Met Office College – Course Notes

3Formation Requirements

1. 
   A PRE-EXISTING LOW PRESSURE AREA Since the minimum criteria for a tropical depression is 22 knots it follows that a low born of some tropical disturbance must already be present. The disturbance may be found within the monsoon trough or associated with tropical waves or TUTT vortices extending to the surface. Such disturbances are commonplace in summer but fortunately only about 10% are likely to develop into tropical cyclones.
   
2. 
   SEA SURFACE TEMPERATURE OF AT LEAST 26C This provides sufficient water vapour to produce the warm core during the condensation process. The warmer the sea, the more conducive it is to tropical cyclogenesis which helps explain why summer is the Hurricane Season (along with the disturbances also prevalent at this time of year). Although empirical, the 26C value has proved so critical that even developed tropical cyclones will weaken if they move over waters cooler than this.
   
3. 
   A MINIMUM VALUE OF LATITUDE OF 5 DEGREES Tropical cyclones require enough coriolis force to produce a cyclonic spiralling circulation. This means they cannot form equatorward of 5 degrees and are rare equatorward of 8 degrees particularly in the Atlantic. So, even though a cyclonic disturbance with enhanced convective activity can form close to the equator, it will not develop into a tropical cyclone unless it moves poleward to the required amount of latitude.
   
4. 
   UPPER LEVEL DIVERGENCE Without outflow aloft the low level inflow would create a build up of air resulting ultimately in rising pressure. Upper level divergence acts rather like an extractor fan, removing the rising air from the top of the tropical cyclone ‘chimney’. A Tropical Depression or Storm passing under a ridge of high pressure at the 200 hPa level will often develop to Hurricane/Typhoon status.
   
5. 
   LITTLE OR NO VERTICAL WIND SHEAR Significant shear prohibits the formation of the warm core thus preventing development. The critical value is thought to be about 20 knots between 950 and 200 hPa. If there are westerly winds below the 300 hPa level, wind shear would probably prevent tropical cyclone formation i.e. deep easterly component winds extending to at least 30000ft are necessary for development and such conditions only occur in the summer months.
   

4Life-cycle

1. 
   FORMATIVE This represents the transition from disturbance to Tropical Depression (TD). Surface pressure begins falling, though remaining above 1000 hPa and winds increase near the centre. The strongest winds are normally found on the poleward side between the centre and the sub-tropical high, but they will still be less than gale force during this stage.
   
2. 
   INTENSIFICATION This is the period of major deepening to Tropical Storm (TS) or Hurricane strength with pressures rapidly falling below 1000 hPa, possibly to as low as 900 hPa 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 maximum speed annulus actually shrinks during intensification and with large systems the ring of highest winds may contract from a diameter of hundreds of miles to 60 nm or less. However, the winds within this ring continue to increase and the region of strong winds beyond 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, although it may fluctuate in the persistent storms if they interact with upper troughs extending into the tropics from higher latitudes.
   
4. 
   DECAYING The decaying stage represents the period when the system either fills and loses its identity, or it assumes extra-tropical characteristics. Tropical cyclones moving inland will fill rapidly as their energy supply is cut off from below, particularly if the terrain is rough. Moving over seas colder than 26 deg C will also promote decay as will a low level feed of cold air such as incursion of the winter monsoon from Asia into the system’s circulation (it should be noted, however, that cold bursts of polar air can have the reverse effect, creating intense cyclogenesis in storms which have become extra-tropical). When tropical cyclones recurve into higher latitudes they generally assume extra-tropical characteristics i.e. polar air is drawn into the system causing the ‘eye’ and tight band of maximum winds to become less defined.
   

5Structure and Characteristics

Figure 1 shows a schematic cross-section of a mature tropical cyclone. The right hand side shows the convection increasing towards the centre becoming most marked at what is termed the ‘eye wall’. The left hand side depicts the common occurrence where convection is also concentrated in a weather band some distance from the wall with an interruption of activity between the two, often called the ‘annular zone’. The ‘inflow layer’ comprises the bottom 3km of the storm, most of the inflow occurring below one km. Outflow occurs at the top of the storm with subsidence at the outer limit and in the annular zone. The ‘outflow layer’ extends from about 8km to the top of the storm, the maximum outflow in mature storms occurring near 12km. Some descent also takes place within the eye, helping to maintain the raised temperature in the mid-tropospheric levels and causing it to be relatively cloud free.

Figure 1. Schematic cross section of vertical motion, clouds, and pressure for the idealised hurricane/typhoon.

6Surface Pressure

Mature storms have very low central pressures dropping below 900 hPa at times although 920 to 930 hPa are more typical and some quite intense hurricanes may have central pressures around 950 hPa. A barograph trace of a tropical cyclone passing close to the observation point is shown at Figure 2. Note the diurnal variation prior to the main pressure fall which takes place over a period less than 12 hours. Pressure falls prior to this were not particularly significant, thus demonstrating the limited value of pressure tendency as a long term forecasting aid. Once pressure falls are noticed it is likely that the ship or station is already well within the tropical cyclone circulation.

Figure 2. Barograph trace during the passage of a severe cyclone in the western South Indian Ocean, 1959.

7Wind

The low level wind-field surrounding tropical cyclones has 3 distinct regions:

1. 
   The outer region extending from the periphery to the ring of maximum winds. The radius of gale force winds tends to be greater on the poleward side in the stronger pressure gradient between the tropical cyclone and the subtropical high. Also as the storm itself moves poleward, the radius of gales increases from typically 100 to 200 nm at latitudes 20 or less to double these values at latitude 30 to 35.
   
2. 
   The maximum wind belt surrounds the eye peaking within the inner margin of the eye-wall cloud. The width of the band is of the order of 5 to 10 nm but the radius of hurricane force winds may lie between 20 and 80 nm.
   
3. 
   The eye is the region of lightest winds decreasing rapidly from the wall cloud to the centre. The radius of the eye can vary from 10 nm or less, to 40 nm in the very large tropical cyclones.
   

Vector addition of the tropical cyclone’s own speed of movement to the winds means that they are normally strongest in the right hand semicircle (NH), relative to the direction of movement, with localised maxima in the right rear quadrant. Figure 3 illustrates this asymmetry of the wind field and is based on mean data derived from reconnaissance flights into 13 mature Pacific tropical cyclones.

Figure 3. Total wind speed (knots) near 1000 feet. Arrow indicates direction of storm movement.

8Cloud and Precipitation

The basic structure has already been seen in Figure 1. Because of the subsidence at the periphery the precursor of the close approach of the tropical cyclone is unusually warm and fine weather. The first cloud signs would be thin cirrus gradually thickening and lowering until the bands of cumulus cloud become evident. These too increase in vertical extent as the system draws nearer. The eye wall represents the densest cloud with tops extending to 15km or more and is accompanied by very heavy precipitation and rapid pressure falls. The eye is often cloud free apart from some patchy low stratus and perhaps some upper cloud. The intense precipitation in the wall and outer bands of cloud (sometimes called feeder bands) greatly reduce visibility and lower cloud base although the hurricane force winds reduce the visibility even more from blowing spray. The banded structure shows well on radar. To observe such a structure, radar wavelengths need to be of the order of 100 metres. Lesser wavelengths would be severely attenuated and the old fashioned 3cm weather radar would probably not penetrate the first band with sufficient energy to produce detectable returns from subsequent bands.

At sea, wind and sea states are the prime hazards, but inland and at coastal stations the main killers are landslips and flooding due to excessive precipitation and abnormal rises in sea-level. The accumulated rainfall from the passage of a tropical cyclone can be staggering. As much as 4000 mm (100 inches) has fallen from a single typhoon in the Philippines and amounts of more than 250 mm (10 inches) over a 12 hour period at a single station are not uncommon. Intense rain bands may supply over 100 mm (4 inches) in one hour. To put these figures in perspective London’s average annual rainfall is about 600 mm (24 inches).

Figure 4. Representation of swell emanating from a tropical cyclone.

9Sea and Swell

Despite limited fetches in the maximum wind zones, the very high speeds still manage to produce phenomenal wave heights in excess of 15m, particularly in the right hand semicircle (NH) where winds are strongest. Constructive interference may even produce the odd wave in excess of 30m. Obviously such seas can inflict severe damage and are capable of capsizing ships even of frigate or destroyer size. In fact, it is thought that typhoons inflicted more damage to the US Pacific Fleet during WW2 than the entire Japanese Navy. One particular typhoon, encountered by the US 3rd Fleet, caused 3 destroyers to be capsized and lost, 146 aircraft blown overboard or damaged beyond repair, 13 other vessels requiring major repairs and 9 others minor repairs.

At low latitudes tropical cyclones move at speeds of the order of 10 knots or less. It follows, therefore, that waves will move out of the storm area as swell ahead of the system. The waves propagate radially outwards as shown in Figure 4 with the largest ones, generated in the right rear quadrant, moving out almost parallel with the storm track. Thus, swell is often a forerunner of the tropical cyclone. For coastal stations there is the additional hazard of storm surges, sometimes called hurricane tides. As the tropical cyclone approaches the land, the winds in the right hand semicircle (NH) will pile up water along the coastline to the right of the storm track. If this occurs at the top of the normal astronomical tide, abnormal rises in the sea level will occur, particularly if the water is piling up in narrowing inlets. The resulting flooding in low lying areas can be catastrophic. In one particular typhoon in Hong Kong in 1937, 11000 people were killed, largely by drowning in such a surge situation.

10Movement

As a broad generalisation, tropical cyclones form on the equatorward side of the sub-tropical highs and then move in the anticyclonic flow. They thus move westward in the early stages, normally at about 10 knots. Depending on the configuration of the high the tropical cyclone may continue its westward movement, usually with a poleward component, or recurve round its western periphery. Figure 5 shows generalised mean tracks around the world but it must be emphasised that individual tracks can be highly erratic.

Figure 5. Generalised mean tracks of tropical storms for the various development areas.

The strength of the steering currents depends on the intensity of the sub-tropical high. When these currents are very weak the tropical cyclones become slow moving or almost stationary. In this event there is a tendency, due to the tropical cyclones own internal forces, for a poleward drift.

The presence of another tropical cyclone within about 800 nm can create interaction between the two where they dumb-bell around each other cyclonically. This is known as the ‘Fujiwhara Effect’ in the Pacific where the higher occurrence of tropical cyclones makes such interaction a not unusual happening. The interaction is complex and its extent is dependent on the strength of the steering currents so it is not really feasible for the ship borne forecaster to make accurate allowances for it.

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 and have been known to move eastward at speeds up to 40 knots as they become extra-tropical.

11Occurrence

Figure 6 shows the average number of tropical cyclones per year which reach tropical storm intensity or greater plus the percentage of the global total. As can be seen none develop within 5 deg of the equator and certain regions, notably the S Atlantic and SE Pacific do not experience any. Vertical wind shear considerations alone can explain these gaps, but also the trade wind trough does not migrate far enough from the equator to produce embryonic disturbances with the required degree of latitude for tropical cyclogenesis. Additionally, sea temperatures are too cool to sustain storms in the SE Pacific and make them rare in the central N Pacific.

Figure 6. Average annual number (and global percentage) of tropical storms in each development area.

The greatest number occur in the NW Pacific which experiences about 30% of the global total. Of these about two thirds are likely to reach typhoon intensity in comparison with ~ in the NE Pacific while in the N Atlantic about 60% reach hurricane strength. Almost 75% of the world’s tropical cyclones occur in the northern hemisphere.

Another interesting feature is the latitude of occurrence. In the NW Pacific early and late season storms tend to form within 5 to 15 deg N compared to 10 to 25 deg N mid-season corresponding to the northward migration of the monsoon trough. In the N Atlantic, formation occurs at higher latitudes, even as high as 25 to 35 deg N, which reflects the fact that tropical cyclones in this region are more likely to form from tropical waves or TUTT disturbances.

Figure 7. Average frequency of tropical cyclones (excluding tropical depressions). Abscissae start with January in the northern hemisphere and July south of the equator.

The seasonal variation for the oceans of the world is depicted at Figure 7. In general tropical cyclones are most frequent in the summer and autumn although there are some interesting departures. Conditions are such that they can occur in any month in the NW Pacific though they are considerably rarer in winter. Some regions also display a double peak frequency structure. This is primarily due to the transition of the monsoon trough across those seas during the appropriate months, moving poleward in early summer and equatorward later. In the Arabian Sea and Bay of Bengal vertical wind shear also inhibits tropical cyclone formation in the mid-summer months although 1 or 2 depressions can form in the extreme northern part of the Bay of Bengal.

12Forecasting by Centres

Forecasting the movement of tropical cyclones has improved dramatically over the last 2 or 3 decades largely as a result of improved input data ie. accurate positioning of the centre using satellite imagery. However, storms are still renowned for their erratic movement and so forecasting remains problematical and better left to warning centres ashore which have access to large quantities of data, not to mention the experience of their forecasters. Such centres normally use several objective forecasting techniques which would probably include the following:

1. 
   Persistence: Simple extrapolation of recent past movement.
   
2. 
   Climatology: The mean tracks taken by tropical cyclones in that position at that time of year.
   
3. 
   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.
   
4. 
   Statistical Techniques: These use various regression equations to facilitate selection of statistical predictors, from grid-point fields, of such parameters as surface pressure, contour heights at various levels, winds, temperatures, thickness values etc.
   
5. 
   Dynamic Techniques: These normally use the concept of a point vortex being steered in the overall current after the removal (by vector subtraction) of the influence of the tropical cyclone circulation. This is usually done in 6 hour time steps out to, perhaps, 72 hours for all standard levels. Such methods may also have built-in feedback i.e. the previous 12 hour forecast error is computed by examining the actual and old forecast position, and then used to apply a bias correction for subsequent forecast positions.
   
6. 
   Analog Techniques: These scan historical data for similar cyclones (within certain accepted limits) to the current one. Forecast positions are then computed first assuming no recurvature and then taking a recurving situation.
   

The various results of all these objective techniques are then assessed by the forecaster and a track is selected whereupon a tropical cyclone warning is issued to all readers of the broadcast. Frequency of warnings may be 6 or even 3 hourly and they contain information on storm intensity and location, wind fields and forecast positions out to at least 48 hours.

Initial detection and location is normally by satellite imagery. Further monitoring and analysis may be carried out by radar networks and aircraft reconnaissance. The USAF Weather Reconnaissance Squadrons use C 130s (Hercules) to enter tropical cyclones at the 700 hPa level keeping the wind on the port quarter to facilitate a spiralling penetration of the eye where drop-sondes are released. Specialised techniques of analysis of satellite pictures, particularly enhanced JR imagery, also enable estimation of central pressure, wind strength and likely future development.

13The Ship-borne Forecaster

Upon receipt of a warning the METOC or other responsible officer should immediately plot the position and forecast tracks, regularly updating it, thereafter, on receipt of subsequent warnings. He must, of course, keep the Command informed at all times and be ready to advise on the best way of avoiding the storm. Occasionally there may be warnings from more than one centre and it is not unusual for these to be conflicting thus requiring judgement on which is likely to be most reliable. It is best to plot the information from different centres separately and probably prudent to consider the most threatening tracks. If the forecasts differ greatly then the steering currents are probably weak and none is likely to be too reliable. As a general rule, more confidence can be placed on forecast positions when the storm has definite movement, say of 8 knots or more, than when it is slow moving. Naturally the short term forecasts will be more reliable than the longer term ones.

With the continuous satellite coverage and a highly efficient warning network it is most unlikely that ships would ever be caught out by a tropical cyclone in modern times. Thus, observation techniques are not as crucial as they once were, although it would be prudent to remain alert to the warning signs of an approaching storm:

1. 
   A surface pressure 5 hPa or more below normal or a 24 hour change of 4 hPa or more - but bear in mind comments on surface pressure (Section 6).
   
2. 
   An unusual wind direction after accounting for land/sea breezes or other topographical effects.
   
3. 
   Wind speeds 25% or more above normal, particularly if the flow curves cyclonically.
   
4. 
   Long period swells especially from an unusual direction (Moderate NE Monsoon swell is common in the S China Sea and any departure from this should be viewed with suspicion).
   
5. 
   A solid overcast of Cirrostratus thickening and lowering particularly if observed at several adjacent reporting stations.
   

14Avoidance Rules

If appropriate heed is paid to warnings, then ships are unlikely to be caught in the circulation of a tropical cyclone. However, there are avoidance rules for ships in such a situation, in the Mariners Handbook and the Admiralty Manual of Navigation. The rules relate to the sectors of a tropical cyclone relative to its track as shown in Figure 8.

Figure 8. Tropical Cyclone Quadrant/Semicircle Nomenclature (Northern Hemisphere).

The dangerous semicircle is the right hand one in the northern hemisphere (left hand in the SH) and is so named because it has the strongest winds and highest seas. The ‘safer’ half is called the navigable semicircle. The dangerous quadrant is the front right (NH) and is to be avoided if at all possible. Here any ship unable to make steerage way would be drawn into the path of the storm and if there is going to be any deviation from the forecast track, then the storm is more likely to curve towards the dangerous quadrant.

Basically the rules are:

1. 
   Ensure there is plenty of sea room to avoid being blown aground. This is particularly important and will require early decision making if the ship is in coastal waters that have no hurricane or typhoon havens.
   
2. 
   If possible, get on the safe side of the storm to avoid danger from recurvature.
   
3. 
   If caught in the dangerous semicircle, place the wind on the starboard bow and keep it there.
   
4. 
   If caught in the navigable semicircle place and maintain the wind on the starboard quarter.
   

15Tropical Cyclone Warnings

Examples of operational messages are shown at Annex A.

16Acknowledgements

These notes form part of the material included in a Tropical meteorology course developed at the Royal Navy School of Meteorology and Oceanography (RNSOMO). The College is grateful for permission to reproduce and use the material.

17Further reading

A Synoptic Meteorology of the Tropical Oceans, Director of Naval Oceanography and Meteorology Hydrographic Department, Ministry of Defence, prepared by Captain T A Marshall BSc RN (retd.).

Atkinson. Forecaster’s Guide to Tropical Meteorology, Technical Report 240.

Herbert Riehl 1954. Tropical Meteorology. McGraw-Hill

Herbert Riehl 1979. Climate and Weather in the Tropics. Academic Press.

Maurice A. Garbell. Tropical and Equatorial Meteorology. Pitman Publishing Corporation.

Annex A

EXAMPLES OF TROPICAL CYCLONE WARNINGS

MIAMI WARNING

HURRICANE CLAUDETTE WARNING NO.23

1. HURRICANE CENTRE LOCATED NEAR 36.5N4 35.0W8 OR 380 NM ESE OF LAJES, AZORES AT 152200Z.

POSITION ACCURATE WITHIN 40 MILES BASED ON SATELLITE.

PRESENT MOVEMENT TOWARDS THE EAST OR 085 DEGS AT 2OKT.

A. MAXIMUM SUSTAINED WINDS 65 KT WITH GUSTS TO 80 KTS.

B. RADIUS OF 64 KT WINDS 50NE 755E 75SW 5ONW.

C. RADIUS OF 50 KT WINDS 75NE 100SE 100SW 75NW.

D. RADIUS OF 34 KT WINDS 100NE 1755E 175SW 100NW.

E. REPEAT CENTER LOCATED AT 36.5N4 35.0W8 AT 152200Z.

2. FORECAST VALID 160600Z. 37.ONO 32.0W5.

A. MAXIMUM SUSTAINED WINDS 55 KTS WITH GUSTS TO 70 KTS.

B. RADIUS OF 50 KT WINDS 75NE 100SE 100SW 75NW.

C. RADIUS OF 34 KT WINDS 100NE 150SE 150SW 100NW.

3. FORECAST VALID 161800Z. 39.0N2 26.0W8.

A. MAXIMUM SUSTAINED WINDS 45 KTS NEAR CENTER

B. RADIUS OF 334 KT WINDS 100NE 150SE 150SW 100NW.

4. OUTLOOK VALID 171800Z. EXTRATROPICAL.

5. REQUEST 3 HOURLY SHIP REPORTS WITHIN 300NM OF 36.5N4 35.0W8.

6. NEXT WARNING 160400Z.

HONG KONG WARNING

FROM ROHK

TO HMS ARK ROYAL

TROPICAL CYCLONE WARNING FOR SEVERE TROPICAL STORM TIP ISSUED AT 130120Z JUL

AT 130000Z SEVERE TROPICAL STORM TIP (8314) HAD BEEN DOWNGRADED TO A TROPICAL STORM WITH CENTRAL PRESSURE 994 MBS AND WAS CENTRED WITHIN 30 N MILES OF ONE NINE POINT TWO (19.2) DEGREES NORTH, ONE ONE ONE POINT FOUR (111.4) DEGREES EAST.

TROPICAL STORM TIP IS FORECAST TO MOVE NORTHWEST AT ABOUT 10 KNOTS AND GRADUALLY WEAKEN.

MAXIMUM SUSTAINED WINDS NEAR THE CENTRE ARE ESTIMATED AT 40 KNOTS.

RADIUS OF 34 KNOTS OR GREATER IS 100 N MILES.

24 HOUR FORECAST POSITION FOR 140000Z 22 NORTH 109 EAST

48 HOUR FORECAST POSITION FOR 150000Z DISSIPATED OVERLAND

BT

FROM ROHK

TO HMS ARK ROYAL

TROPICAL CYCLONE WARNING FOR TROPICAL STORM VERA ISSUED 130125Z.

AT 130000Z TROPICAL STORM VERA (8315) HAD INTENSIFIED TO A SEVERE TROPICAL STORM WITH CENTRAL PRESSURE 986 MBS AND WAS CENTRED WITHIN 60 N MILES OF ONE ONE POINT EIGHT (11.8) DEGREES NORTH, ONE TWO EIGHT POINT FOUR (128.4) DEGREES EAST.

SEVERE TROPICAL STORM VERA IS FORECAST TO MOVE WEST AT ABOUT 10 KNOTS AND INTENSIFY FURTHER

MAXIMUM SUSTAINED WINDS NEAR THE CENTRE ARE ESTIMATED AT 50 KNOTS

RADIUS OF 47 KNOTS OR GREATER 1560 N MILES

RADIUS OF 34 KNOTS OR GREATER IS 180 N MILES

24 HOUR FORECAST POSITION FOR 140000Z 12 NORTH 125 EAST

48 HOUR FORECAST POSITION FOR 150000Z 13 NORTH 120 EAST

BT


Crown Copyright. Permission to quote from this document must be obtained from The Principal, Met Office College, FitzRoy Road, Exeter, Devon, EX1 3PB

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