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Chapter 7. Land surface parametrization
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Chapter 1. Overview
Chapter 2. Radiation
Chapter 3. Turbulent diffusion and interactions with the surface
Chapter 4. Subgrid-scale orographic drag
Chapter 5. Convection
Chapter 6. Clouds and large-scale precipitation
Chapter 7. Land suface parametrization
Chapter 8. Methane oxidation
Chapter 9. Climatological data
REFERENCES
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7.1 Introduction
The parametrization scheme described in this chapter represents the surface fluxes of energy and water and, where appropriate, corresponding sub-surface quantities. Fig. 7.1 summarizes the main features of the land part of the model; hereafter the scheme will be referred to as the TESSEL (Tiled ECMWF Scheme for Surface Exchanges over Land) scheme. At the interface between the surface and the atmosphere, each grid-box is divided into fractions (tiles), with up to 6 fractions over land (bare ground, low and high vegetation, intercepted water, shaded and exposed snow) and up to 2 fractions over sea and freshwater bodies (open and frozen water). Each fraction has its own properties defining separate heat and water fluxes used in an energy balance equation solved for the tile skin temperature. Special attention is devoted to the different physical mechanisms limiting evaporation of bare ground and vegetated surfaces.
Over land, the skin temperature is in thermal contact with a four-layer soil or, if there is snow present, a single layersnow mantle overlying the soil. The snow temperature varies due to the combined effect of top energy fluxes, basal heat flux and the melt energy. The soil heat budget follows a Fourier diffusion law, modified to take into account the thermal effects of soil water phase changes. The energy equation is solved with a net ground heat flux as the top boundary condition and a zero-flux at the bottom.
Snowfall is collected in the snow mantle, which in turn is depleted by snowmelt, contributing to surface runoff and soil infiltration, and evaporation. A fraction of the rainfall is collected by an interception layer, where the remaining fraction (throughfall) is partitioned between surface runoff and infiltration. Subsurface water fluxes are determined by Darcy's law, used in a soil water equation solved with a four-layer discretization shared with the heat budget equation. Top boundary condition is infiltration plus surface evaporation, free drainage is assumed at the bottom; each layer has an additional sink of water in the form of root extraction over vegetated areas.
Finally, open water points have a fixed surface temperature. When present, frozen water occupies a fraction of the grid box, with a prognostic ice temperature evolving in the forecast following the heat budget of a four-layer ice model in thermal contact with an underlying ocean at freezing temperature.
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05.04.2002 |
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