Geoscience Reference
In-Depth Information
addition, satellites have a data-collection system (DCS)
that relays data on numerous environmental variables
from ground platforms or ocean buoys to processing
centres; GOES can also transmit processed satellite
images in facsimile, and the NOAA polar orbiters have
an automatic picture transmission (APT) system that is
used at about 1000 stations worldwide.
Sutcliffe and others, providing the theoretical basis
of synoptic forecasting. In this way, the position and
intensities of low- and high-pressure cells and frontal
systems were predicted.
Since 1955 in the United States - and 1965 in the
United Kingdom - routine forecasts have been based on
numerical models. These predict the evolution of
physical processes in the atmosphere by determinations
of the conservation of mass, energy and momentum.
The basic principle is that the rise or fall of surface
pressure is related to mass convergence or divergence,
respectively, in the overlying air column. This predic-
tion method was first proposed by L. F. Richardson,
who, in 1922, made a laborious test calculation that gave
very unsatisfactory results. The major reason for this
lack of success was that the net convergence or
divergence in an air column is a small residual term
compared with the large values of convergence and
divergence at different levels in the atmosphere (see
Figure 6.7). Small errors arising from observational
limitations may therefore have a considerable effect on
the correctness of the analysis.
Numerical weather prediction (NWP) methods
developed in the 1950s use a less direct approach. The
first developments assumed a one-level barotropic
atmosphere with geostrophic winds and hence no
convergence or divergence. The movement of systems
could be predicted, but not changes in intensity. Despite
the great simplifications involved in the barotropic
model, it has been used for forecasting 500-mb contour
patterns. The latest techniques employ multi-level
baroclinic models and include frictional and other
effects; hence the basic mechanisms of cyclogenesis are
provided for. It is noteworthy that
fields
of continuous
variables, such as pressure, wind and temperature, are
handled and that fronts are regarded as secondary,
derived features. The vast increase in the number of
calculations that these models perform necessitated a
new generation of supercomputers to allow the prepara-
tion of forecast maps to keep sufficiently ahead of the
weather changes!
Forecast practices in the major national weather
prediction centres around the globe are basically similar.
As an example of the operational use of weather for-
casting models we discuss the methods and procedures
of the National Centers for Environmental Prediction
(NCEP) in Washington, DC, established in 1995. NCEP
currently runs a global spectral model operationally.
The Global Forecast System (GFS) model (formerly
D NUMERICAL WEATHER PREDICTION
General circulation models of all kinds are also applied
operationally to the day-to-day prediction of weather at
centres around the world. Modern weather forecasting
did not become possible until weather information could
be collected, assembled and processed rapidly. The first
development came in the mid-nineteenth century with
the invention of telegraphy, which permitted immediate
analysis of weather data by the drawing of synoptic
charts. These were first displayed in Britain at the Great
Exhibition of 1851. Severe storm events and loss of life
and property prompted the development of weather
forecasting in Britain and North America in the 1860s
to 1870s. Sequences of weather change were correlated
with barometric pressure patterns in both space and time
by such workers as Fitzroy and Abercromby, but it was
not until later that theoretical models of weather systems
were devised, notably the Bjerknes' depression model
(see Figure 9.7).
Forecasts are usually referred to as short-range
(up to approximately three days), medium-range (up to
approximately fourteen days) and long-range (monthly
or seasonal) outlooks. For present purposes, the first two
can be considered together as their methodology is
similar, and because of increasing computing power
they are becoming less distinguishable as separate types
of forecast.
1 Short- and medium-range forecasting
During the first half of the twentieth century, short-range
forecasts were based on synoptic principles, empirical
rules and extrapolation of pressure changes. The
Bjerknes' model of cyclone development for middle
latitudes and simple concepts of tropical weather (see
Chapter 11) served as the basic tools of the forecaster.
The relationship between the development of surface
lows and highs and the upper-air circulation was worked
out during the 1940s and 1950s by C-G. Rossby, R.C.
