- 1. What is a tropical cyclone?
A tropical cyclone is defined as a non-frontal low pressure system of synoptic scale developing over warm waters having organised convection and a maximum mean wind speed of 34 knots or greater extending more than half-way around near the centre and persisting for at least six hours.
Every cyclone is unique varying according to a number of factors including life cycle, intensity, movement, size and impact (wind, storm surge and flooding).
Satellite image of cyclone Kalunde showing a well developed eye surrounded by the eye wall and spiral bands of convection.
- 2. What is the tropical cyclones intensity scale? How is this different from the USA intensity scale?
The severity of a tropical cyclone is described in terms of categories ranging from 1 (weakest) to 5 (strongest) related to the maximum mean wind speed as shown in this table.
Note: corresponding approximate wind gusts and central pressure are also provided as a guide. Stronger gusts may be observed over hilltops, in gullies and around structures.
Category Maximum Mean Wind (km/h) Typical Strongest Gust (km/h) Central Pressure (hPa) Typical Effects 1 63 - 88 < 125 > 985 Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings 2 89 - 117 125 - 164 985 - 970 Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small craft may break moorings. 3 118 - 159 165 - 224 970 - 955 Some roof and structural damage. Some caravans destroyed. Power failures likely. (e.g. Winifred) 4 160 - 199 225 - 279 955 - 930 Significant roofing loss and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures. (e.g. Tracy, Olivia) 5 > 200 > 279 < 930 Extremely dangerous with widespread destruction. (e.g. Vance)
Further detail: Tropical cyclone intensity
The USA uses the Saffir-Simpson hurricane intensity scale for the Atlantic and Northeast Pacific basins. US Atlantic Oceanographic and Meteorological Laboratory's Hurricane FAQ: The Saffir-Simpson hurricane intensity scale
The diagram below provides a comparison of the USA hurricane scale with the Bureau of Meteorology's 5-point tropical cyclone categories. The demarcation points are not precise - this table is intended to provide a rough comparison only. It is based on winds at coastal crossing - converting from the US 1-minute averaged winds. For more information of wind averaging issues see the following WMO report: https://www.wmo.int/pages/prog/www/tcp/documents/WMO_TD_1555_en.pdf.
- 3. How is a severe tropical cyclone different from a non-severe cyclone?
Tropical cyclones are classified as severe when they are producing 'very destructive winds' having sustained surface winds of at least 118 km/h near the centre and gusts of at least 165 km/h. This corresponds to cyclone categories 3, 4 and 5.
Category Sustained winds(km/h) Strongest gust (km/h) Typical effects 1 Tropical Cyclone 63 - 88 Below 125 Damaging winds 2 Tropical Cyclone 89 - 117 125 - 164 Destructive winds 3 Severe Tropical Cyclone 118 - 159 165 - 224 Very destructive winds 4 Severe Tropical Cyclone 160 - 199 225 - 279 5 Severe Tropical Cyclone Over 200 Over 280
- 4. What is the difference between Australian tropical cyclones and cyclones, typhoons and hurricanes in other parts of the world?
For historical reasons tropical cyclones are called different names in different parts of the world. The terms hurricane and typhoon are regionally specific names for a severe tropical cyclone (sustained winds of more than 118 km/h (64 knots). Hurricane is used in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E. Typhoon is used in the Northwest Pacific Ocean west of the dateline.
For many parts of the world a non-severe tropical cyclone is referred to as a tropical storm and assigned a name.
- 5. How are tropical cyclones different from tornadoes?
While both tropical cyclones and tornadoes are atmospheric vortices, they have little in common. Tornadoes have diameters on the scale of hundreds of metres and are usually produced from a single thunderstorm. A tropical cyclone, however, has a diameter on the scale of hundreds of kilometres and contains many thunderstorms. Tornadoes are primarily an over-land phenomena as solar heating of the land surface usually contributes toward the development of the thunderstorm that spawns the vortex (though over-water tornadoes have occurred). In contrast, tropical cyclones are purely an oceanic phenomena - they die out over-land due to a loss of a moisture source. Lastly, tropical cyclones have a lifetime that is measured in days, while tornadoes typically last on the scale of minutes.
