Hurricanes: Their Nature and Impacts on Society - Climate Science ...

CHAPTER 4
Hurricane Forecasts
4.1 TROPICAL CYCLONE MOVEMENT
Tropical cyclones move because the storm is embedded in a larger-scale region
of moving air, referred .to as the steering current, which tends to move the
10w-1eve110w pressure center, upper-level high pressure <strong>and</strong> associated cluster
of thunderstorms in the direction of that flow (e.g. see Riehl <strong>and</strong> Burgner
1950; Riehl <strong>and</strong> Shafer 1946; Simpson 1946). Tropical cyclones of different
intensity are steered by winds at different levels in the troposphere (Figure
4.1).1 The cyclone itself, of course, is part of the large-scale flow, <strong>and</strong> its
motion is also influenced by its own internal circulation. This sets up a
complex process of interaction that is a challenge to predict. Yet accurate
prediction of a hurricane's movement is central to short-term decisions to
protect life <strong>and</strong> property.
4.2 EXTERNAL FLOW: THE STEERING CURRENT
If the steering flow were fixed in time, hurricane track forecasting would be
comparatively simple. Unfortunately, this is not the case, as the orientation
<strong>and</strong> strength of the steering current changes in response to the position of
large-scale pressure features. Contrary to popular conception, however, in the
Atlantic most tropical cyclones have fairly regular, well-defined tracks because
the location <strong>and</strong> orientation of the Bermuda <strong>and</strong> Azores high pressure
systems, which determine the track of most Atlantic tropical cyclones, usually
change only slowly during the hurricane season. However, the difficulty in
predicting a storm track occurs either when the typical climatological steering
1 G.J. Holl<strong>and</strong> of the Bureau of Meteorology in Melbourne, Australia suggests
using the winds
averaged within a concentric b<strong>and</strong> of 125-250 miles (200-400 kIn) from the storm center. In
another study, tropical cyclones were found to move about 2-4 miles per hour (1-2 meters per
second) faster <strong>and</strong> 10 to 20 degrees to the left of the mean wind flow between about 5000 feet
(~1.5 kIn) <strong>and</strong> 30 000 feet (~9 kIn) averaged over an area within a 5 to 7 degree of latitude radius
centered on the storm (McElroy 1996).