Comprehensive Risk Assessment for Natural Hazards - Planat

Chapter 2
METEOROLOGICAL HAZARDS
2.1 INTRODUCTION
Despite tremendous progress in science and technology,
weather is still the custodian of all spheres of life on earth.
Thus, too much rain causes flooding, destroying cities, washing
away crops, drowning livestock, and giving rise to
waterborne diseases. But too little rain is equally, if not more,
disastrous. Tornadoes, hail and heavy snowfalls are substantially
damaging to life and property. But probably most
alarming of all weather disturbances are the low pressure
systems that deepen and develop into typhoons, hurricanes or
cyclones and play decisive roles in almost all the regions of the
globe. These phenomena considerably affect the socioeconomic
conditions of all regions of the globe.
The objective of this chapter is to provide a global
overview of these hazards; their formation, occurrence and
life-cycle; and their potential for devastation. It must, however,
be pointed out that these phenomena in themselves are
vast topics, and it is rather difficult to embrace them all in
their entirety within the pages of this chapter. Thus, because
of the severity, violence and, most important of all, their
almost unpredictable nature, greater stress is laid upon tropical
storms and their associated secondary risks such as
storm surge, rain loads, etc.
2.2 DESCRIPTION OF THE EVENT
2.2.1 Tropical storm
In the tropics, depending on their areas of occurrence, tropical
storms are known as either typhoons, hurricanes,
depressions or tropical storms. In some areas they are given
names, whereas in others they are classified according to
their chronological order of occurrence. For example, tropical
storm number 9015 means the 15th storm in the year
1990. The naming of a tropical storm is done by the warning
centre which is responsible for forecasts and warnings in the
area. Each of the two hemispheres has its own distinct storm
season. This is the period of the year with a relatively high
incidence of tropical storms and is the summer of the
hemisphere.
In some regions, adjectives (weak, moderate, strong, etc.)
are utilized to describe the strength of tropical storm systems.
In other regions, tropical storms are classified in ascending
order of their strength as tropical disturbance, depression
(moderate, severe) or cyclone (intense, very intense). For
simplicity, and to avoid confusion, all through this chapter only
the word tropical storm will be used for all tropical cyclones,
hurricanes or typhoons. Furthermore, the tropical storm cases
dealt with in this chapter are assumed to have average wind
speed in excess of 63 km/h near the centre.
During the period when an area is affected by tropical
storms, messages known as storm warnings and storm bulletins
are issued to the public. A storm warning is intended
to warn the population of the impact of destructive winds,
whereas a storm bulletin is a special weather message providing
information on the progress of the storm still some
distance away and with a significant probability of giving
rise to adverse weather arriving at a community in a given
time interval. These bulletins also mention the occurring
and expected sustained wind, which is the average surface
wind speed over a ten-minute period; gusts, which are the
instantaneous peak value of surface wind speed; and duration
of these.
Extratropical storms, as their names suggest, originate
in subtropical and polar regions and form mostly on fronts,
which are lines separating cold from warm air. Depending
on storm strength, warning systems, which are not as systematically
managed as in the case of tropical storms, are
used to inform the public of the impending danger of strong
wind and heavy precipitation.
Warnings for storm surge, which is defined as the difference
between the areal sea level under the influence of a
storm and the normal astronomical tide level, are also
broadcast in areas where such surges are likely to occur.
Storm procedures comprise a set of clear step-by-step rules
and regulations to be followed before, during and after the
visit of a storm in an area. These procedures may vary from
department to department depending on their exigencies.
Tropical storms are non-frontal systems and areas of low
atmospheric pressure. They are also known as “intense vertical
storms” and develop over tropical oceans in regions with
certain specific characteristics. Generally, the horizontal scale
with strong convection is typically about 300 km in radius.
However, with most tropical storms, consequent wind (say 63
km/h) and rain start to be felt 400 km or more from the centre,
especially on the poleward side of the system. Tangential wind
speeds in these storms may range typically from 100 to 200
km/h (Holton, 1973).Also characteristic is the rapid decrease
in surface pressure towards the centre.
Vertically, a well-developed tropical storm can be
traced up to heights of about 15 km although the cyclonic
flow is observed to decrease rapidly with height from its
maximum values in the lower troposphere. Rising warm air
is also typical of tropical storms. Thus, heat energy is converted
to potential energy, and then to kinetic energy.
Several UN-sponsored symposia have been organized
to increase understanding of various scientific aspects of the
phenomena and to ensure a more adequate protection
against the destructive capabilities of tropical storms on the
basis of acquired knowledge. At the same time, attempts
have been made to harmonize the designation and classification
of tropical storms on the basis of cloud patterns, as
depicted by satellite imagery, and other measurable and
determinable parameters.
2.2.2 Necessary conditions for tropical storm genesis
It is generally agreed (Riehl, 1954; Gray, 1977) that the conditions
necessary for the formation of tropical storms are: