Comprehensive Risk Assessment for Natural Hazards - Planat

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Chapter 2 — Meteorological hazards
Figure 2.2 — Satellite
picture of Hurricane Mitch
on 26 October 1998
(weakest) to 8 (strongest) as shown in Figure 2.3. The units
for the abscissa of Figure 2.3 are:
1st line — T number (unitless) corresponding to the
Dvorak scale;
2nd line — mean 10 minutes wind (km/h);
3rd line — sustained 1 minute wind (km/h);
4th line —
5th line —
gusts (km/h);
central pressure measured in units of hectopascal
(hPa).
Perhaps the most important features to be considered
during cyclone watch are the direction and speed of movement.
These are also the most difficult features to obtain as
no specific method of estimating them has been shown to
be perfect, even in the global weather model run at World
Meteorological Centres. This is probably due to a lack of
data at all levels in the oceans. Some of the methods used or
tried are briefly mentioned in the following.
Rules of thumb for forecasting movement using satellite
imagery indicate that, when deep convective cloud clusters
(CCC) develop around the cloud system centre (CSC), which
itself is located at the centre of the eye, then the cyclone is
believed to move towards these CCCs. Elongation of the
cyclone cloud system or the extension of the cirrus shield are
also indications of direction of cyclone movement. Some of
the other methods used by different national Meteorological
Services (NMSs) are: persistence and climatology method;
statistical-dynamic method; dynamic-space mean method;
fixed and variable control-point method; analog method; and
global prediction model. These methods are extensively dealt
with in WMO (1987) Report No. TCP-23.
For the prediction of intensity and wind distribution, a
reconnaissance analysis is conducted in addition to surface
and satellite information. In several cases, real strengths of
tropical storms could not possibly be gauged for several reasons.
The obvious reason is that no direct measurement of
central pressure or maximum wind speed can be made as
the storms evolve mostly over oceans where surface observations
are inadequate. Secondly, cases have been reported
when anemometers gave way after having measured the
highest gust. In Mauritius, for example, the highest ever
recorded gust was on the order of 280 km/h during the passage
of tropical storm Gervaise in February 1975. After this,
the anemometer gave way.
2.4.2 Statistical methods
For the providers of storm and other meteorological hazard
information and appropriate warning, efficient and judicious
use of tried and tested tools is important. The primary
tool in risk analysis and assessment is the application of the
concept of probability. This permits the compilation of the
consequences of various actions in view of the possible outcomes.
If the probability of each possible outcome can be
estimated, then an optimum action can be found that minimizes
the otherwise expected loss. Furthermore, risk
assessment of secondary effects of the hazards previously
described, besides being used for operational hazard forecasting,
can also be useful for informing the public of the
risks involved in developing certain areas and for long-term
land-use planning purposes.
It is, therefore, important to analyse past data of the
magnitude and occurrence of all the hazards and their constituents.
One of the simplest statistical parameters utilized
in meteorology is the return period, Tr. This is defined as the
average number of years within which a given event of a certain
magnitude is expected to be equalled or exceeded. The
event that can be expected to occur, on an average, once
every N years is the N-year event. The concepts of return
period and N-year event contain no implication that an
event of any given magnitude will occur at constant, or even
approximately constant, intervals of N-years. Both terms