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Bài giảng khí hậu học chương 9

G304 – Physical Meteorology and Climatology

Chapter 9
Tropical storms and typhoons

By Vu Thanh Hang, Department of Meteorology, HUS

9.1 Typhoon characteristics and structure
• Extremely strong tropical storms go by a number of different

names, depending on where they occur.
• Over the Atlantic and the eastern Pacific they are known as
• Those over the western Pacific are called typhoons.
• Those over the Indian Ocean and Australia are cyclones.
• In structure, the three kinds of storms are essentially,
although typhoons tend to be larger and stronger than others.
• The region having the greatest number of storms is the
western part of North Pacific.

9.1 Typhoon characteristics and structure

9.1 Typhoon characteristics and structure

9.1 Typhoon characteristics and structure

9.1 Typhoon characteristics and structure
• Typhoons have sustained wind speeds of 120 km/hr and are

typically about 600 kilometers wide.
• Sea level pressure near the center of a typical typhoon is
around 950 mb, but pressures as low as 870 mb have been
observed for extremely powerful typhoons.
• Typhoons obtain most of their energy from the latent heat
released by condensation and are most common where a
deep layer of warm water fuels them.
• August and September are the prime typhoon months in the
NH, while January to March is the main season in the SH.

9.1 Typhoon characteristics and structure

9.1 Typhoon characteristics and structure
• Typhoons do not consist of only one uniform convective cell.

• They contain a large number of thunderstorm arranged in a
pinwheel formation, with bands of thick clouds and heavy
thundershowers spiraling counterclockwise (in NH) around the
storm center.

• The bands of heavy convection are seperated by areas
weaker uplift and even descending air and less intense
• The wind speed and the intensity of precipitation both
increase toward the center of the system (eye), reaching a
maximum 10 to 20km away from the center (eye wall)

9.1 Typhoon characteristics and structure

A cross section of a typical typhoon.

9.1 Typhoon characteristics and structure
• The typhoon eye is a region of relatively clear skies, slowly

descending air, and light winds.
• Along the margin of the eye lies the eye wall, the zone of
most intense storm activity with the strongest winds, thickest
cloud cover, and most intense precipitation of the entire

9.2 Typhoon formation
• Tropical

disturbances are disorganized groups of
thunderstorms having weak pressure gradients and little or no

• When a tropical disturbance develops to the point where
there is at least one closed isobar on a weather map, the
disturbance is classified as a tropical depression.
• If the depression intensifies further and maintains wind
speeds above 60 km/hr, it becomes a tropical storm.
• A further increase in sustained wind speeds to 120 km/hr
creates a true typhoon.

9.2 Typhoon formation (cont.)
• Conditions necessary for typhoon formation:
- the ocean has a deep surface layer with temperatures
above 27°C.
- the coriolis force must be strong enough to prevent
filling of the central low pressure (5o-20o).
- unstable conditions throughout the troposphere.
- an absence of strong vertical wind shear, which
disrupts the vertical transport of latent heat.
• The release of latent heat within the cumulus clouds causes
the air to warm and expand upward Æ supports upper level
divergence Æ draws air upward and promotes low pressure
and convergence at the surface.

9.3 Typhoon movement and dissipation
• The movement of tropical systems is related to the stage in

their development.
• Tropical disturbances and depressions are guided mainly by
the trade winds Æ tend to migrate westward.
• The influence of the trade winds often diminishes after the
depressions intensify into tropical storms.
• The upper level winds and the spatial distribution of water
temperature more strongly determine their speed and direction
Æ tending to move toward warmer seas.
• Typhoon and TS often move in wildly erratic ways.
• After making landfall, a TS may die out within a few days, it
can bring very heavy rainfall hundreds of kilometers inland.

9.3 Typhoon movement and dissipation (cont.)

9.3 Typhoon movement and dissipation (cont.)

9.3 Typhoon movement and dissipation (cont.)

9.4 Typhoon forecasts and advisories
• The standard computer models for conventional weather and
typhoon forecasting can be divided into three categories:
statistical, dynamical, and hybrid.
• Statistical models apply information on past typhoon tracks
and use those tracks as predictors for current storms.
• Dynamical models take information on current atmospheric
and sea surface conditions and apply the governing laws of
physics to current data.
• Hybrid models combine elements of statistical and dynamical

9.4 Typhoon forecasts and advisories (cont.)
• The models repeatedly forecast the movement and internal
changes of typhoons for short time increments Æ give
information on projected storm positions, air pressure, and
wind at 6hr intervals.
• Model forecasts become less accurate as lead time
increases and are unreliable for more than 72 hours.
• When forecasters predict that an approaching typhoon will
reach land in more than 24 hours Æ issue a typhoon watch.
• If it is expected to make landfall within 24 hours Æ issue a
typhoon warning.

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