Modelling the weather 79Figure 5.2Schematic
illustrating the parameters
and physical processes
involved in atmospheric
models.Water vapour is important because of its associated latent heat (in
other words, it gives out heat when it condenses) and also because the
condensation of water vapour results in cloudformation, which modifies
substantially the interaction of the atmosphere with the incoming energy
from the Sun. Variations in both these energy inputs modify the atmo-
spheric temperature structure, causing changes in atmospheric density
(since warmed gases expand and are therefore less dense). It is these
density changes that drive atmospheric motions such as winds and air
currents, which in their turn alter and feed back on atmospheric density
and composition. More details of the model formulation are given in the
box below.
To forecast the weather for several days ahead a model covering the
whole globe is required; for example, the southern hemisphere circula-
tion today will affect northern hemisphere weather within a few days and
vica versa. In a global forecasting model, the parameters (i.e. pressure,
temperature, humidity, wind velocity and so on) that are needed to de-
scribe the dynamics and physics (listed in the box below) are specified
at a grid of points (Figure 5.3) covering the globe. A typical spacing
between points in the horizontal would be 100 km and about 1 km in
the vertical; typically there would be twenty levels or so in the model in
the vertical. The fineness of thespacing is limited by the power of the
computers currently available.
Having set up the model, to generate a forecast from the present, it is
started off from the atmosphere’s current state and then the equations are
integrated forward in time (see box below) to provide new descriptions of
the atmospheric circulation and structure up to six or more days ahead.
For a description of the atmosphere’s current state, data from a wide
variety of sources (see box below) have to be brought together and fed
into the model.