Geoscience Reference
In-Depth Information
index, but rather to use the laws of physics that explain how and why air moves. For
example, the presence of an air pressure gradient is required, which could be
provided by a type of model that works on a larger scale (smaller for Geographers!).
In conjunction with a Digital Terrain Model (DTM) that has been created, a smaller
model will take the information given by the larger model, and will give a wind
map, taking into account different problems associated with relief into
consideration. If the general pressure gradient (synoptic scale) is zero and if we
want to work in a non hydrostatic atmosphere, the pressure gradient needs to be
generated so that breezes that are experienced in reality can be produced. To
generate this pressure gradient, in other words differences in local pressure, the
meteorological model needs to be capable of producing differences in temperature
on the surface of the ground, which means that the energy budget needs to be
calculated for each point. So that the energy budget can be measured, the following
variables also need to be measured: latent heat (evaporation-condensation),
convection, conduction in the ground fluxes, and the radiation budget, including
visible radiation (solar) and infra-red radiation (from the Earth and the atmosphere).
The energy balance depends heavily on the vegetation that is growing on the ground
(and on its color), and on its orientation in relation to the where the sun is (such as
slope and exposure), etc. The main problem associated with such a task is the fact
that the more information required on a more detailed level, the more the
calculations and data storage increase at an unbelievable rate. If the resolution of a
grid is doubled, this means that the time and effort required to work out the
calculations are increased by a scale of four. Another important point often
overshadowed by the calculations (which do not stop increasing), is the sheer
amount of information that is required for the model to work efficiently. Generally,
the information required is geographical information, including the type and
characteristics of soils, vegetation and the roughness of the ground's surface.
Climatic information is also required, such as the limpidity coefficient of the sky,
for each point. Further information on this topic can be seen in Chapter 4 of this
topic.
The absence of information means that only estimations will be given and as a
result, the precision of the model becomes increasingly weaker.
Other less complex solutions can also be used. However, they are less accurate
and cannot be used under certain conditions; this is the case in some hydrostatic
models that use linear equations [NAN 85; CAR 94]. The advantage of using these
models is that they work at a quicker rate than some of the other models, owing to
their simple principles: the relief of an area is considered as being an obstacle that
the synoptic flow of air must overcome, given some constraints in terms of the
thickness of the limit layer, the roughness of the surface and the vertical stability of
the air, etc. (Figure 8.1).
The problem with such a model is that it is incapable of simulating any thermal
breeze for any area at any given time: as a matter of fact, the thermal wind is due to
the relief and to the topographic contrasts that exist and which must not be seen as
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