Chapt - 36.pdf - po ang - Januszek66

CHAPTER 35

TROPICAL CYCLONES

DESCRIPTION AND CAUSES

3500. Introduction

closed isobars and a distinct rotary circulation. The highest

sustained (1-minute mean) surface wind speed is 34 to 63

knots.

When fully developed, a hurricane or typhoon has

closed isobars, a strong and very pronounced rotary

circulation, and a sustained (1-minute mean) surface wind

speed of 64 knots or higher.

A tropical cyclone is a cyclone originating in the

tropics or subtropics. Although it generally resembles the

extratropical cyclone of higher latitudes, there are

important differences, the principal one being the concen-

tration of a large amount of energy into a relatively small

area. Tropical cyclones are infrequent in comparison with

middle and high latitude storms, but they have a record of

destruction far exceeding that of any other type of storm.

Because of their fury, and because they are predominantly

oceanic, they merit special attention by mariners.

A tropical storm may have a deceptively small size,

and beautiful weather may be experienced only a few

hundred miles from the center. The rapidity with which the

weather can deteriorate with approach of the storm, and the

violence

3502. Areas of Occurrence

Tropical cyclones occur almost entirely in six distinct

areas, four in the Northern Hemisphere and two in the

Southern Hemisphere, as shown in Figure 3502. The name

by which the tropical cyclone is commonly known varies

somewhat with the locality.

the

fully

developed

tropical

cyclone,

are

North Atlantic. A tropical cyclone with winds of 64

knots or greater is called a hurricane .

difficult to imagine if they have not been experienced.

On his second voyage to the New World, Columbus

encountered a tropical storm. Although his vessels suffered

no damage, this experience proved valuable during his

fourth voyage when his ships were threatened by a fully

developed hurricane. Columbus read the signs of an

approaching storm from the appearance of a southeasterly

swell, the direction of the high cirrus clouds, and the hazy

appearance of the atmosphere. He directed his vessels to

shelter. The commander of another group, who did not heed

the signs, lost most of his ships and more than 500 men

perished.

Eastern North Pacific. The name hurricane is used

as in the North Atlantic.

Western North Pacific. A fully developed storm

with winds of 64 knots or greater is called a

typhoon or, locally in the Philippines, a baguio .

North Indian Ocean. A tropical cyclone with winds

of 34 knots or greater is called a cyclonic storm .

South Indian Ocean. A tropical cyclone with winds

of 34 knots or greater is called a cyclone .

Southwest Pacific and Australian Area. The name

cyclone is used as in the South Indian Ocean. A

severe tropical cyclone originating in the Timor Sea

and moving southwest and then southeast across the

interior of northwestern Australia is called a willy-

willy .

3501. Definitions

Tropical cyclones are classified by form and intensity

as they increase in size.

A tropical disturbance is a discrete system of

apparently organized convection, generally 100 to 300

miles in diameter, having a nonfrontal migratory character,

and having maintained its identity for 24 hours or more. It

may or may not be associated with a detectable disturbance

of the wind field. It has no strong winds and no closed

isobars i.e., isobars that completely enclose the low.

At its next stage of development it becomes a tropical

depression . A tropical depression has one or more closed

isobars and some rotary circulation at the surface. The highest

sustained (1-minute mean) surface wind speed is 33 knots.

The next stage is tropical storm . A tropical storm has

Tropical cyclones have not been observed in the South

Atlantic or in the South Pacific east of 140

3503. Origin, Season and Frequency

See Figure 3503a and Figure 3503b . Origin, season,

and frequency of occurrence of the tropical cyclones in the

six areas are as follows:

North Atlantic: Tropical cyclones can affect the

entire North Atlantic Ocean in any month. However, they

are mostly a threat south of about 35

N from June through

November;

August,

September,

and

October

are

the

503

504

TROPICAL CYCLONES

Figure 3502. Areas in which tropical cyclones occur. The average number of tropical cyclones per 5˚ square has been

analyzed for this figure. The main season for intense tropical storm activity is also shown for each major basin.

months of highest incidence. See Figure 3503b . About 9

or 10 tropical cyclones (tropical storms and hurricanes)

form each season; 5 or 6 reach hurricane intensity (winds

of 64 knots and higher). A few hurricanes have generated

winds estimated as high as 200 knots. Early and late

season storms usually develop west of 50

September are the months of highest incidence. These

storms differ from their North Atlantic counterparts in that

they are usually smaller in size. However, they can be just

as intense.

