Tropical Revolving Storm (TRS) Explained

Definition of 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; whereas 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,

  • “Cyclones” is used on the Bay of Bengal and the Arabian Sea.
  • “Hurricane” is used on the western side of the North Atlantic and South Pacific.
  • “Cordonazo” is used on the Eastern Side of the North Pacific.
  • “Typhoon” is used on the western side of the North Pacific.
  • “Willy- willy” is used on the eastern side of the South Indian Ocean.

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.

  • Low pressure is the low area, where winds are less than or equal to 17 Kts.
  • Well marked low, where winds are within 18 Kts to 21 Kts.
  • Depression, where winds are within 22 Kts to 27 Kts, with a radius of disturbances 44 km.
  • Deep Depression, where winds are within 28 Kts to 33 Kts, with a radius of disturbances 48 km.
  • Cyclonic Storm, where winds are within 34 Kts to 47 Kts, with a radius of disturbances 54 km.
  • Severe Cyclonic Storm, where winds are within 48 Kts to 63 Kts, with a radius of disturbances 64 km.
  • Severe Cyclonic Storm with a Core of Hurricane wind, where winds are within 64 Kts to 118 Kts, with a radius of disturbances 74 km.
  • Super Cyclone, where winds are 119 Kts or more, with a radius of disturbances 84 km.

Basic Requirements to form Tropical Revolving 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.

  • 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 meters. These warm waters are necessary to fuel the heat engine of the tropical cyclone.
  • 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.
  • 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%.
  • Adequate value of Coriolis Effect: It has been observed that TRS does not form within 3 degrees latitude from the equator, as the Coriolis effect is negligible at the equator. So a minimum distance of 500 km from the equator is normally needed from TRS.
  • Largest low pressure: Pressure must be lowered with the largest amount and this LP area must be surrounded by areas of HP, which is required for preexisting near-surface disturbance.
  • 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.

Also Read, Definition Of Various Terms Under Meteorology

Structure of a Tropical Storm

Parts of a tropical revolving storm are described as below:

Birds-Eye-view-of-TRS-Blog-Post-Image

Eye or Vortex

Eye or vortex is a roughly circular area of comparatively light winds and fair weather, available at the center of a severe tropical cyclone. The 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 the 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

Eyewall 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 a diameter between 100 miles or more, into the atmosphere. Changes in the structure of the eye and eyewall can cause changes in the wind speed, which is an indicator of the storm’s intensity. The pressure gradient in the eyewall is very steep and barograph trace will show a near-vertical curve line.

Tropical Revolving Storm (TRS) Explained 1

Outer Storm Area

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

Here the angle of the indraft of wind is about 45º and this gradually decreases to 0º in the eyewall. 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 areas.

Cirrus Canopy

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 eyewall. This forms a huge canopy over the cyclone, making the satellite location of the TRS system center difficult during the early development stages before the eye shows through the canopy.

ertical-Cross-Section-of-a-TRS-created-in-Northern-Hemisphere-Blog-Post-Image

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 the negligible Coriolis effect, tropical cyclones rarely form or move within about 5 degrees of the equator. Tropical cyclones move slowly westward when near the equator and then intensify as they move further.
  • Initially, TRS travels between W and WNW in the Northern Hemisphere and between W and WSW in the Southern Hemisphere. During their passages, they curve away from the equator, which is 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 Tropical Revolving Storm does not recurve at all and continues on the same route until it crosses the coastline. The 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 the speed of 40 knots or even more has been reported.
Route-of-Tropical-Revolving-Storm-TRS

Route of a TRS is two types:

  • Track – The route over which a TRS is already passed.
  • Path – The predicted route, over which, there is a possibility of the TRS passing in the near future.

