tropical revolving storm

M A R I T I M E I N S T I T U T E

These bad weather systems originate on the Equatorial side of the oceanic sub-tropical anticyclones, on the periphery of the Trade Wind flow.

‘Trade Wind inversion’

plays a significant part in their formation.

Subsiding air in the anticyclone heats adiabatically at the dry lapse rate and a marked inversion forms at around 500m on the Eastern side of the ocean.

Air below inversion is moist and tends towards instability.

The trade winds are a pattern of wind that are found in bands around the

Earth's equatorial region. The trade

winds are the prevailing winds in the tropics, blowing from the high-pressure

area in the horse latitudes towards the low-pressure area around the

equator. The trade winds blow

predominantly from the northeast in the northern hemisphere and

from the southeast in the southern hemisphere

THE TRADE WINDS

Downstream, the inversion weakens as the degree of subsidence decreases,

the inversion level rises, and

‘cumulus towers’

penetrate through the inversion spreading moisture into the dry air above.

Tropical waves, or easterly waves, also known

as African easterly waves in the Atlantic region,

are a type of atmospheric trough,

an elongated area of relatively low air pressure, oriented north to south,

which move from east to west across

the tropics causing areas of

cloudiness and thunderstorms.

West-moving waves can also form from

the tail end of frontal zones in the subtropics and tropics and may be referred to as easterly waves,

but these waves are not properly called tropical waves; they are a form of inverted trough

sharing many characteristics with fully tropical waves.

All tropical waves form in the easterly flow

along the southern side of the subtropical ridge or belt of high pressure which lies

north and south of the Intertropical Convergence Zone (ITCZ).

Tropical waves are generally carried

westward by the prevailing

easterly winds along

the tropics and subtropics

near the equator.

They can lead to the formation of

tropical cyclones

in the north Atlantic and

northeast Pacific basins.

Weather Sequence of an Easterly Wave

In the ridge ‘ahead of the trough’:fine weather, scattered cumulus, some haze.

‘Close to the trough’ :Well developed cumulus, occasional showers, improving visibility.

‘Behind the trough’ : Veering of wind, heavy Cu/Cb, moderate/heavy showers, good visibility except in rain.

Waves in the trade winds in the Atlantic Ocean

Few easterly wave depressions

develop into closed

isobaric structures

with cyclonic votices.

2. Tropical Depressions

Principal conditions for the formation of cyclonic motion…

a. Latitude north/south of 5-6 deg N/Sb. Surfcae water temp. at least 27deg C.c. Presence of groups of small islands

increases convective lift due to higher insolation.

d. ITCZ should be well removed from the Equator.

Conditions b and c are found on the Western sides of the N Atlantic,

N and S Pacific and S Indian ocean (where the water is shallower and warmer than on the eastern sides of these oceans).

Condition d occurs during late summer when the ITCZ is farthest removed from equator.

Tropical depressions are almost unknown in the S Atlanticand in the eastern side of the S Pacific for the following reasons :

a)Sea surface temps, are low due to the cool currents brought about by upwelling of cold water from below.

b) There are no island archipelagoes.

c) The ITCZ never extends south of the equator in these regions.

75%

of all

tropical depressions

occur in

the Northern Hemisphere.

3. Tropical Hurricanes :

…on the western side of the Trade Wind where the inversion is higher and less active,the surface temp. is highest, Tropical Depressions

may develop into hurricanes

In meteorology, a tropical cyclone (also referred to as a tropical depression, tropical storm, typhoon, or hurricane depending on strength and geographical context) is a type of low pressure system which generally forms in the tropics. While they can be highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the higher latitudes.

DEFINITION of a TRS :

A tropical revolving storm may be defined as a roughly circular atmospheric

vortex, originating in the tropics or subtropics, wherein the winds which blow

in converging spiral tracks ( anticlkws in NH and clockws in SH)

Reach or exceed gale force 8.

Intensity of winds in tropical depressions :

1.‘Tropical depression’ less than force 7.

2.‘Moderate tropical storm’ force 8 to 9.

3.‘Severe tropical storm’ force 10 to 11.

4.‘Hurricane’ reach force 12 and above.

Pre-requisite for development of a tropical storm:

1.Sea surface temp. in excess of 26.5deg C

2.Latitude must not be less than 5deg N or S.

3.Small vertical wind shear.

4.A pre-existing depression.

Model vertical profile of a mature typhoon. The central portion is the eye and eye wall.

Model vertical profile of a mature typhoon. The central portion is the eye and eye wall.

…A distinctive feature of the hurricane is the

warm vortex,which developes through

the action of large number of Cb towers

releasing latent heat of condensation into the atmosphere

…the warm core is vital to hurricane growth because

it intensifies the upper anticyclone,and stimulates the low level influx

of heat and moisture,which further intensifies

convective activity and therefore the upper level high pressure system.

…In the eye, which has a diameter of 30-40 km,

adiabatic warming of descending airaccentuates the high temperaturesand results in calm conditions and

sometimes clear skies.The temp. of the descending air

in the eye may be up to 18deg warmer than the temp in the same plane

in the ascending air

…The supply of heat and moisture combined with

low frictional drag at the sea surface,the release of latent heat through

condensation, and the removal of air aloft,

are essential conditions for the maintenance of hurricane intensity.As soon as one of these ingredients

is lost, the storm decays

…If the vortex passes overland increased friction hastens

the process of filling,while the cutting off of the moisture supply removes one of the major sources of heat.

If the vortex passes over a cool sea,cooler air drawn into the circulation

results in a rapid decay of the system.

…Radar and satellite photographyreveal that cloud and precipitationwithin a hurricane are arranged in spiral bands whose axes are

parallel to the surface wind streamlines.

Saffir-Simpson Hurricane ScaleCategory Wind speed Storm surge

mph(km/h)

ft(m)

5 ≥156(≥250)

>18 (>5.5)

4 131–155(210–249)

13–18(4.0–5.5)

3 111–130(178–209)

9–12(2.7–3.7)

2 96–110(154–177)

6–8(1.8–2.4)

1 74–95(119–153)

4–5(1.2–1.5)

Additional classificationsTropical

storm39–73(63–117)

0–3(0–0.9)

Tropicaldepression

0–38(0–62)

0

Wind speed record of Typhoon York Sept 1999

Indications of a Tropical Hurricane in the Vicinity

a) Swell

b) Change in Trades

c) Irregularity in semi-diurnal range

d) Oppressive atmosphere

a)A heavy swell approaching from the storm field…

Large waves generated by the storm pass beyond the limits of the disturbance, and travel great distances.

The swell travels at a much greater velocity than the storm centre, an observer may be forewarned of the existence of the storm by 2 or 3 days, and thus giving first warning and the storm bearing.

b) A change in

the intensity or

direction of the Trades.

c) Irregularity in

the semi-diurnal range,

or the barometric pressure

more than 3 mbs,

below seasonal normal for

the particular region and

time of day.

d) An oppressive atmosphere

with increasing humidity, and

an ugly, lurid sky

at sunset /sunrise.

Sunset view of Hurricane rain-bands

Storm tracks worldwide

Tracks followed by tropical revolving storms

(A)….Movement of Tropical Hurricanes :

a) They tend to move parallel to isobars which lie outside the storm field at about 10 to 12 kts.

b) They re-curve pole-ward around the western margin of the sub-tropical high –pressure cells, entering the circulation of the westerlies.

Track of a typhoon

(B)….Movement of Tropical Hurricanes :

c) After re-curvature, they tend to accelerate to 15 to 20 kts and gradually die out or degenerate into extra-tropicaldepressions.

d) Occasionally, they fail to re-curve and cross continental coast wreaking heavy damage especially where the land is low.

(C)….Movement of Tropical Hurricanes :

e) Cirri-form clouds, up to 1000kms from the centre, in thick bands converging on the centre.

f) PRESSURE… From 500m to 120m from centre:

slow fall of pressure from 120m to 60m from centre:

distinct fall of pressure from 60m to centre :

a rapid fall.

g) Rain showers give way to torrential rain,With visibility seriously reduced.

Typical barogram of a hurricane

1. To find thebearing ofthe stormcentre:By BuysBallot’s Lawand facingthe wind…

Plotting the probable path of the centre

Plotting the probable path of the centre…

Radio reports of the presence of a tropical hurricane will indicate the present position course and speed of the centre.

As the information is correct up to the last observation, the master can only make an approximate estimation of the future path.

In any one 24 hour period, the centre will normally be expected to move into an area covered by a sector extending 40 deg either side of its present path, so the master should re-plot the centre on receipt of every message, and endeavor to avoid that area.

Use of safety sector for keeping a shipclear of a tropical stormin the NorthernHemisphere.

Avoiding tropical hurricanes

Practical Rules for Avoiding Tropical Hurricanes…

1. In the dangerous semi-circle,

make speed with the wind on the

starboard BOW and haul round to

starboard as the wind veers, to

clear the centre.

Practical Rules for Avoiding Tropical Hurricanes…

2. In the direct path of the storm

make speed with the wind abaft

the starboard beam, and run into

the navigable semi-circle,

hauling to port as the wind backs.

Practical Rules for Avoiding Tropical Hurricanes…

3. In the navigable semi-circle run with

the wind on the starboard quarter,

hauling to port as the wind backs.

Note…

If the sea and swell are too heavy

to carry out these manoeuvres,

Heave-to with the wind on

the starboard bow.

Note…

The shift of the wind is very important,

and care must be exercised to avoid

being taken unawares by the storm

centre re-curving.

Note…

Care must also be exercised to avoid running before increasingly heavy seas,

as the dangers of broaching in small ships particularly, cannot be over-

stressed. The results of failing to take timely avoiding action are evidenced

by marine disasters ( example… US fleet December 1944)

Movement of a ship in a tropical storm near the point of re-curvature (NH)

Avoiding the Hurricane Centre :

before taking avoiding action…

1.Find the bearing of the centre

2.Establish the semi-circle in which the ship lies

3. Plot the probable path of the centre.

Dangerous semi-circle…

is that in which,

if a vessel runs with the wind aft,

she will become further involved

with the centre

Navigable semi-circle…

Is that which,

if the vessel runs

with the wind aft,

she will tend to run

clear of the centre.

Semi-circle…

a) If the wind shifts to the right (veers),vessel is in the right-hand semi-circle.

b) If the wind shifts to the left (backs), vessel is in the left-hand semi-circle.

c) If the wind direction remains steady,

Semi-circle…(continued)

c) If the wind direction remains steady,vessel is either :

1.In the direct path of the storm being overtaken by it (falling barometer)

2. Running parallel to it, at the same speed as the storm (steady pressure)

3. Converging with the storm (falling pressure)

Semi-circle…(continued)

If necessary, stop the vessel and establish the real shift of the wind.

The rule of the semi-circle applies to both hemispheres…

NH--- RH semi-circle is dangerousSH--- LH semi-circle is dangerous