Interestingly, tropical cyclones near landfall often provide the conditions necessary for tornado formation. As the strong onshore surface winds move over land they weaken, but above the surface (> 1 km) winds are not affected. This creates strong wind shear that is conducive to tornado formation, especially on the cyclone's left forward quadrant (in the southern hemisphere). It is believed that most of the tornadoes that do form occur in the outer rain bands some 80-300 km from the centre, but some have been documented to occur in the inner core of the eye wall. It is possible that the extreme damage produced from winds in the eye wall are actually due to tornadoes.
- 6. How are tropical cyclones different to mid-latitude cyclones?
To a first approximation a tropical cyclone is like a heat engine - it derives its energy from the heat that is released when water vapour that has been evaporated from the ocean surface (assisted by high winds and low pressure) condenses in the middle of the atmosphere. Mid-latitude cyclones (low pressure systems associated with fronts) primarily get their energy from horizontal gradients in temperature.
Another important difference between the two is that tropical cyclones have their strongest winds near the surface while mid-latitude systems have their strongest winds many kilometres above the surface near the top of the atmosphere.
- 7. What is storm surge?
Storm surge is a large mound of water that accompanies a tropical cyclone as it comes ashore. The intense winds of the cyclone pile up the ocean into a dome of water that is pushed onshore as the cyclone strikes the coast. The low pressure of the cyclone adds to the height of the mound of water, though this is a secondary effect. When the height of a storm surge is discussed it does not take into account the height of the large waves on top of the mound of water.
Further details: Storm Surge
- 8. What is the difference between a storm surge and storm tide?
The combination of storm surge and astronomical tide is known as 'storm tide'. The worst impacts occur when the storm surge arrives on top of a high tide. When this happens, the storm tide can reach areas that might otherwise have been safe.
The graphic below shows the actual tide (top line, blue) and the astronomical tide (red) at Exmouth during cyclone Vance (1999). The black line is the storm surge component that peaked at 3.5 metres. At this time the predicted tide was 1.4 metres so the resultant peak storm tide was 4.9 metres. If the peak surge had of occurred at the time of high tide the actual tide would have been 6.0 m or 2.6 metres above the highest astronomical tide (pink line).
Data courtesy of WA Department of Planning and Infrastructure.
- 9. What about tsunamis?
A tsunami is a series of ocean waves with very long wavelengths (typically hundreds of kilometres) caused by large-scale disturbances of the ocean, such as:
- volcanic eruptions
These disturbances can either be from below (e.g. underwater earthquakes with large vertical displacements, submarine landslides) or from above (e.g. meteorite impacts). They are not caused by tropical cyclones.
See: Tsunami warnings
- 10. What do the terms damaging winds, destructive winds and very destructive winds mean?
The term damaging winds refers to wind gusts in excess of 90 km/h. The term destructive winds refers to wind gusts in excess of 125 km/h. The term very destructive winds refers to wind gusts in excess of 165 km/h.
Category Sustained winds (km/h) Strongest gust (km/h) Typical effects 1 Tropical Cyclone 63 - 88 Below 125 Damaging winds 2 Tropical Cyclone 89 - 117 125 - 164 Destructive winds 3 Severe Tropical Cyclone 118 - 159 165 - 224 Very destructive winds 4 Severe Tropical Cyclone 160 - 199 225 - 279 5 Severe Tropical Cyclone Over 200 Over 280
- 11. What does 'maximum sustained winds' mean? How does it relate to wind gusts in tropical cyclones?
The Bureau of Meteorology uses a 10 minute averaging time for reporting the sustained (i.e. relatively long-lasting) winds. The maximum sustained wind are the highest 10 minute surface winds occurring within the circulation of the cyclone. These surface winds are those observed (or, more often, estimated) to occur at the standard meteorological height of 10 m having an unobstructed exposure.