Western North Pacific: More tropical cyclones form

in the tropical western North Pacific than anywhere else in

the world. More than 25 tropical storms develop each year,

and about 18 become typhoons. These typhoons are the larg-

est and most intense tropical cyclones in the world. Each

year an average of five generate maximum winds over 130

knots; circulations covering more than 600 miles in diameter

are not uncommon. Most of these storms form east of the

Philippines, and move across the Pacific toward the Philip-

pines, Japan, and China; a few storms form in the South

China Sea. The season extends from April through Decem-

ber. However, tropical cyclones are more common in the

off-season months in this area than anywhere else. The peak

of the season is July through October, when nearly 70 per-

cent of all typhoons develop. There is a noticeable seasonal

shift in storm tracks in this region. From July through Sep-

tember, storms move north of the Philippines and recurve,

while early- and late-season typhoons move on a more west-

erly track through the Philippines before recurving.

W; during

August and September, this spawning ground extends to

the Cape Verde Islands. These storms usually move

westward or west northwestward at speeds of less than 15

knots in the lower latitudes. After moving into the

northern Caribbean or Greater Antilles regions, they

usually either move toward the Gulf of Mexico or recurve

and accelerate in the North Atlantic. Some will recurve

after reaching the Gulf of Mexico, while others will

continue westward to a landfall in Texas or Mexico.

Eastern North Pacific: The season is from June

through October, although a storm can form in any month.

An average of 15 tropical cyclones form each year with

about 6 reaching hurricane strength. The most intense

storms are often the early- and late-season ones; these form

close to the coast and far south. Mid season storms form

anywhere

wide

band

from

the

Mexican-Central

American

coast

the

Hawaiian

Islands.

August

and

TROPICAL CYCLONES

505

Figure 3503a. Storm tracks.The width of the arrow indicates the approximate frequency of storms; the wider the arrow

the higher the frequency. Isolines on the base map show the resultant direction toward which storms moved. Data for the

entire year has been summarized for this figure.

AREA AND STAGE

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANNUAL

NORTH ATLANTIC

TROPICAL STORMS

0.1

0.4

0.3

1.0

1.5

1.2

0.4

4.2

HURRICANES

0.3

0.4

1.5

2.7

1.3

0.3

5.2

TROPICAL STORMS AND HURRICANES

0.2

0.7

0.8

2.5

4.3

2.5

0.7

0.1

9.4

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANNUAL

EASTERN NORTH PACIFIC

TROPICAL STORMS

1.5

2.8

2.3

1.2

0.3

9.3

HURRICANES

0.3

0.6

0.9

2.0

1.8

1.0

5.8

TROPICAL STORMS AND HURRICANES

0.3

2.0

3.6

4.5

4.1

2.2

0.3

15.2

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANNUAL

WESTERN NORTH PACIFIC

TROPICAL STORMS

0.2

0.3

0.2

0.4

0.5

1.2

1.8

1.5

1.0

0.8

0.6

7.5

TYPHOONS

0.3

0.2

0.7

0.9

1.2

2.7

4.0

4.1

3.3

2.1

0.7

17.8

TROPICAL STORMS AND TYPHOONS

0.4

0.5

0.9

1.3

1.8

3.9

5.8

5.6

4.3

2.9

1.3

25.3

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANNUAL

SOUTHWEST PACIFIC AND AUSTRALIAN AREA

TROPICAL STORMS

2.7

2.8

2.4

1.3

0.3

0.2

0.1

0.4

1.5

10.9

HURRICANES

0.7

1.1

1.3

0.3

0.1

0.3

0.5

3.8

TROPICAL STORMS AND HURRICANES

3.4

4.1

3.7

1.7

0.3

0.2

0.1

0.7

2.0

14.8

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANNUAL

SOUTHWEST INDIAN OCEAN

TROPICAL STORMS

2.0

2.2

1.7

0.6

0.2

0.3

0.8

7.4

HURRICANES

1.3

1.1

0.8

0.4

0.5

3.8

TROPICAL STORMS AND HURRICANES

3.2

3.3

2.5

1.1

0.2

0.3

0.4

1.4

11.2

1 Winds

* Less than 0.05

48 Kts.

Monthly values cannot be combined because single storms overlapping two months were counted once in each month and once in the annual.

Figure 3503b. Monthly and annual average number of storms per year for each area.

506

TROPICAL CYCLONES

AREA AND STAGE

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANNUAL

NORTH INDIAN OCEAN

TROPICAL STORMS

0.1

0.3

0.5

0.4

0.6

0.5

0.3

3.5

CYCLONES 1

0.1

0.5

0.2

0.1

0.4

0.6

0.2

2.2

0.1

0.3

0.7

0.6

0.4

0.5

1.0

1.1

0.5

5.7

TROPICAL STORMS AND CYCLONES 1

1 Winds

* Less than 0.05

48 Kts.

Monthly values cannot be combined because single storms overlapping two months were counted once in each month and once in the annual.

Figure 3503b. Monthly and annual average number of storms per year for each area.

North Indian Ocean— Tropical cyclones develop in

the Bay of Bengal and Arabian Sea during the spring and

fall. Tropical cyclones in this area form between latitudes

(tropical storms and hurricanes) form each season, and

about 4 reach hurricane intensity. The season is from

December through March, although it is possible for a

storm to form in any month. Tropical cyclones in this

region

N, except from June through September, when

the little activity that does occur is confined north of about

N and 15

usually

form

south

The

latitude

N. These storms are usually short-lived and weak;

however, winds of 130 knots have been encountered. They

often develop as disturbances along the Intertropical

Convergence Zone (ITCZ); this inhibits summertime

development, since the ITCZ is usually over land during

this monsoon season. However, it is sometimes displaced

southward, and when this occurs, storms will form over the

monsoon-flooded plains of Bengal. On the average, six

cyclonic storms form each year. These include two storms

that generate winds of 48 knots or greater. Another 10

tropical cyclones never develop beyond tropical

depressions. The Bay of Bengal is the area of highest

incidence. However, it is not unusual for a storm to move

across southern India and reintensify in the Arabian Sea.

This is particularly true during October, the month of

highest incidence during the tropical cyclone season. It is

also during this period that torrential rains from these

storms, dumped over already rain-soaked areas, cause

disastrous floods.

South Indian Ocean— Over the waters west of 100

recurvature usually migrates from about 20

S in January to

around 15

S, these storms

sometimes become intense extratropical lows.

Southwest Pacific and Australian Area— These

tropical waters spawn an annual average of 15 tropical

cyclones 4, of which reach hurricane intensity. The season

extends from about December through April, although

storms can form in any month. Activity is widespread in

January and February, and it is in these months that

tropical cyclones are most likely to affect Fiji, Samoa, and

the other eastern islands. Tropical cyclones usually form

in the waters from 105

S in April. After crossing 30

Storms affecting northern and western Australia often

develop in the Timor or Arafura Sea, while those that

affect the east coast form in the Coral Sea. These storms

are often small, but can develop winds in excess of 130

knots. New Zealand is sometimes reached by decaying

Coral Sea storms, and occasionally by an intense

hurricane. In general, tropical cyclones in this region

move southwestward and then recurve southeastward.

E to 160

W, between 5

and 20

to the east African coast, an average of 11 tropical cyclones

ANATOMY OF TROPICAL CYCLONES

3504. Formation

conditions needed to generate hurricane circulation, and the

relationships between large- and small-scale atmospheric

processes. But scientists today, treating the hurricane sys-

tem

Hurricane formation was once believed to result from

an intensification of convective forces which produce the

towering cumulonimbus clouds of the doldrums. This view

of hurricane generation held that surface heating caused

warm moist air to ascend convectively to levels where

condensation produced cumulonimbus clouds, which, after

an inexplicable drop in atmospheric pressure, coalesced

and were spun into a cyclonic motion by Coriolis force.

This hypothesis left much unexplained. Although some

hurricanes develop from disturbances beginning in the dol-

drums, very few reach maturity in that region. Also, the high

incidence of seemingly ideal convective situations does not

match the low incidence of Atlantic hurricanes. Finally, the

hypothesis did not explain the drop in atmospheric pressure,

so essential to development of hurricane-force winds.

