Tropical Revolving Storm(TRS)

Tropical Revolving Storm(TRS)

Tropical revolving storm (TRS) is a warm-core, low-pressure system, around which the air circulation is anti-clockwise, in the Northern Hemisphere (NH) and clockwise, in the Southern Hemisphere (SH). It consists of a rotating mass of warm and humid air and creates thunderstorms with strong winds, flooding rain, high waves, damaging storm surge etc. It devolves over large bodies of warm water and normally dies, when moves over land. For this reason, coastal regions receive significant damages from a tropical cyclone; where as inland regions are relatively safe from this.

The diameter of a tropical storm is normally less than 500 nm even can be only 100 nm at its early stage of development.

TRS is common in various places of the world, but they can be called as below local trams,
1. “Cyclones” is used in the Bay of Bengal and Arabian Sea.
2. “Hurricane” is used in the western side of north Atlantic and south Pacific.
3. “Cordonazo” is used in the eastern side of North Pacific.
4. “Typhoon” is used in the western side of North Pacific.
5. “Willy - willy” is used in the eastern side of South Indian Ocean.

On the night of 29th April 1991, a powerful tropical revolving storm struck the coast of Bay of Bangle, at Chittagong, Bangladesh, with wind speed of around 250 km/h. This storm forced a storm surge of 6 meter, over a wide area and killed at least 138,000 people, with heavy damage of estimated amount 1.5 billion US dollars. The high velocity wind and the storm surge completely crushed the
large coastline, houses, lands, structures etc.

Cyclone Sidr was another strongest TRS for Bangladesh. It formed in the central Bay of Bengal and quickly strengthened to reach winds of 260 km/h and finally hit the land of Bangladesh on 15th November, 2007. As per Red crescent society report, the estimated number of deaths were 10,000. The large areas of Patuakhali, Barguna and Jhalokati districts were hit by the storm surge of over
5 meters. About a quarter of the world heritage sites of Sunderbans ware seriously damaged. Total damages of this Sidr were approximately 450 million US dollars.

At Bay of Bengal, tropical revolving storms are most likely to develop in May, October and November but may occur in any month.

The following terms are used to describe the cyclones.
1. Low pressure are or low, where winds are less than or equal to 17 Kts.
2. Well marked low, where winds are within 18 Kts to 21 Kts.
3. Depression, where winds are within 22 Kts to 27 Kts, with radius of disturbances 44 km.
4. Deep Depression, where winds are within 28 Kts to 33 Kts, with radius of disturbances 48 km.
5. Cyclonic Storm, where winds are within 34 Kts to 47 Kts, with radius of disturbances 54 km.
6. Severe Cyclonic Storm, where winds are within 48 Kts to 63 Kts, with radius of disturbances
    64 km.
7. Severe Cyclonic Storm with a Core of Hurricane wind, where winds are within 64 Kts to 118 Kts,      with radius of disturbances 74 km.
8. Super Cyclone, where winds are 119 Kts or more, with radius of disturbances 84 km.

Prediction of cyclonic storm in the Bay of Bengal and issuance of timely warning is the job of the Storm warning centre (National weather forecasting centre), Dhaka. The cyclone warning system is well known in Bangladesh. Following information are normally included in each warning,
1. Position of storm centre.
2. Direction and rate of movement.
3. Areas likely to be affected with the names of Upazillas (administrative unit in Bangladesh), if       
    possible.
4. Approximate time of commencement of storm.
5. Maximum wind speed expected.
6. Approximate height of storm surge/tide.
Storm Warning Signals in Bangladesh: The meteorological department uses separate codes of signals, for maritime and river ports. Code of signals, which are used at maritime ports of Bangladesh, are as follows,

Basic requirements to form tropical storms:
There are six main requirements to develop tropical revolving storms. These are the basic requirements, but do not give any guarantee of forming such cyclones.
1. Sufficiently warm temperatures: Normally an ocean temperature of 26.5°C is the minimum requirement for TRS formation. Warm ocean water must exist over a sufficient depth of at least 50 meter. These warm waters are necessary to fuel the heat engine of the tropical cyclone.
2. Potentially unstable atmosphere: There must be an atmosphere that cools quickly with height, so that it becomes potentially unstable. If the air is unstable, then it will continue rising and the disturbance will grow. This is required to maintain convection for an extended period of time.
3. High relative humidity : A necessary amount of relative humidity must be present in the lower to middle levels of the troposphere. The required amount of humidity is about 50 to 60%.
4. Adequate value of Coriolis Effect: It has been observed that TRS does not form within 3 degrees latitude from the equator, as Coriolis effect is negligible at the equator. So a minimum distance of 500 km from the equator is normally needed to from TRS.
5. Largest low pressure: Pressure must be lowered with largest amount and this LP area must be surrounded by areas of HP, which is required for preexisting near-surface disturbance.
6. Limited vertical wind shear: Limited vertical wind shear can be positive for tropical cyclone formation. This amount should be less than 10 m/s between the surface and the tropopause. On other hand, strong wind shear can blow the tropical cyclone apart.

Structure: 
Parts of a tropical storm are described as below,

Eye or vortex is a roughly circular area of comparatively light winds and fair weather, available at the center of a severe tropical cyclone. Weather in the eye is normally calm but the sea can be extremely violent. There is little or no precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest surface pressure than the surrounding environment. The diameter of an
eye can be around 40 km and also can range from under 10 km to over 100 km. In severe cyclones, the eye usually looks like a circular hole in the central cloud mass.

Eye wall consists of a dense ring of cloud and tall thunderstorms that produces heavy rains and usually the strongest winds (about force 6 or 7) at about circular path. This wall is about 15 km in height, with diameter between 100 mile or more, into the atmosphere. Changes in the structure of the eye and eye wall can cause changes in the wind speed, which is an indicator of the storm's intensity.
The pressure gradient in the eye wall is very steep and barograph trace will show as near vertical curve line. (Shown at below figure)

Outer storm area is the area, which is out side of the eye-wall and can extend up to 1000 km from the cyclone centre and contain heavy rain and wind squalls with wind force 6 to 7, as well as tornadoes. Here pressure gradient is much less than eye wall.

Here angle of indraft of wind is about 45º and this gradually decreases to 0º in the eye wall. In this area, the cirrus cloud can be form of strands or filaments with aligned conditions and points towards the storm centre. Here visibility is excellent, except in occasional shower’s areas.
Cirrus Canopy (in other words CDO or Central dense overcast) is a massive outflow of cirrus cloud in the upper atmosphere, which is produced by the extremely vigorous uplift of moist air, within the clouds of the eye wall. This forms a huge canopy over the cyclone, making satellite location of the TRS system centre difficult during the early development stages, before the eye shows through the canopy.

Route of TRS: 
Most of the TRS systems form between 10 and 30 degrees away of the equator and 87% form no farther away than 20 degrees of latitude, on both hemispheres. On the other hand, due to negligible Coriolis effect, tropical cyclones rarely form or move within about 5 degrees of the equator. Tropical
cyclones move slowly westward when near equator and then intensify as they move further.

Initially TRS travels between W and WNW in the NH and between W and WSW in the SH. During their passages, they curve away from the equator, which are N, then NE in NH and S, then SE in SH. These recurvings are normally done at about 30º N and 30º S. Again sometimes a TRS does not recurve at all and continues on same route, until cross the coast line.

The of speed of TRS is usually about 10 knots in their early stages, which increases a little with latitude, but it seldom exceeds 15 knots before recurving, but thereafter 20 to 25 in usual, though speed of 40 knots or even more have been reported.

