Global Warming

Modelling theweather 81

Figure 5.3Illustration of

a model grid. The levels in

the vertical are not equally

spaced; the top level is

typically at about 30 km in

altitude.

Since computer models for weather forecasting were first introduced,
their forecast skill has improved to an extent beyond any envisaged by
those involved in the development of the early models. As improvements
have been made in the model formulation, in the accuracy or coverage
of the data used for initialisation (see box above) or in the resolution of
the model (the distance between grid points), the resulting forecast skill
has increased. For instance, for the British Isles, three-day forecasts of
surface pressure today are as skilful on average as two-day forecasts of
ten years ago, as can be seen from Figure 5.5.
When looking at the continued improvement in weather forecasts, the
question obviously arises as to whether the improvement will continue or
whether there is a limit to the predictability we can expect. Because the
atmosphere is a partially chaotic system (see box below), even if perfect
observations of the atmospheric state and circulation could be provided,
there would be a limit to our ability to forecast the detailed state of the
atmosphere at some time in the future. In Figure 5.6 current forecast skill
is compared with the best estimate of the limit of the forecast skill for
the British Isles (similar results would be obtained with any other mid-
latitude situation) with a perfect model and near perfect data. According
to that estimate, the limit of significant future skill is about 20 days
ahead.