Gusts are a wind peak lasting for just a few seconds. Typically, in a cyclone environment the value for a peak gust is about 25 % higher than a 10 minute sustained wind. Barrow Island and Mardie sustained wind and wind gust profile during TC Monty (2004).
NOTE: USA agencies, who have responsibility for issuing tropical cyclone warnings in the Atlantic and Northeast Pacific tropical cyclone basins, use a 1 minute averaging time for sustained winds. While one can utilize a simple ratio to convert from peak 10 min. wind to peak 1 min. wind (roughly 12% higher for the latter), such systematic differences tend to make inter-basin comparison of tropical cyclones around the world problematic.
- 12. How does the amount of damage caused by a cyclone increase as the wind speed increases?
Or to rephrase the question: Would a cyclone with wind gusts of 280 km/h cause twice the damage of a similar sized cyclone with wind gusts of 140 km/h? No - it would cause hundreds of times more damage.
As wind speed increases the power of the wind to do damage increases exponentially. Hence a category 5 severe tropical cyclone (with wind gusts > 280 km/h) has the potential to do around 250 times the damage of a Category 3 severe tropical cyclone (with wind gusts of 165 km/h). This underscores the importance of the category system.
Graphical representation of the variation of damage with wind speed.
Image provided by Michael Drayton, Risk Management Solutions Inc. after George Walker, Australian residential, post 1980 construction.
- 13. Why and how are cyclone names chosen?
Tropical cyclones are named to provide ease of communication between forecasters and the general public regarding forecasts, watches, and warnings. Having a name also raises the profile of the cyclone heightening the public's awareness. Since the storms can often last a week or longer and that more than one can be occurring in the same region at the same time, names can also reduce the confusion about what storm is being described.
The Bureau of Meteorology maintains a list of names (arranged alphabetically and alternating male and female). A name remains on the list until its corresponding cyclone severely impacts the coast (e.g. Larry and Vance). The name is then permanently retired and replaced with another (of the same gender and first letter). It can take over 10 years from the time a name is put on the list to when it is first used to name a cyclone.
- 14. When did the naming of cyclones begin?
The convention of naming Australian tropical cyclones began in 1964. The first Western Australian named cyclone was Bessie that formed on 6 January 1964. Female names were used exclusively until the current convention of alternating male and female names commenced in 1975.
The naming of weather systems in Australia began much earlier than the 1960s, however. The flamboyant Clement Wragge, Government meteorologist in Queensland from 1887 until 1902, initiated the practice by naming weather systems after anything from mythological creatures to politicians who may have annoyed him.
During World War II, US Navy and Army Air Corps meteorologists informally gave tropical cyclones women's names. The Northwest Pacific basin tropical cyclones were given women's names officially starting in 1945 and men's names were also included beginning in 1979. Beginning on 1 January 2000, tropical cyclones in the Northwest Pacific basin were named using a new and very different list of names. The new names are Asian names and were contributed by all the nations and territories that are members of the World Meteorological Organisation's (WMO) Typhoon Committee. These newly selected names have two major differences from the rest of the world's tropical cyclone name rosters. One, the names by and large are not personal names. There are a few men's and women's names, but the majority are names of flowers, animals, birds, trees, or even foods, etc, while some are descriptive adjectives. Secondly, the names will not be allotted in alphabetical order, but are arranged by contributing nation with the countries being alphabetized.
The Southwest Indian Ocean tropical cyclones were first named during the 1960/1961 season but in the North Indian Ocean region tropical cyclones are not named.
Characteristics and formation
- 1. Why do tropical cyclones form?
The sun heats the tropical areas more than the polar regions. If there were no wind, then the tropics would keep getting hotter and hotter, and the poles would get colder and colder. The atmosphere's basic function is to redistribute heat from the equator to the poles, and tropical cyclones are one mechanism by which this occurs. However it is still quite remarkable that such a thing as a tropical cyclone should arise. It has been said that if we had not actually observed tropical cyclones then, despite all we know about the physics of the atmosphere, we would never have guessed at their existence.
