Geoscience Reference
In-Depth Information
Hurricane Katrina, August 2005
HUMAN IMPACT
In August 2005 one of the strongest hurricanes in the last hundred years headed towards the Gulf of Mexico coast.
Unfortunately the part of the coast affected was the low-lying Mississippi delta and the city of New Orleans. The
combination of strong winds, heavy rain, a storm surge and flooding as some of the sea defences gave way brought
devastation to the area, with 80 per cent of the city under water. Wind speeds of over 140 mph were recorded at
landfall in south-eastern Louisiana and gusts of over 100 mph were observed in New Orleans. Rainfall was estimated
at between 200 mm and 250 mm along much of the hurricane's track, with intensities of over 25 mm hr
-1
for three
consecutive hours at many locations.
With this severe barrage of the elements, it is not surprising that the area around New Orleans suffered. At least
1,836 people are estimated to have lost their lives and damage has been estimated at over $81 billion. Immediately
after the storm there was little power for the city, roads had been damaged, oil refineries closed and many offshore
oil platforms affected. Lack of power meant pumps needed to remove the flood water were inoperative. Parts of
the coastline were affected, with some small islands being washed away and severe erosion along the coast line.
The storm and its track had been correctly predicted but the evacuation process took too long for the large number
of people affected. Some were accommodated in large public buildings such as the Superbowl for protection during
and after the storm. Many lessons have been learnt about the impact that severe tropical storms can have on our
economy and environment.
is not true. It appears that small deviations can seriously
affect the development of weather-forming systems. New
predictions have to be made on a daily basis to incorporate
small-scale changes which could become very important.
The problem is that we do not have enough information
(or large enough computers) to solve the equations
accurately.
Efforts are being made to improve our techniques of
long-period forecasting but success has been limited. In
the United Kingdom monthly forecasts of temperature
and precipitation used to be prepared on the basis of
previous weather analogues. For example, if the atmos-
pheric circulation and sea surface temperature patterns in,
say, July 2007 were very similar to those of July 1964, it
could be assumed that the weather in August 2007 should
be the same as that in August 1964. Other controlling
factors such as ice cover, the state of the El Niño-Southern
Oscillation and the Quasi-biennial Oscillation (a feature
of the wind circulation in the tropical upper troposphere
and lower stratosphere) are also included to improve the
accuracy of the forecast. However, even if the basic
circulation pattern is correctly predicted, slight errors
in the tracks of cyclones or anticyclones can produce
markedly different weather. Future developments in
monthly forecasting are likely to be built around
numerical ensemble methods, used to generate probability
forecasts. The ensembles are produced as a set of runs
from a global numerical model, all slightly different. Their
output is interpreted in terms of the probability of the
occurrence of particular circulation types.
HAZARDS
It is possible to predict many of the weather hazards
discussed earlier, but we have to distinguish between large-
scale and small-scale hazards. The longer the time scale
of development the larger will be the area affected. Major
droughts, as the result of a reduction in the number of
rain-generating systems, usually affect a large area and take
many months to develop, though our techniques of long-
term forecasting are not good at predicting when the
drought will finish. Tropical and temperate-latitude
cyclones can be predicted reasonably well, so that we
know approximately the areas they are likely to affect. On
a smaller scale, the warnings of tornado formation are
announced for a large area, but precisely where the funnel
clouds will touch down on the surface is not known. It is
probably impossible to forecast such conditions for more
than a few minutes ahead. Flash floods from a single
thunderstorm are in a similar category. We have to accept
them as one of the micro-scale features of our atmosphere
that occasionally may cause devastation over a small area.
The chance of any one site being affected by them is very
small.