Another point on the route is called the Vertex, which is the westernmost point, of the TRS, when recurving takes place. The above attitude of speed and route is not 100% correct for all storms. Some examples of abnormal track of TRS in Bay of Bengal are given in the next diagram

Tropical Revolving Storm (TRS) Explained 2

Semi Circles

If a storm is divided along the route, at which the storm is passing, then we get 2 parts, which are:

  • Right-hand semicircle (RHSC): It is the half of the storm, which lies to the right of the observer, who faces along the route of the storm. For a stationary observer, here the wind veers steadily.
  • Left-hand semicircle (LHSC): It is the half of the storm, which lies to the left of the observer, who faces along the route of storm. For a stationary observer, here the wind backs steadily.
Semi-circles-Tropical-Revolving-Storm

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 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 at the below diagram, then we can see both winds are about in line with the wind direction of right-hand semicircles and against the wind direction of left-hand semicircles.

Tropical Revolving Storm (TRS) Explained 3

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 judgement, semicircles are two types, which are:

  • 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.
  • 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 center, with maximum apparent wind speed.

Tropical Revolving Storm (TRS) Explained 4

Life cycle of a TRS

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 a gigantic vertical heat engine, which is also powered by the 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 center.

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 center is well defined and subsequently, an eye may begin to form. Satellite and radar observations of the system shown as a distinctive spiral banding pattern.

The premature Stage of a tropical cyclone can cause devastating wind and storm surge effects upon coastline, but damage occurs usually within a small area.

Mature Stage: If the ocean and atmosphere environment continue 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 center and the disappearance of major curved convective bands.

Indications of approaching TRS

The signs and symptoms Of TRS/cyclone are as follows:

  • Heavy and long swell from Cyclone center.
  • Pressure will be very much lower than normal. TRS becomes confirm when the barometric pressure is lower than 5 Mb along with other TRS confirmation factors.
  • Cirrus cloud will be in the sky.
  • At sunset time cloud color will be red or copper.
  • Clouds will be dense and heavy with threatening appearances.
  • Frequent lighting will be experienced.
  • Availability of storm warnings from the local authority.
  • Occurrence of squalls, which are increasing in frequency and intensity.
  • Possibility of rain with violent torrential character.
  • Sea waves are becoming heavy and dangerous.
  • The shift of wind direction, in accordance with the rotation of winds of cyclonic spin.

Actions when TRS is conformed

Procedures to find out probable direction and distance:

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

Procedures to find out probable semi circle of the storm

  • For probable semi-circle, the followings can be done:
    • Bring the ship in stationery or “heave to” (moving very slowly in one direction) to the position.
    • Carefully monitor and write downwind directions, every after 2 hours.
    • If the wind direction changes clockwise (wind veers), then the vessel is at Right-hand semicircle (RHSC) and if the wind direction changes anti-clockwise (wind backs), then the vessel is at Left-hand semicircle (LHSC), in
      both hemispheres.

Procedures for avoiding actions

  • Actions in TRS, at port:
    If possible, first try to go to sea at a safe distance with plenty of sea room and sufficient depth of water. Otherwise do the followings:
    • Double the moorings;
    • Keep Engine standby;
    • All persons to be onboard;
    • Keep all LSA at stand-by position;
    • Rig lifeline at fore and aft;
    • No slag tanks;
    • All hatches should be securely battened down;
    • All derricks should be lowered and secured;
    • Adequate fenders should be placed between the ship and the jetty.
  • Actions in TRS, at the anchorage:
    If possible, first try to go to sea at safe distance with plenty of sea room and sufficient depth of water or shift to a safe anchorage with enough shelter. Otherwise do the followings:
    • Drop both anchors with several cables in the water;
    • Keep Engine standby;
    • All persons to be onboard;
    • Keep all LSA at stand-by position;
    • Rig lifeline at fore and aft;
    • No slack tanks;
    • All hatches should be securely battened down;
    • All derricks should be lowered and secured;
    • All bridge equipment (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 a 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 points for high-speed ship (more than 12kts).
    • If the vessel is in the 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, then the vessel should be in a “heave to” condition, with the wind as above.
ship-in-tropical-revolving-storm-blog-image

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)

Also Read, Difference between TRS and TLD

Reference

Marine Meteorology by Capt. Harry Subramaniam