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Why Does A Tropical Storm Re-Curve?

Out at sea, it is a matter of vital importance that the presence of a storm is detected as soon as signs appear or should I say that the storm has started developing. It is equally important that the location of its centre is established at the earliest. It is preferred but not always possible to keep well clear of the centre, at least off by 100 miles. Having identified the storm, it is now important to know correctly, the predicted path and speed of the centre. If a ship has at least 20 knots, at her disposal and manages a course that will take her most rapidly away from the storm, a big problem is averted. Hopefully, this should happen before the wind starts dominating her movement. Sometimes, a tropical storm moves so slowly that a vessel, if behind it, can easily outpace it. Never must, a tropical storm be taken lightly. Its devastating effects are amongst one of the worst ones to record.

Fall of Pressure:

A warning message from a coast station would normally, be the first information received. Sometimes, you might be the first one at sea, to experience the process of cyclogenesis in your area. Appreciable change in the direction and strength of the wind, with approaching swell from the direction of the storm centre; increase of wind speed as pressure falls, invariably supplemented with lightning and threatening sky; are amongst the usual indications of the presence of a TRS in vicinity.

A positive indication of TRS in vicinity is when the barometer has fallen by 5 millibars (corrected for diurnal variation) below the normal pressure or the wind has increased to force 6 when the barometer has fallen at least 3 millibars. Then she should act as recommended in the following text, until the barometer has risen duly and the wind has decreased below force 6. Invariably, an increase of pressure is what brings smile on mariners’ faces.

Tropical Cyclones are categorized according to maximum sustained wind speed close to the storm’s centre. Tropical Storm has winds speeding to 34 – 47 knots. Severe Tropical Storm has winds from 48 up to 63 knots. Typhoon and Super Typhoon are the names given for winds speed up to 128 knots and over 128 knots respectively.

Following conditions favour formation of cyclone

  1. Low Level Relative Vorticity – A strong correlation exists between the location of tropical storms and large values of low level vorticity. In a developing tropical disturbance, the effect of intense convection is to generate a convergent low level wind field as air flows towards convection; this convergence produces an increase of relative vorticity. The regions of low level positive vorticity are associated with enhanced upward motion; cumulus convection and release of latent heat. The increased heating leads to increase in horizontal convergence which in turn increases the relative vorticity.
    (Note: In continuum mechanics, the vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along with the flow. Continuum mechanics is a branch of mechanics that deals with the mechanical behavior of materials modeled as a continuous mass rather than as discrete particles.)
  2. Coriolis Parameter – Since, the Coriolis parameter is very small near the equator the tropical storms do not form in that region.
  3. Weak Vertical Wind Shear – Low vertical wind shear is essential for a disturbance to develop as the latent heat generated during the convective processes is not advected (advection: transfer of heat horizontally in the atmosphere) away from the circulation field.
  4. Sea Surface Temperature and Depth of Warm Water –The threshold value of the sea surface temperature below which tropical cyclones do not form is 26.5°C. Since, cooler temperatures prevail in the southeast Pacific and south Atlantic, no cyclones from in these regions.
  5. Degree of Convective instability – Since, deep cumulus convection is essential for mature tropical cyclones; it would appear that strong convective instability would be associated with tropical cyclogenesis.
  6. Large value of relative Humidity in the lower and middle troposphere – Regions with low relative humidity in the middle trosphere are unfavorable for cyclogenesis.

To understand the process of re-curving it is important to understand the four stages of the life cycle of a tropical storm:

1. FORMATIVE: This represents the transition from disturbance to Tropical Depression (TD). Surface pressure begins falling, though remaining above 1000 millibars. The strongest winds are normally found on the pole-ward side between the centre and the sub-tropical high.

2. INTENSIFICATION: This is the period of major deepening to Tropical Storm (TS) or Hurricane strength with pressures rapidly falling below 1000 millibars, possibly to as low as 900 millibars or even less. This stage may take several days or occur explosively within 24 hours depending on the conditions prevailing at the time. The tropical cyclone becomes clearly evident in its pressure and wind fields both horizontally and vertically and it is during this stage that the eye develops. The region of strong winds also expands slowly during the intensification stage. Convective clouds increase in amount and vertical extent and form a band-like structure. Showers and squalls increase and rain becomes widespread near the centre.

3. MATURE: This stage is the period when the tropical cyclone remains at or near its maximum intensity although this may vary greatly from storm to storm. Some mature as small intense systems while others develop into the most powerful of typhoons or hurricanes. The mature stage can last from a few hours to a week or more during which time the central pressure remains roughly constant.

4. DECAYING: The decaying stage represents the period when the system either fills or loses its intensity or it assumes extra- tropical characteristics. Tropical cyclones moving inland will fill rapidly as their energy supply (moisture with heat) is cut off from below.

General movement

As we know, after the initial latitudes (5°- 8°) the tropical storm, generally travels westwards with some polar component till it reaches 12° to 20° of latitude, from where a sudden turning towards the pole and then eastward turning or re-curving is caused. Although, avoiding actions in respect of tropical storms are in place and a prudent Master taking appropriate action must find the rules beneficial, a sharper lookout and vigilance is necessary when re-curving has a high probability.

Should it be certain, however, that the vessel is behind the storm, or in the navigable semicircle, it will evidently be sufficient to alter course away from the centre.

Avoiding action:

In the Northern hemisphere:

a)   If the wind is veering (clockwise turning of wind for stationary observer) the ship must be in the dangerous semicircle. The ship should proceed with all available speed with the wind 10° to 45°, depending on speed, on the starboard bow. As the wind veers the ship should turn to starboard, thereby tracing a course relative to the storm.

b)   If the wind remains steady in direction, or if it backs, (anti-clockwise turning of wind for stationary observer) so that the ship seems to be nearly in the path or in the navigable semicircle respectively, the ship should bring the wind well on the starboard quarter and proceed with all available speed.  As the wind backs the ship should turn to port.

In the Southern hemisphere:

  1. If the wind is backing (anti-clockwise turning of wind for stationary observer) the ship must be in the dangerous semicircle.  The ship should proceed with all available speed with the wind 10° to 45° depending on speed, on the port bow.  As the wind backs the ship should turn to port thereby tracing a course relative to the storm.
  2. If the wind remains steady in direction, or if it veers, so that the ship seems to be nearly in the path or in the navigable semicircle respectively, the ship should bring the wind well on the port quarter and proceed with all available speed.  As the wind veers the ship should turn to starboard.

If there is insufficient sea room to run, when in the navigable semicircle, and it is not practicable to seek shelter, the ship should heave to with the wind on her starboard bow in the N and on her port bow in the S hemisphere.

If in harbour, when a tropical storm approaches, it is preferable to put to sea. If this can be done in time to avoid the worst of the storms. Riding out a tropical storm, the centre of which passes within 50 miles or so, in a harbour or anchorage, even if some shelter is offered, could be unpleasant and hazardous experience. Even if berthed alongside, or if special moorings are used, a ship cannot feel entirely secure. The water level may suddenly rise in rivers and very strong winds with tidal stream adding to drift may break the moorings or drift an anchored ship.

As previously stated, tropical cyclones generally move at around 10 knots in the early stages but during intensification they slow down particularly if the deepening is rapid. Recurving storms also slow down during the actual recurvature process but then speed up again as they engage the higher latitude westerlies. They have been known to move eastward at speeds up to 40 knots as they become extra-tropical.

Forecasting the Movement

Forecasting centres normally use several objective forecasting techniques which would probably include the following:

a. Persistence: Simple extrapolation of recent past movement.

b. Climatology: The mean tracks taken by tropical cyclones in that position at that time of year.

c. Blended Persistence and Climatology: A combination of the methods in a and b. This may be either a simple average of the two, or more weight may be placed on persistence in the short term and climatology in the long term.

d. Statistical Techniques: These use various regression equations to facilitate selection of statistical predictors using parameters as surface pressure, contour heights at various levels, winds, temperatures, thickness values etc.

e. The size of storm system, with neighboring pressure systems: Coriolis force available is inter-related. The system is like a rotating cracker used in festivals. The moment, equilibrium is lost it flies off along a straight line. Thus, in case of a tropical storm that has reached reasonably high latitude, owing to the prevailing conditions ahead, losses on the westward component. The energy gets utilised in the poleward motion, causing the storm to turn.

f. Gyroscopic effect: In one particular theory it is said that the tropical revolving storm system is a rotatory pattern and the angular momentum vectors are thus forced to precess the pattern. When subjected to a global torque the precession toward the earth’s pole in the respective hemisphere occurs.

The disturbance and equilibrium of the system could also be attributed to the higher difference in Coriolis force between the poleward semicircle and the equator-ward semicircle. A distinct shade of clouds in the satellite imagery of advanced dangerous quadrant is a sure indication of recurving.

A Master must be well-versed with the system; cautious and vigilant about the prevailing situation to take a proper care of his ship. He must also study the behavior of his ship in the type of wave system he is facing. Sometimes, at a certain speed and in a certain direction the vessel may behave wonderfully well. 

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