rate?

The surface energy balance responds in a highly non-linear fashion to cloud shading leading to different sensible and latent heat flux partitioning on average compared to surfaces interacting with a cloud-free boundary layer. The evaporative fraction increases by about 2-3% in the presence of shallow cumuli. As expected, the solar irradiance reduction drives the surface response. However, the turbulence and the secondary circulations induced by the cloud dynamics increase surface flux spatial variability. Although less than 1.5 km in horizontal scale, the cloud-induced surface heterogeneities impact the atmospheric vertical heat and moisture fluxes to heights well above the surface layer. The cloud roots amplify the drying and the cooling of the subcloud layer; effects that buoyantly compensate each other, which result in average entrainment rates at large time scales that remain unaffected by shallow cumuli.