SCIENCE REVIEW 1987 - Bedford Institute of Oceanography

Research
earth as rain, freezing rain, hail, or snow
depending on the temperature of the air
through which it falls.
The Norwegian model of extra-tropical
cyclonic storms, formulated more than half
a century ago, is still used by meteorologists
to describe the large-scale features of
storms. The weather maps carried by
North American newspapers and television
show them as low pressure systems,
typically 1000 km in size, with their warm
fronts, cold fronts, and precipitation zones,
much as originally described by the
Norwegian meteorologists.
More recent research has shown that
small-scale features also exist within
storms. These features were not described
by earlier investigators who relied on
widely-spaced weather observers on the
ground. Modern research tools, such as
weather radar and sensitive instruments
carried into the atmosphere by balloons
and aircraft, have revealed that within
storms there are cells and bands of intense
precipitation only 10-50 km wide. The
wind patterns, when viewed in detail, are
much more complex than implied by the
large-scale Norwegian model. Narrow
zones of high winds are found along the
warm and cold fronts. These small-scale
storm features may have devastating effects
in terms of property damage and loss of
life, but cannot be predicted using presentday
weather observing and forecasting
methods.
The effects of storms on the ocean.
The ocean responds readily to atmospheric
conditions at its surface (Figure 3). Wind
blowing over the sea generates waves on its
surface, from ripples less than a centimeter
high, to storm-driven waves that can reach
more than 15 m in height. The force of the
wind also causes motion in the water
below the sea surface. The large-scale
general circulation of the oceans is maintained
by the long-term average pattern of
wind over the whole globe. Storm winds,
however, generate locally-intense currents
which may be several times stronger than
the average wind-driven current.
Fluctuations in sea level are also caused
by the surface wind, and by atmospheric
pressure over the ocean. Near coasts,
storm-driven currents may transport water
either toward or away from the land,
Fig. 3 The ocean responds readily to atmospheric conditions at its surface. (a) Wind blowing over the
sea generates surface waves, which grow in proportion to the wind speed and the time and distance
over which it blows. (b) The force of the wind also causes complex motions in the water below the sea
surface. (c) Near coasts, storm-driven currents can transport water toward the land causing sea level
to rise during and after the storm (storm surge). The low atmospheric pressure (L) at the centre of
storms allows water to be forced in from the surrounding region of high air pressure (H), contributing
to storm surge.
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causing extraordinary sea level changes
during and after storms. The low atmospheric
pressure at the centre of storms
allows water to be forced in from the
surrounding region of higher air pressure,
producing a rise in sea level. The sea
surface movement associated with storms,
known as “storm surge,” each year causes
thousands of deaths around the world as
surges flood low-lying coastal areas.
The importance of storm forecasting.
A thorough understanding of storms and
their effects on the ocean is of value for
several reasons. Accurate weather forecasting
obviously benefits everyone whose
activities are affected or controlled by the
weather. In addition to the general public,
this includes, for example, those involved
with public safety, agriculture, transportation
and construction. On Canada’s coasts,
reliable forecasts of the weather and sea
conditions are important to the safety of
thousands of fishermen and workers on
offshore drilling rigs.
Extremes in the weather and the associated
sea conditions also determine the
design of marine structures such as
wharves, breakwaters, ships, and drill rigs.
Civil engineers and naval architects therefore
need to know the levels of extreme
weather that are likely to occur, and the
surface waves, currents, and sea levels that
would result from those extremes. Studies
of the what would have occured under a
given set of circumstances are known as
“hindcasts.”
Computer models of the atmosphere and
ocean are used to make forecasts and
hindcasts. A computer model is a set of
mathematical statements of the physical
laws that govern the motion and heating of
the atmosphere and ocean, and, in the case
of the atmosphere, the formation of clouds
and precipitation. The computer applies
the laws to a known starting state (defined
by observations) and predicts the state for a
short time later. This process is repeated for
a large collection of points in the atmosphere
or ocean, and for a large number of
time steps totalling up to several days. The
accuracy of such computer models is
limited because the governing laws are not
fully understood, and by the speed and size
of available computers. Advances in
understanding the laws of physics and in
computer technology are being made