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4.3 Notably, the understanding of key climate processes
and their representation in models (such as the inclu-
sion of sea-ice dynamics and more realistic ocean heat
transport) has improved in the past few years. Many
models now give satisfactory simulations of climate
without the need for non-physical adjustments of
heat and water fluxes at the ocean-atmosphere inter-
face used in earlier models. Moreover, simulations
that include estimates of natural and anthropogenic
forcing are well able to reproduce observed large-scale
changes in surface temperature over the twentieth
century. This large-scale consistency between models
and observations lends confidence in the estimates
of warming rates projected over the next century. The
simulations of observed natural variability (e.g.
ENSO, monsoon circulations, the North Atlantic
Oscillation) have also improved
4.4 On the other hand, systematic errors are still all too
apparent, e.g. in simulated temperature distributions
in different regions of the world or in different parts
of the atmosphere, in precipitation fields, clouds (in
particular marine stratus). One of the factors that
limits confidence in climate projections is the uncer-
tainties in external forcing (e.g. in predicting future
atmospheric concentrations of carbon dioxide and
other greenhouse gases, and aerosol loadings)
4.5 As with NWP and seasonal forecasts, ensembles of
climate projections are also extremely important.
Ensembles enable the magnitude and effects of
natural climate variability to be gauged and affect its
impact on future projections, and thereby permit any
significant climate change signal to be picked out
more clearly statistically (the magnitude of natural
climate variability will be comparable with that of
climate change for the next few decades).
5. Dissemination to end-users
5.1 The weather forecasts have to be communicated to
a vast array of users such as emergency managers, air
traffic controllers, flood forecasters, public event
managers, etc. in a timely and user-applicable form.
This in itself poses another major challenge that is
increasingly benefiting from advances in information
technology. Predictions at seasonal to interannual
timescales and climate projections are also being used
by an increasingly wide range of users
5.2 The value of forecasts to decision makers is greatly
enhanced if the inherent uncertainty can be quanti-
fied. This is particularly true of severe weather, which
can cause such damage to property and loss of life
that precautions may be well advised even if the event
is unlikely, but possible. Probabilities are a natural
way of expressing uncertainty. A range of possible
outcomes can be described with associated proba-
bilities and users can then make informed decisions
allowing for their particular costs and risks
5.3 Forecasts expressed as probabilities, or ensembles,
contain much more information than deterministic
forecasts, and it is difficult to convey it all to users.
Broadcast forecasts can only give a broad picture of
the most likely outcome, with perhaps some idea of
important risks. Each user’s decision may be based
on the probabilities of a few specific occurrences.
What these are, and the probability thresholds for
acting on the forecasts, will differ. So for important
user decisions it is necessary to apply their particu-
lar criteria to the detailed forecast information.
6. Conclusions
6.1 The skill in weather forecasting has advanced
substantially since the middle of the twentieth
century, largely supported by the advancement of
computing, observation and telecommunications
systems, along with the development of NWP models
and the associated data-assimilation techniques. This
has been greatly facilitated because of the vast expe-
rience of both forecasters and decision makers in
producing and in using forecast products.
Nevertheless, each component within the science and
technology of weather forecasting and climate projec-
tion has its own uncertainties. Some of these are
associated with a lack of a complete understanding
of, or an inherent limitation of, the predictability of
highly complex processes. Others are linked still to
the need for further advances in observing or comput-
ing technology, or to an inadequate transfer between
research and operations. Finally, one cannot under-
estimate the importance of properly communicated
weather forecasts to well educated users
6.2 Without a doubt, significant benefits will result from
continued attention to scientific research and the
transfer of knowledge gained from this work into the
practice of forecasting. Furthermore, a recognition of
the limitations of weather forecasts and climate
projections, and when possible, an estimate of the
degree of uncertainty, will result in the improved use
of forecasts and other weather information by deci-
sion makers. Ultimately the objective is for the
scientific and user communities to work better
together, realizing even greater benefits.