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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
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7.3 the surface energy balance and coupling to the soil
A skin temperature Tsk forms the interface between the soil and the atmosphere. As detailed in Section 3.5, it is calculated for each grid box tiles separately, by scanning the surface energy balance solver over the 8 tiles, assuming a complete coverage of the specific tile. For a single tile, this procedure is very similar to the derivation of the Penman-Monteith equation in which the skin temperature is eliminated from the surface energy balance equation. The numerical approach used in TESSEL has the advantage that the feedback of skin temperature on net radiation and ground heat flux is included (see Section 3.5). The input radiation and reference atmospheric temperature (TL), specific humidity (qL) and wind speed (UL) are identical for each tile. The surface fluxes "seen" by the atmosphere are calculated as an area-weighted average over the tiles (see Eqs. (7.17) and (7.18)). For the high vegetation with snow underneath, the skin temperature is that of the high vegetation; the temperature of the underlying snow is calculated separately.
The energy balance equation solved for each tile takes into account partial absorption of net short-wave radiation, 1-fRs,i, in the skin layer (see Table 7.2). The remaining energy is directly passed to the soil or snow:
|
(7.19) |
where i denotes the tile index, Rs and RT are downward short-wave radiation and long-wave radiation, respectively,
is the Stefan-Bolzman constant, T1 the temperature
of the upper soil or snow layer, Hi the sensible heat
flux, and Lv,sEi the latent heat flux
from the skin layer, and 
, the skin conductivity for tile i.
Latent heat of evaporation, Lv, is used for all evaporation
terms except snow evaporation, while Ls, the latent heat
of sublimation, is used for evaporation of snow (i.e., tile 5 and the contribution
Esn,7 from tile 7, defined by Eq.
(7.16)).The tiled surface is thermally coupled to the snow deck, when present, and to a single soil profile. The net flux into the soil is a weighted average of the flux from each tile.
The solution of Eq. (7.19) is performed inside the code for turbulent exchanges in the atmosphere (Chapter 3). The atmospheric vertical diffusion equations yield a tridiagonal system of equations, with the coupling to the skin temperature given by the matrix row corresponding to the lowest model level. The first step for the solution of the system of equations, an LU decomposition, is followed by the solution of Eq. (7.19) before back-substitution. Details of the computations can be found in Chapter 3.
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05.04.2002 |
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