- 2. How do tropical cyclones form?
For a cyclone to form several preconditions must be met:
- Warm ocean waters (of at least 26.5°C) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m). Warm waters are necessary to fuel the heat engine of the tropical cyclone.
- An atmosphere which cools fast enough with height (is "unstable" enough) such that it encourages thunderstorm activity. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.
- Relatively moist layers near the mid-troposphere (5 km). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity.
- A minimum distance of around 500 km from the equator. Some of the earth's spin (Coriolis force) is needed to maintain the low pressure of the system. (Systems can form closer to the equator but it's a rare event)
- A pre-existing disturbance near the surface with sufficient spin (vorticity) and inflow (convergence). Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organised system with sizeable spin and low level inflow.
- Little change in the wind with height (low vertical wind shear, i.e. less than 40 km/h from surface to tropopause). Large values of wind shear tend to disrupt the organisation of the thunderstorms that are important to the inner part of a cyclone.
Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favourable conditions do not develop.
- 3. What is the life-cycle of a tropical cyclone?
Tropical cyclones have a distinct life cycle. For cyclones that reach at least severe (category 3 or higher having wind gusts of at least 165 km/h) the life-cycle may be divided into four stages. For non-severe cyclones, their development is constrained by one or more of a number of factors such as being located in an unfavourable atmospheric environment, movement over cooler water or making landfall.
1. The formative stage
On satellite images the disturbance appears as an unusually active, but poorly organised, area of convection (thunderstorms). The circulation centre is usually ill-defined but sometimes curved cumulus cloud bands spiralling towards an active area of thunderstorms indicate the location of the centre. Initially the amount of convection near the centre is dependent upon the normal diurnal cycle of tropical convection, increasing overnight and subsiding during the day. As development occurs the convection persists throughout the day. The strongest surface winds may be well removed from the centre, tend to occur in disorganised squalls and are often confined to one quadrant, for example the northwesterly monsoon winds to the north of the centre. Apart from local squalls the maximum wind is usually less than gale force. When formative stage tropical cyclones move inland they produce little or no damage on landfall but are often associated with heavy rain and sometimes flooding over northern Australia.
2. The immature stage
In this stage the area of convection persists and becomes more organised. Intensification occurs simultaneously. The minimum surface pressure rapidly drops below 1000 hPa and convection becomes organised into long bands spiralling inwards. Gale-force winds develop with the strengthening pressure gradient, and the maximum winds (which now may be storm-force or more) are concentrated in a tight band close to the centre. The circulation centre is well defined and subsequently an eye may begin to form. In satellite images several well organised curved bands of active convection may be seen spiralling in towards a central dense mass of clouds covering the focal point of the banding, or surrounding the centre. The eye (if it exists) may be masked by a canopy of cirrus cloud, which itself may contain curved striationsassociated with the outflow at the top of the tropical cyclone. The immature tropical cyclone can cause devastating wind and storm surge effects upon landfall, although damage is usually confined to a relatively small area. In this stage of development very rapid intensification can occur and the associated structural changes observed when the cyclone is under radar surveillance can sometimes be confusing.
3. The mature stage
During this stage the tropical cyclone acquires a quasi-steady state with only random fluctuations in central pressure and maximum wind speed. However, the cyclonic circulation and extent of the gales increase markedly. Asymmetries in the wind field may also become more pronounced. In satellite images the cloud field is highly organised and becomes more symmetrical. The more intense cyclones are characterised by a round central dense overcast containing a well-centred, distinct round eye. The surrounding convective bands are tightly coiled and quasi-circular. Typically a cyclone spends just a day or so at maximum intensity until it begins to weaken, unless the cyclone remains in a highly favourable environment.
4. The decay stage
The warm core is destroyed during this stage, the central pressure rises, and the belt of maximum wind expands away from near the centre. Decay may occur very rapidly if the system moves into an unfavourable atmospheric or geographic environment, but sometimes only the tropical characteristics are modified while the cyclonic circulation moves on to higher latitudes.
