Interactions of the land-surface with the atmospheric boundary layer

We study daytime land-atmosphere interaction using a one-dimensional (column) coupled land-surface - atmospheric boundary-Iayer (ABL) model and data sets gathered at Cabauw (1978, central Netherlands) and during the Hydrological and Atmospheric Pilot Experiment - Modélisation du Bilan Hydrique (HAPEX-MOBILHY, 1986, southwest France). The sensitivity of this interaction to the parameterization of soil hydraulic processes shows that the effects on surface fluxes and boundary layer development are largest for dry to moderate values of soil moisture, particularly for bare soil conditions. Boundary-Iayer clouds are controlled by the evolution of relative humidity (RH) at the boundary-Iayer top, which involves the interaction of soil moisture, surface heating, initial ABL conditions, and the moisture content and temperature stratification above the ABL with a number of competing feedback mechanisms. A fractional cloud cover formulation is developed based on a Gaussian distribution of total-water RH at the ABL top, where the distribution includes both turbulent vari ations (as a function of ABL-top dry air entrainment) and mesoscale variations (as a function of horizontal domain size); the modeled cloud cover is found to be more sensitive to the specified mean vertical motion than to the adjustable coefficients in the cloud cover formulation. Various improvements are made to the land-surface model and tested against data in off-line model runs without parameter tuning; using this improved land-surface model, coupled ABL - land-surface model runs yield realistic daytime surface fluxes and atmospheric profiles. Finally, it is shown in coupled landatmosphere modeling, analytically, and with data that the effect of soil moisture is to increase ABL-top RH tendency and thus cloud cover only if the stability above the ABL is not too weak (and there is sufficient initial ABL RH, and air above the ABL not too dry), while for weak stability above the ABL, drier soils yield a greater ABL-top RH tendency and thus cloud cover (again, with sufficient initial ABL RH, and air above the ABL not too dry).