Geoscience Reference
In-Depth Information
The global weather forecast model in use at Meteo-France known as ARPEGE
has a variable horizontal resolution (see Figure 4.5). This makes it possible to
provide a more detailed description of predicted weather over certain parts of the
world. In its operational version, it has a resolution of 20 km over Western Europe,
while over the antipodes resolution decreases down to 200 km.
Limited-area models, which are used to address regional weather or climate, are
generally used in conjunction with global models, to which they are coupled, and
from which they take their boundary conditions. They allow a better description and
understanding of meteorological phenomena on a smaller scale than in global
models. Examples of such models include: ALADIN and AROME in use at Meteo-
France, HIRLAM, DWD COSMO and NCAR MM5 models, etc.
4.4. Role and description of the Earth's surface
4.4.1.
Continental surfaces
The role of the surface in both nature and in numerical weather forecast models
is to exchange energy and momentum with the atmosphere. The exchanges take
place as fluxes of sensible heat, latent heat (evaporation) and momentum.
The different components of surface budgets (e.g. exchange of energy between
the soil, the biosphere and the atmosphere) are described by specific models based
on balanced:
- surface energy budget: Rn = H + LE + G
- surface radiative budget: Rn = (1-alpha)Rg + Eps (Rat-sigma Ts
4
)
- surface water balance: P = LE + R +D
where
- Rn is net radiation; H is sensible heat flux; LE is latent heat flux; G is sensible
heat flux into the ground;
- alpha is the albedo of the ground; Rg is the global radiation; Eps is the
emissivity of the ground with respect to the long wave downward radiation; Rat is
the long wave downward radiation; sigma is the Boltzman constant; Ts is the surface
temperature;
- P is precipitation; R is surface runoff; and D is a drainage term.
SVAT (soil-vegetation-atmosphere) transfer schemes such as the ISBA scheme
in use at Meteo-France [NOI 96] have been developed to describe all surface
processes. Given a description of the atmospheric forcing, and data describing soil
and vegetation properties, they enable us to calculate net radiation, sensible and
latent heat fluxes, soil heat flux, soil moisture, runoff and drainage and surface
temperature.
These models generally need the following parameters:
i) parameters characterizing the soil:
