Sensitivity of Land‐Atmosphere Exchanges to Overshooting PBL Thermals in an Idealized Coupled Model

Abstract

The response of atmospheric carbon dioxide to a given amount of surface flux is inversely proportional to the depth of the planetary boundary layer (PBL). Overshooting thermals that entrain free tropospheric air down into the boundary layer modify the characteristics and depth of the mixed layer through the insertion of energy and mass. In addition, entrainment “dilutes” the effects of surface fluxes on scalar quantities (temperature, water vapor, carbon dioxide, etc.) in the PBL. Therefore, incorrect simulation of PBL depth can lead to linear errors in estimates of carbon dioxide fluxes in inverse models. Dilution by entrainment directly alters the surface‐air gradients in scalar properties, which serve as the “driving force” for surface fluxes. In addition, changes in near‐surface temperature and water vapor affect surface fluxes through physiological processes in plant canopies (e.g. stomatal conductance). Although overshooting thermals are important in the physical world, their effects are unresolved in most regional models. We explore the sensitivity of surface fluxes and PBL scalars to the intensity of PBL top entrainment by manipulating its strength in an idealized version of the coupled SiB‐RAMS model. An entrainment parameterization based on the virtual potential temperature flux at the surface is implemented into SiB‐RAMS to produce a warmer and drier mixed layer, to alter the surface fluxes, and to increase the depth of the PBL. These variations produce modified CO 2 concentrations and vary with the strength of the parameterized entrainment.