Route of a TRS is 2 types,
1. Track – The route over which a TRS is already passed.
2. Path – The predicted route, over which, there is a possibility of the TRS passing at near future.
Another point on route is called the Vertex, which is the western most point, of the TRS, when recurving takes place.
Above attitude of speed and route, is not 100% correct for all storms. Some examples of abnormal track of TRS are given at next diagram, (Source: Bay of Bengal Pilot)

Semi circles: If a storm is divided along the route, at which the storm is passing, then we get 2 parts, which are,
1. Right hand semicircle (RHSC): It is the half of the storm, which lies to right of the observer, who faces along the route of storm. For a stationary observer, here the wind veers steadily.
2. Left hand semicircle (LHSC): It is the half of the storm, which lies to left of the observer, who faces along the route of storm. For a stationary observer, here the wind backs steadily.

In the northern hemisphere (NH), conditions on the right-hand side of storms are more severe than those on their left-hand sides. For that reason, in NH, RHSC is called the "dangerous semicircle” and LHSC is called the "navigable semicircle”.
There are several reasons to make these conditions of dangerous and navigable situations.

In NH, normally a TRS forms at latitudes between 5º and 20º N and recurves at about latitudes 30º N. On the other hand, both westerlies and north-east trade winds blow from 30º N at NH. So if we look to below diagram, then we can see both winds are about inline with the wind direction of right hand semicircles and against the wind direction of left hand semicircles.

The TRS spins and travels at the same time, which also creates stronger wind conditions on RHSC for NH. In the SH, the situation is reversed.
So again, as per navigator’s judgment, semicircles are two types, which are,
1. Navigable semicircle— It is the side of a tropical cyclone, which lies to the left of the direction of movement of the storm in the Northern Hemisphere (to the right in the Southern Hemisphere), where the winds are weaker and better for the navigation purpose, although all parts of TRS are more or less dangerous to mariners.
2. Dangerous semicircle— It is the side of a tropical cyclone, which lies to the right of the direction of movement of the storm in the Northern Hemisphere (to the left in the Southern Hemisphere), where the storm has the strongest winds and heaviest seas.

Below is one more term, which is also used for safe navigation purpose at TRS.
Dangerous quadrant: It is the forward quadrant of RHSC in NH and LHSC IN SH. In this quadrant, the wind’s rotation pulls us toward the centre, with maximum apparent wind speed.

Life cycle of a tropical cyclone:
The complete life cycle of a tropical cyclone usually spans about 9 days but can be only 2 or 3 days or more than 20 days. It has 4 stages, which are described as below,

Formation:
The formation of a tropical cyclone is dependent upon six favorable environmental conditions (described before), which are available in the Inter Tropical Convergence Zone. Tropical cyclones gain energy from latent heat, driven by significant thunderstorm activity and condensation of moist air. In other words, tropical cyclone formation can be called as a gigantic vertical heat engine, which is also powered by earth's gravity and rotation. On satellite images, this stage appears as an unusually active, but poorly organized area of convection (thunderstorms). Sometimes curved cumulus clouds band towards an active area of thunderstorms, which indicates the location of the centre. At this stage, if tropical cyclones move inland, then they make little or no damage
but may form heavy rain and flooding in some areas.

Premature Stage: In this stage the area of convection continues and becomes more organized. Also strengthening occurs simultaneously. The minimum surface pressure rapidly drops well below than normal level. Gale-force winds also develop with the strengthening pressure gradient. The circulation centre is well defined and subsequently an eye may begin to form. Satellite and radar observations of the system show as the distinctive spiral banding pattern.
Premature Stage of tropical cyclone can cause devastating wind and storm surge effects upon coast line, but damage occurs usually within a small area.

Mature Stage: If the ocean and atmosphere environment continues to be favorable, the cyclone may continue to intensify to this stage. This is the severe cyclone stage, where the cyclone is most dangerous. Approximately half of the cyclones can come up to this stage. During this stage, the cyclonic circulation and extent of the gales increase markedly. In satellite images, the cloud fields look highly organized and become more symmetrical, with a well-centered, distinct round eye. This stage remains for a day or so with maximum intensity, unless the cyclone remains in a highly
favorable environment.

Decay Stage: At this stage, the warm core of TRS is destroyed, as the central pressure increases and the maximum surface winds are weaken. Decay may occur very rapidly, if the system moves into an unfavorable atmospheric or geographic environment. At this stage the heavy or medium rain can be
available.
In satellite images, the decaying stage is distinguished by the weakening of organized convection near the centre and disappearance of major curved convective bands.

