Comprehensive Risk Assessment for Natural Hazards - Planat

Comprehensive risk assessment for natural hazards
i.e. over 10 km. They are mainly characterized by a violent
whirl of air affecting a circle of about a hundred metres in
diameter, but in which winds of over 300 km/h or more
blow near the centre. The whirl is broadest just under the
cloud base and tapers down to where it meets the ground.
Visually, it appears as a dark curly funnel-shaped cloud. The
darkness is due to the presence of thick clouds, torrential
rain, dust and debris. Despite modern means at the disposal
of meteorologists, tornadoes give little time for evacuation
or preparation. They form in a short time and often move
quickly along unpredictable tracks. Thus, for every event,
lives and property could be placed in jeopardy.
Conditions favourable to the formation of tornadoes
are when maritime polar air overruns maritime tropical air
leading to high atmospheric instability. The most violent
and frequent tornadoes have been observed to form in the
middle west and central plains of the USA, where hundreds
of millions of dollars worth of damage are inflicted to property
every year. They also occur in other localities mainly in
temperate zones but are of a lesser intensity.
Tornadoes within tropical storms are much more common
than was once assumed. Jarrell (1987) suggests that
tornadoes are expected for about half of the landfalling
tropical storms. Analysis of proximity soundings show that
the large-scale feature associated with tornadoes is very
strong vertical shear of the vertical wind between the surface
and 1.5 km. This shear is estimated to be around 23 m/s
compared to about half the value in tropical storms without
tornadoes.
2.3.10 Heatwaves
Temperatures where humans can live in comfort, without
the need for heating or artificial air conditioning, is generally
accepted to be in the range of 20 to 28°C. Below 20°C,
the need to be dressed in warm clothes is required, whereas
above 28°C artificial cooling of the surrounding air
becomes necessary. However, the human ability to adapt
provides another two degrees of tolerance on either side of
this range.
Temperatures above 30°C are very hard on society,
especially the elderly, the sick and infants. Exposures to such
temperatures may affect the output of people at work. At
temperatures above 35 to 40°C, human health is threatened.
Persistent occurrence of such high temperatures over a
period of time ranging from days to a week or so is known
as a heatwave. Heatwaves have been found to be a major
threat to human health and well-being and are most prevalent
over large land masses and megacities of the world
during the warmer months of the year.
The threat to human health from the impact of heat
stress is one of the most important climate-related health
issues facing all nations. Every year several hundreds die as
a result of heatwaves, with the elderly being the most affected.
This was clearly apparent in the summer of 1960, when during
a heatwave event, the number of deaths in New York
soared well above the average.
In June 1998 at the peak of northern summer, the north
of India witnessed a higher-than-usual number of deaths
linked to dehydration and the extreme heat of 45 to 50°C
which persisted for several days. Those who survive such
heatwaves definitely emerge affected. This is also reflected in
the economy as national production is reduced. Output of
people at work and yield from crops greatly suffers during
such events.
The phenomenon of a heatwave is not always apparent.
There is a need to devise methods, which will filter the wave
from the predicted general temperature patterns of predicted
meteorological conditions. Projects within meteorology
geared to the mitigation of the potential impacts of heatwaves
would be useful.
2.4 TECHNIQUES FOR HAZARD ANALYSIS AND
FORECASTING
2.4.1 Operational techniques
As mentioned earlier, tropical storms can encompass huge
areas. Figure 2.2 shows the cloud mass and wind field associated
with tropical storm Hurricane Mitch on 26 October
1998 using the GOES 8 satellite. The figure shows the spiralling
tentacles of cloud bands that cover a large portion of
the Caribbean Sea. Different methods, depending on
regions, are used to analyse and assess the “content” of such
events. This would consist of:
(a) Analysis of the central position, intensity and wind
distribution;
(b) 12-, 24- and 48-hour forecasts of the central position;
(c) Forecasts of intensity and wind distribution; and
(d) Diagnostic reasoning and tendency assessment, if
applicable.
The central position of tropical storms can be extrapolated
based on the persistence of the storm movement in
the hours immediately preceding the given moment and a
reliable,accurate current position.Ifwithin “reach”ofa
radar(s), fairly accurate positions can be obtained.
Reconnaissance flight observations also provide useful
information for position determination. Satellite analysis is
another efficient tool, especially with geostationary
satellites.
The assessment of tropical cyclone intensity can be performed
by using the empirical relation between central
pressure and maximum wind given by the equation:
V m = 12.4 (1010-P c ) 0.644 (2.1)
Vm = maximum sustained (one minute) wind speed (km/h)
Pc = minimum sea-level pressure (hectopascal).
Reconnaissance flight analysis is an additional tool for
intensity assessment and consists of different types of data —
eye data, drop sonde data, peripheral data and flight report.
Tropical storm intensity analyses are, in some regions,
conducted using the technique described by Dvorak (1984).
This technique uses pictures both in the infrared and visible
wavelengths. It considers whether or not the tropical storm
has an eye, the diameter of this eye, and cloud band widths
and spiralling length. The result of analysis culminates
in a digital value (T No.) or Dvorak scale ranging from 1
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