Idealized model for equilibrium boundary layer over land

An idealized equilibrium mixed layer (ML) model is used to explore the coup ling between the surface, the ML. and the atmosphere above. It shows that M L depth increases as vegetative resistance to evaporation increases. The su rface radiative forcing also increases ML depth: the ML radiative and evapo rative cooling processes reduce ML depth. The model largely uncouples mean ML structure from the mean ML fluxes. The upper boundary condition controls ML potential temperature and mixing ratio but does not affect the fluxes; it is the surface radiative forcing and the radiative and evaporative cooli ng terms within the ML (together with the vegetative resistance R-v) that c ontrol the surface fluxes and evaporative fraction. Furthermore, for a give n R-v, the radiative and evaporative cooling terms in the ML control the su rface sensible heat flux. and the surface radiative forcing then controls t he surface latent heat flux. The solutions show that, except for extreme hi gh values of vegetative resistance and very dry air above the ML. this idea lized equilibrium ML is capped by shallow cumulus clouds. as over the ocean . At the same time as R-v increases, the ML structure and depth shift from the oceanic limit toward a warmer, drier boundary layer. It is shown that s urface evaporation controls equilibrium near-surface relative humidity and not vice versa. The equilibrium solutions also give insight into how the gr adient of mean mixing ratio across, the Mississippi River basin is linked t o changes in surface pressure as well as vegetative resistance to evaporati on. The equilibrium model is oversimplified. and the nonlinearities introdu ced by the diurnal cycle have nor been addressed, but nonetheless the solut ions are a plausible zero-order fit to daily mean model data for the Missou ri and Arkansas-Red River basins and to summer composites from the First In ternational Land-Surface Climatology Project Field Experiment.