Environmental Engineering Reference
In-Depth Information
models can trace their origins. The earliest
weather forecasts depended very much on the
interpretation of atmospheric conditions by an
experienced observer who used his accumulated
knowledge of past events to predict future short-
term changes in weather conditions. Particular
skills in that field were often attributed to sailors,
farmers and others who were regularly exposed
to the vagaries of the weather in their work. This
subjective approach continued well into modern
times, but by the end of World War I the first of
the modern predictive models was being
developed. This was the mid-latitude frontal
model which grew out of the work of the
Norwegian school of meteorology. With its
combination of air masses, anticyclones and
frontal depressions it was the mainstay of
weather forecasting in mid-latitudes for half a
century. Past experience with such systems
coupled with observations of their on-going
development allowed local weather conditions
to be predicted twelve to twenty-four hours
ahead. As knowledge of the working of the
atmosphere grew in the 1940s and 1950s, it
became clear that such models over-simplified
the complex dynamics of the atmospheric system,
and new methods of prediction were sought.
In these new models, atmospheric physical
processes were represented by a series of
fundamental equations. Three of the equations
were derived from the laws of conservation of
momentum, mass and energy, which dealt with
motion in the atmosphere. In addition, the models
included an equation of state derived from the
gas laws of classical physics—which related
atmospheric pressure, density and temperature—
plus a moisture equation (Washington and
Parkinson 1986). When solved repeatedly for a
series of small but incremental changes, at
selected grid points across the study area, these
equations provided a forecast of the future state
of the atmosphere. L.F.Richardson, a British
meteorologist, was a pioneer in this field,
publishing the results of his work in 1922 in his
book, Weather Prediction by Numerical Process
(Ashford 1981). At that time his methods were
not practicable. The complexity of the
computations, and the existence of only
rudimentary methods of mechanical calculation
meant that the predictions could not keep ahead
of changing weather conditions. As a result no
real forecast could be made. The inadequacies
of existing upper-air observations also
contributed to the failure of this first attempt at
numerical forecasting (Ashford 1992).
Following this initial setback, little was done
to develop numerical prediction further until the
1950s and 1960s when advances in computer
technology and improved methods of observation
led to the development of models capable of
predicting changes in the essential meteorological
elements across the globe. Since then these
methods have been widely adopted, and
numerical modelling is currently the most
common method of weather forecasting for
periods of several hours to 5 or 6 days ahead.
Problems remain, however, despite the growing
sophistication of the systems used. Gaps exist in
the data required to run the models, for example.
Ocean coverage is thin and the number of
observing stations in high latitudes is small.
Problems of scale may also arise depending upon
the horizontal and vertical resolution of the
model. A coarse resolution, for example, will miss
the development of small scale phenomena such
as the shower cells associated with local
convective activity. A fine resolution model will
provide greater accuracy, but only at a cost. Even
the simplest weather models require a billion
mathematical operations before they can produce
a single day's forecast (Levenson 1989). In setting
up the grid reference points used in the
calculations, therefore, a compromise has to be
struck between the accuracy required and the cost
of running the program. The UK Meteorological
Office, for example has developed two variants
of its forecasting model. The global version has
a horizontal grid with a resolution of 1.5° latitude
by 1.875° longitude, and includes fifteen levels
stretching from the surface into the stratosphere.
In contrast, a finer grid with a resolution of 0.75°
latitude and 0.9375° longitude is only available
for an area covering the North Atlantic and
Western Europe (Barry and Chorley 1992).
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