"Numerical Weather Prediction" is the name given to the technique
used to forecast the weather by computer from its present, measured
state up to several days ahead.
The weather is governed by physical laws
The behaviour of the atmosphere is governed by a set of physical
laws which can be expressed as mathematical equations. These take
into account how atmospheric quantities or fields (such as temperature,
wind speed and direction or humidity for example) will change from
their values at the present time. If we can solve these equations,
we will have a description of the future state, a forecast, of the
atmosphere, derived from a current state (initial values), which
we can interpret in terms of "weather" - rain, temperature, sunshine
and wind.
Computers can be used to calculate changes to the atmosphere
However, these equations are complex (they are 'non-linear partial
differential equations'). There is no exact solution that can give
us the future values. Instead, numerical modelling techniques are
employed to provide approximate solutions. In these numerical models
the fields are represented by a finite set of numbers. By using
approximate form of the equations,equations, we can calculate the
future values of the numbers with a computer. Representing fields
with approximate numerical values is called 'discretization' which
emphasises the limits of the numerical approach. The smaller the
set of numbers the coarser the discretization and the less detail
we will have about the future state of the atmosphere. On the other
hand, the finer the discretization, the larger the amount of numbers
we have to deal with and the more expensive in terms of computer
time the solution becomes.
Modelling for a limited area for short-range forecasting by Member
States
The task can be made more manageable if we forecast not the whole
atmosphere but only for a local area, for example part of Europe.
We have then a Limited Area Model. These models can produce a very
detailed forecast, but they are useful only in the range several
hours to about two days into the future - what is happening outside
the treated area influences the weather inside it, the more so the
longer the forecast interval in which we are interested.
Medium-range forecasting
ECMWF predicts the behaviour of the atmosphere in the medium-range
up to ten days ahead. In this time the future state of the atmosphere
at any point can be influenced by phenomena at very distant geographical
locations. Many applications of medium-range forecasting, for example
ship routeing, or pollution dispersion, are not confined to limited
areas of the globe. Therefore the whole atmosphere must be included
in the model - a model for medium-range forecasting must be global
and must describe the atmosphere from the earth's surface to a height
of 30 km. The discretization we can afford depends on the power
of the computer we have available and how efficiently we use this
power.
Small-scale effects have to be taken into account
Some important factors influencing the evolution of the atmosphere
occur on a very small scale. These include the heating of the soil
by the sun, the turbulence of the air near the ground and at high
levels in the atmosphere, for example when air flows over mountains,
and cumulus cloud systems. These cannot be represented properly
by the discretization we can afford in even the most powerful computers
available. We must represent their effects by taking into account
their influence on the behaviour of the parameters of the large
scales. This "parametrization" is one of the areas where much effort
has to be put in order to improve our forecasts.
More powerful computers allow us to use finer grids
The Centre has three Fujitsu systems, a 100-processor VPP5000, a
116-processor VPP700 and a 48-processor VPP700E. The aggregate sustained
performance of these three machines is about 400 Gflops (400 thousand
million floating point operations per second). The Centre has also
an IBM RS/6000 based Data Handling System together with various servers
from IBM, HP and SGI.
The the resolution of the discretization of the Centre's current
model is equivalent to having gridpoints separated by about 60 km
around the globe. The points are evenly distributed geographically.
This network of points is then repeated at 31 levels in the vertical.
The model forecasts the wind, the temperature and the humidity
at 4,154,868 points throughout the atmosphere, plus several other
fields at 134,028 points on the earth's surface.
With this resolution it is possible, for example, to distinguish
the French Massif Central from the Alps, and the Po valley in northern
Italy is identified. With this detail the Centre's model can produce
a realistic forecast of the near surface weather parameters, such
as local winds, and the temperature at the level of the measurement
stations.
Making the forecast
In order to start the computer model, initial or starting conditions
are required. Observations are used to calculate the weather (wind
etc.) at each point throughout the model atmosphere. The forecast
is made in short steps, of about 20 minutes ahead, with each forecast
providing initial conditions for the next forecast step.
The preparation of initial conditions is both a delicate and demanding
task which in the ECMWF forecasting system requires almost as much
computer resources as a ten day forecast.
Initial conditions for the ECMWF global model are prepared by making
an appropriate synthesis of observed values of atmospheric fields
taken over a 24 hour period and short-range forecasts provided by
the global model itself. This synthesis is a process of assimilating
observed values into a model. The use of both observations and model
forecasts in the construction of initial values is required. High
quality data are sparsely and irregularly distributed over the globe.
Short-range model forecasts carry forward in time knowledge of earlier
observations and also provide a crucial background for extracting
useful information from expensive satellite observations.
Austin.Woods@ecmwf.int
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