There is still no exact understanding of the triggering

mechanism involved in hurricane generation, the balance of

atmospheric

heat

engine,

present

comprehensive and convincing view.

They begin with a starter mechanism in which either

internal or external forces intensify the initial disturbance.

The initial disturbance becomes a region into which low-

level air from the surrounding area begins to flow,

accelerating the convection already occurring inside the

disturbance. The vertical circulation becomes increasingly

well organized as water vapor in the ascending moist layer

is condensed (releasing large amounts of heat energy to

drive the wind system), and as the system is swept into a

counterclockwise cyclonic spiral. But this incipient

hurricane would soon fill up because of inflow at lower

levels, unless the chimney in which converging air surges

upward is provided the exhaust mechanism of high-altitude

winds.

TROPICAL CYCLONES

507

Figure 3504. Pumping action of high-altitude winds.

These high-altitude winds pump ascending air out of

the cyclonic system, into a high-altitude anticyclone, which

transports the air well away from the disturbance, before

sinking occurs. See Figure 3504 . Thus, a large scale vertical

circulation is set up, in which low-level air is spiraled up the

cyclonic twisting of the disturbance, and, after a trajectory

over the sea, returned to lower altitudes some distance from

the storm. This pumping action-and the heat released by the

ascending air may account for the sudden drop of

atmospheric pressure at the surface, which produces the

steep pressure gradient along which winds reach hurricane

proportions.

It is believed that the interaction of low-level and high-

altitude wind systems determines the intensity the hurricane

will attain. If less air is pumped out than converges at low

levels, the system will fill and die out. If more is pumped out

than flows in, the circulation will be sustained and will

intensify.

Scientists have found that any process which increases

the rate of low-level inflow is favorable for hurricane

development, provided the inflowing air carries sufficient

heat and moisture to fuel the hurricane’s power system. It has

also been shown that air above the developing disturbance, at

altitudes between 20,000 and 40,000 feet, increases 1

3505. Portrait of a Hurricane

In the early life of the hurricane, the spiral covers an

area averaging 100 miles in diameter with winds of 64

knots and greater, and spreads gale-force winds over a 400-

mile diameter. The cyclonic spiral is marked by heavy

cloud bands from which torrential rains fall, separated by

areas of light rain or no rain at all. These spiral bands

ascend in decks of cumulus and cumulonimbus clouds to

the convective limit of cloud formation, where condensing

water vapor is swept off as ice-crystal wisps of cirrus

clouds. See Figure 3505 . Thunderstorm electrical activity is

observed in these bands, both as lightning and as tiny

electrostatic discharges.

In the lower few thousand feet, air flows in through the cy-

clone, and is drawn upward through ascending columns of air

near the center. The size and intensity decrease with altitude,

the cyclonic circulation being gradually replaced above 40,000

feet by an anticyclonic circulation centered hundreds of miles

away, which is the exhaust system of the hurricane heat engine.

At lower levels, where the hurricane is more intense,

winds on the rim of the storm follow a wide pattern, like the

slower currents around the edge of a whirlpool; and, like

those currents, these winds accelerate as they approach the

center of the vortex. The outer band has light winds at the

rim of the storm, perhaps no more than 25 knots; within 30

miles of the center, winds may have velocities exceeding

130 knots. The inner band is the region of maximum wind

velocity, where the storm’s worst winds are felt, and where

ascending air is chimneyed upward, releasing heat to drive

the storm. In most hurricanes, these winds reach 85 knots,

and more than 170 knots in severe storms.

In the hurricane, winds flow toward the low pressure in

the warm, comparatively calm core. There, converging air

is whirled upward by convection, the mechanical thrusting

of other converging air, and the pumping action of high-

altitude circulations. This spiral is marked by the thick

in temperature about 24 hours before the disturbance

develops into a hurricane. But it is not known whether low-

level inflow and high-level warming cause hurricanes. They

could very well be measurable symptoms of another effect

which actually triggers the storm’s increase to hurricane

intensity.

The view of hurricanes as atmospheric engines is

necessarily a general one. The exact role of each contributor

is not completely understood. The engine seems to be both

inefficient and unreliable; a myriad of delicate conditions

must be satisfied for the atmosphere to produce a hurricane.

Their relative infrequency indicates that many potential

hurricanes dissipate before developing into storms.

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