Geoscience Reference
In-Depth Information
Requirements in a Soil-Vegetation-Atmosphere Transfer (SVAT) scheme: (A) Basic variables that must be
calculated at each model time step by a SVAT if it is used in a meteorological model; (B) Additional required calculations to
allow representation of the hydrological impacts of climate; (C) Additional required calculations to allow representation of
changes in CO 2 (and perhaps other trace gases) in the atmosphere.
Table 24.1
A.
Basic requirements in meteorological models
1. Momentum absorbed from the atmosphere by the land surface - requires the effective area-average aerodynamic
roughness length.
2. Propo r tion of incoming solar radiation captured by the land surface - requires the effective area-average, wavelength
average solar reflection coefficient or albedo.
3. Outgoing longwave radiation (calculated from area-average land surface temperature) - requires the effective
area-average, wavelength average emissivity of the land surface.
4. Effective area-average surface temperature of the soil-vegetation-atmosphere interface - required to calculate longwave
emission and perhaps energy storage terms.
5. Area-average fraction of surface energy leaving as latent heat (with the remainder leaving as sensible heat)
- to calculate this other variables such as soil moisture and/or measures of vegetation status are often required, these either being
prescribed or calculated as state variables in the model.
6. Area-average of energy entering or leaving storage in the soil-vegetation-atmosphere interface (required to calculate the
instantaneous energy balance).
B. Required in hydro-meteorological models to better estimate area-average latent heat and to describe the
hydrological impacts of weather and climate
7. Area-average partitioning of surface water into evapotranspiration, soil moisture, surface runoff, interflow, and baseflow.
C. Required in meteorological models to describe indirect effect of land surfaces on climate through their
contribution to changes in atmospheric composition
8. Area-average exchange of carbon dioxide (and possibly other trace gases).
radiant energy, and through the properties and processes that control how this
energy, once captured, is returned to the atmosphere either as latent or sensible
heat. There are also interactions between the two surface exchanges of energy and
momentum. The aerodynamic roughness of the surface controls not only the
transfer of momentum to the surface but also the efficiency of the exchange of
latent and sensible heat. Similarly, the surface energy balance affects atmospheric
buoyancy and, through this, the efficiency of both momentum and energy transfers.
Over the past four decades, the complexity and realism of the SVATS used in
meteorological and hydrological models has increased dramatically, as described
below. However, when seeking to understand the purpose of this development and
to classify the diversity of the models that have resulted from it, it is helpful to
recognize that SVATS strictly speaking only need to meet a limited number of the
basic requirements at each point in time, albeit these are required as area-averages
over the grid area used in the model. When estimating primary variables, present
day SVATS often also calculate several secondary variables, and are distinguished
by differences in the number of secondary variables calculated and the complexity
used in their calculation. Nonetheless, the motivating purpose for calculating
these additional values remains to define the time evolution of the primary set of
area-average requirements listed in Table 24.1.
 
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