Indications of approaching TRS: 
The signs and symptoms Of TRS/cyclone are as follows,
1. Heavy and long swell from Cyclone center.
2. Pressure will be very much lower than the normal. TRS becomes confirm, when barometric   
     pressure is lower then 5 mb along with other TRS confirmation factors.
3. Cirrus cloud will be at sky.
4. At sunset time cloud colour will be red or copper.
5. Clouds will be dense and heavy with threatening appearances.
6. Frequent lighting will be experienced.
7. Availability of storm warnings from local authority.
8. Occurrence of squalls, which are increasing in frequency and intensity.
9. Possibility of rain with violent torrential character.
10. Sea waves are becoming heavy and dangerous.
11. Shift of wind direction, in accordance with the rotation of winds of cyclonic spin.

Actions when TRS is conformed:
Then do the followings,
a. The probable direction and distance of the storm centre.
b. The probable semi circle of the storm.
c. Take avoiding actions.

a. Procedures to find out probable direction and distance.
 For probable direction, followings can be done -
1. Face the wind, then the storm centre will be within 8 to 12 points on right hand in NH (on left     
    hand  in SH).
2. From the direction of the swell, this indicates roughly the storm centre.
3. From the direction of the densest part of the huge bank of clouds, this also indicates the storm
    centre.
 For probable distance, followings can be done -
1. If barometric pressure falls 5 mb below normal, then there is a possibility that ship is in the well
   developed outer storm area.
2. If barometric pressure falls 20 mb or more below normal, then there is a possibility that ship is
    near the eye of a well developed TRS.

b. Procedures to find out probable semi circle of the storm.
For probable semi circle, followings can be done –
1. Bring the ship in stationery or “heave to” (moving very slowly in one direction) to position.
2. Carefully monitor and write down wind directions, every after 2 hours.
3. If wind direction changes clock wise (wind veers), than the vessel is at Right hand semicircle
    (RHSC) and if wind direction changes anti-clockwise (wind backs), than the vessel is at Left hand
     semicircle (LHSC), in both hemispheres.

c. Procedures for avoiding actions.
 Actions in TRS, at port,
If possible, first try to go to sea at a safe distance with plenty sea room and sufficient depth of water. Otherwise do the followings,
a. Double the moorings.
b. Keep Engine standby.
c. All persons to be onboard.
d. Keep all LSA at standby position.
e. Rig lifeline at fore and aft.
f. No slag tanks.
g. All hatches should be securely battened down.
h. All derricks should be lowered and secured.
i. Adequate fenders should be placed between the ship and the jetty.

 Actions in TRS, at anchorage,
If possible, first try to go to sea at safe distance with plenty sea room and sufficient depth of water or shift to a safe anchorage with enough shelter.
Otherwise do the followings,
a. Drop both anchors with several cables in water.
b. Keep Engine standby.
c. All persons to be onboard.
d. Keep all LSA at standby position.
e. Rig lifeline at fore and aft.
f. No slag tanks.
g. All hatches should be securely battened down.
h. All derricks should be lowered and secured.
i. All bridge equipments (including Radar, fog-horn) and navigational lights (including emergency
    navigational lights) should be in standby mode.

 Actions in TRS, at sea,
If the vessel is in right hand semicircle, try to proceed as fast as possible with the wind 1 to 4 points on the Stbd bow in NH (Port bow in SH), as 1 point for slow speed ship (less than 12kts) and 4 point for high-speed ship (more than 12kts).
If the vessel is in Left-hand semicircle, try to proceed as fast as possible with the wind about 4 points on the Stbd quarter in NH (Port quarter in SH).
This process would continue, until the normal weather condition. If there is no enough sea room to follow the avoiding actions, than the vessel should be with “heave to” condition, with the wind as above.
If the vessel is in the direct path, then she should run with the wind just abaft the starboard beam into the navigable semicircle (which is LHSC for NH and RHSC for SH)