BAMOS
Jun 2018
20
Position Statement
continued...
meteorologist Edward Lorenz to develop the mathematical
theory of chaos. Two of the characteristics of a chaotic system are
that its behaviour is not periodic and that its evolution depends
very strongly on the initial conditions. Although the limits on
predictability depend on the scale of the weather system, one of
the implications of Lorenz’s work is that there is a limit of about
two weeks on the predictability of the synoptic weather systems
in the middle and higher latitudes. However, the time limit for
predictability in the tropics is less well defined.
One way to make more useful forecasts in the face of this
sensitivity is to make a number of separate forecasts, each
forecast starting from a slightly different estimate of the
current state, but with each estimate lying within the range
of the observational uncertainty. By encompassing the range
of possible initial conditions the resulting forecasts should
encompass the range of possible outcomes, thereby providing
an error bar on the forecast. This process is known as ensemble
forecasting, and the forecasts so produced are probabilistic. In
other words, the forecast is not a single, specific forecast, but
instead can be used to estimate the probability of a particular
outcome (for example, rain exceeding a certain amount).
Figure 3. Verification skill scores for 72 hour predictions from