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A model that computes the fluxes of energy and momentum between the land surface and the atmosphere is presented. It is designed to serve as a lower boundary in a mesoscale atmospheric model and is intended to be used to study the influence of the land surface on regional atmospheric circulations and climate. The land surface model contains one vegetation layer, a soil skin layer, and four subsurface soil layers. The shortwave and longwave radiation schemes are based on the two-stream theory. Turbulent transfer is treated in a very simple manner, by considering canopy - air and ground - air exchanges separately. Plant water flow is governed by differences in water potential between the soil and the leaves. The stomatal resistance formulation uses the effective leaf area index and the leaf water potential as key variables. It is shown that the resulting transpiration scheme implicitly accounts for the influence of visible radiation, soil moisture, atmospheric saturation deficit, and leaf temperature. The results of four validation experiments are shown. Parameters were chosen prior to the model runs, and no tuning was involved. These experiments show that the surface model is capable of reproducing observed fluxes within instrumental error including timescales ranging from rapid weather changes up to several months.

The radiation subscheme of a land surface model is presented. It is based on the two-stream approximation and includes explicit calculations for the scattering coefficients of direct and diffuse radiation. It applies to horizontally homogenous vegetation canopies, for both solar and thermal infrared radiation. Leaf orientation is accounted for by a leaf inclination index. An intercomparison with field data indicates that, when using general vegetation type-dependent parameters, the radiation scheme has an absolute accuracy on the order of a few percent for the amount of radiation absorbed by the surface and up to 10% as far as the partitioning between soil and vegetation is concerned. Finally, the radiation scheme is used in order to derive a method to calculate an effective leaf area index for transpiration that is physically based and consistent with observations.

Byline: Koen De Ridder, Clemens Mensink Surface fluxes and atmospheric stability affect atmospheric dispersion through their impact on low-level turbulence, mixing layer height, and mesoscale circulation patterns. In the first part of this paper, a sensitivity study is performed to demonstrate the impact of the land surface on energy fluxes and atmospheric stability. In the second part, a methodology to estimate surface heat fluxes and stability is presented. The method is based on the combined use of a surface energy balance model and satellite remote sensing of green vegetation cover and surface soil moisture, and requires either standard single-level meteorological data, or the use of a mesoscale meteorological model.

Byline: Koen De Ridder, Clemens Mensink Surface fluxes and atmospheric stability affect atmospheric dispersion through their impact on low-level turbulence, mixing layer height, and mesoscale circulation patterns. In the first part of this paper, a sensitivity study is performed to demonstrate the impact of the land surface on energy fluxes and atmospheric stability. In the second part, a methodology to estimate surface heat fluxes and stability is presented. The method is based on the combined use of a surface energy balance model and satellite remote sensing of green vegetation cover and surface soil moisture, and requires either standard single-level meteorological data, or the use of a mesoscale meteorological model.