Tropical Revolving Storms
Tropical Cyclogenesis
• Warm ocean waters - at least 26.5°C
throughout a depth of 50m
• Unstable/moist atmosphere - rapid cooling
with height
• Moist layers near the mid-troposhere
• At least 500km from the Equator with
sufficient Coriolis force to provide
geostrophic rotation
Tropical Cyclogenesis
• Pre-existing near-surface disturbance with
sufficient vorticity & convergence
• Low levels of vertical windshear between
the surface and upper troposphere
Tropical Cyclones
• Low pressure systems deriving energy primarily
from evaporation from a warm sea
• Associated with high winds, lowered pressure and
convective clouds concentrated around the centre
• A tropical cyclone is the generic term for a nonfrontal synoptic scale low-pressure system over
tropical or sub- tropical waters with organised
convection (i.e. thunderstorm activity) and definite
cyclonic surface wind circulation
• The storms tend to affect certain areas:
– They occur mainly on the western edge of oceans
after the winds have had a long sea track and the
air has acquired a large amount of moisture.
– They occur in the summer hemisphere in areas
where the ITCZ has reached a latitude of 10°N or S
or greater and where there is sufficient
geostrophic force to produce cyclonic rotation.
– They do not originate over land.
• The terms "hurricane" and "typhoon" are regionally
specific names for a strong "tropical cyclone".
• Tropical cyclones with maximum sustained surface
winds of less than 17 m/s (34 kt) are called "tropical
depressions".
• Typical tropical cyclones have an “eye” in the centre.
- The "eye" = roughly circular area of comparatively
light winds and fair weather, approx. 30-60km dia.
- Calm winds at the axis of rotation, but strong winds
may extend well into the eye.
- Little or no precipitation and sometimes blue sky or
stars can be seen.
- The eye is composed of air that is slowly sinking.
• Once the tropical cyclone reaches sustained winds of
at least 17 m/s (34 kt) they are typically called a
"tropical storm" and assigned a name.
• If winds reach 33 m/s (64 kt), then they are called:
• "hurricane"
(the North Atlantic Ocean, the Northeast
Pacific Ocean east of the dateline, or the
South Pacific Ocean east of 160E);
• "typhoon"
(the Northwest Pacific Ocean west of the
dateline);
• "severe tropical cyclone"
(the Southwest Pacific Ocean west of 160E
or Southeast Indian Ocean east of 90E);
• "severe cyclonic storm"
(the North Indian Ocean);
• a "tropical cyclone"
(the Southwest Indian Ocean near
Madagascar)
• In West Africa the squally trough is known
locally as a tornado but is not to be confused with
a tornado as described in Chapter 16.
TRS Names
From 1979 individual storms were
allocated
alternate male and female first names
alphabetically starting with A at the
beginning
of each storm season (e.g. Alfred,
Barbara,
Charles, etc.)
TRS Names
• In the Northwest Pacific basin, Asian
names are selected by members of the
WMO Typhoon Committee.
• N. Indian Ocean storms are allocated
numbers
Weather associated with a TRS
• Very strong to extreme wind conditions
• All hazards associated with TS activity
• Severe pressure disturbances making
pressure instruments unreliable
• Torrential rain
• Calm eye with subsidence
Weather associated with a TRS
• Anticylonic outflow above the storm with
subsidence and fine weather around the
edges giving a deceptive calm before
the storm
• Increasing sea swell as the storm
approaches
TRS Tracks and Seasons
HURRICANES
(July - Oct)
CYCLONES
(May - Oct)
TYPHOONS
(May - Nov)
CYCLONES
(Dec - April)
CYCLONES
(Nov - May)
Frequency of Tropical Cyclones
• Tropical Storm/Cyclone averages for the various areas
are:
– N.W.Pacific (South China Sea) about 25 times per
season,
– Atlantic about 9 -10 times per season
– N.E. Pacific (Coast of Central America) about 16 -17
times per season
Frequency of Tropical Cyclones
– N. Indian (Gulf of Arabia and Bay of Bengal); 5 - 6
times per season.
– S.W. Indian Ocean (Madagascar) about 10 - 11 times
per season.
– S.E. Indian Ocean (W. Australia) about 7 times per
season.
– S.W.Pacific (E. Australia) 9 times per season.
– Global Average 84 - 85 per annum.
Easterly wave
Easterly Waves
• The equatorial trough is a permanent series of thermal
lows in a more or less continuous belt around the
earth.
• On both sides of the equatorial low pressure belt,
troughs sometimes extend out towards the sub-tropical
high pressure belts.
• These troughs, known as easterly waves, move
westwards under the influence of the easterly upper
winds.
Easterly Waves
• The Easterly waves are extremely important
phenomena since they are related to the
formation of hurricanes in the north Atlantic.
But not all easterly waves results in a violent
tropical revolving storm.
Easterly Waves
• Easterly waves are most frequent during late
summer when surface water temperature is
highest. They occur mainly in the western
parts of large ocean basins between 5° and
20° latitude.
Easterly Waves
• Easterly waves are usually very apparent over
West Africa where they can be seen from their
formation zone over Sudan/Ethiopia and
westwards.
Easterly Waves
Easterly Waves
• Clusters are often formed east of the Philippines
and north of New Guinea where the water is the
warmest, at about 30°C.
• This is also the area where most tropical
cyclones are generated in the world.
• The clusters formed in this area move westward
towards Borneo but they seldom show the
typical characteristics of an Atlantic Easterly
wave.
Storm Tracks
• What determines the movement of tropical cyclones?
– Generally tropical cyclones tend to follow the circulation around the subtropical highs existing over the oceans.