In satellite images the decaying stage is characterised by the weakening of organised convection near the centre and disappearance of major curved convective bands. The low-level circulation centre may still be very well defined by narrow bands of low clouds. Those cyclones that cross the coast and weaken over land may continue to produce heavy rain a considerable distance inland.
An abbreviated life cycle of tropical cyclone Paul, April 2000.
- Formative stage: 11 April.
- Immature stage: 14 April.
- Mature stage (920 hPa): 15 April.
- Decay stage: 20 April.
- 4. How long do tropical cyclones last?
Track of the long-lived cyclone Rewa. The cyclone briefly weakened about 8 January.
The lifetime of a cyclone is determined by how favourable the atmospheric environment is, movement, sea surface temperatures. While most cyclones undergo a life-cycle of 3-7 days some weak ones only briefly reach gale force while others can be sustained for weeks if they remain in a favourable environment. The longest being Hurricane Ginger (1971) that lasted for 30 days. More recently Hurricane Ivan (2004), that moved through the Gulf of Mexico crossing the Alabama coast, was a very long lasting intense hurricane having estimated winds of at least 200 km/h for over 7 days.
Cyclone Rewa formed northwest of the Solomon islands on 28 December 1993 and finally decayed below cyclone strength in the Coral Sea east of Brisbane 24 days later on 21 January 1994.
- 5. What is the eye and eye wall?
The circular eye or centre of a tropical cyclone is an area characterised by light winds, fine weather and often clear skies. The eye is the region of lowest surface pressure
The size of the eye varies from one cyclone to the next ranging from 10 km to over 100 km. The eye diameter of severe cyclones off the northwest coast tends to be about 20 to 40 km, and are typically smaller than those in some other parts of the world such as the north Pacific. The eye size of Tracy (Darwin, 1974) was just 12 km across. Rosita (Broome, 2000) only had an eye diameter of 20 km.
The eye is surrounded by a dense ring of cloud known as the eye wall. This marks the most dangerous part of the cyclone having the strongest winds and heaviest rainfall. Radar and satellite imagery often show that the eye wall clouds are the inner-most coil of a series of spiral rain-band clouds that extend hundreds of kilometres from the centre and typically produce very strong wind squalls. The eye-wall is not always symmetrical, in fact at any one time it is common for the strongest convection and surface winds to be in one part of the eye wall as shown during Monty (2004).
Gove radar image as Monica (2006) moved past the north. The eye wall is clearly visible surrounding the eye.
Diagram of the eye and eye-wall structure.
US Atlantic Oceanographic and Meteorological Laboratory's Hurricane FAQ: Technical description of the eye and eye wall.
- 6. How big are tropical cyclones?
The size of a cyclone is usually described in terms of the radius of gale-force (sustained winds of at least 63 km/h). Although the distribution of surface winds is never completely symmetrical estimates of the gale-radius provide a reasonable guide on the size of a system. Cyclones off the northwest have an average radius of gales of about 150 to 200 kilometres. It is common for gales to occur well beyond the average radius of gales in one quadrant, for example the northwesterly monsoon winds to the north of the centre.
The size varies considerably. The radius of gales of Rosita (Broome, 2000) was just 70 km compared to the 350-400 km of Orson (1989). The adjacent picture shows the 100 km diameter of gales of Tracy (Darwin, 1974) could fit into the eye of Kerry (Qld, 1979) which had an eye diameter of 180 km. The areal extent of gales is also very important for wave generation and rainfall. The radius of storm-force (sustained winds of at least 90 km/h) and hurricane-force (sustained winds of at least 118 km/h) winds are other important parameters of cyclone size.
- 7. What happens to cyclones as they move further south?
Cyclones off northern Australia typically develop between 5 and 15°S and reach maximum intensity between 10 and 20°S. As they move further south they will weaken over land or, for those remaining over water, weaken as they encounter an unfavourable environment (strengthening wind shear) and move over cooler water. They usually increase in speed as the steering winds in the mid-levels of the atmosphere increase and move to the south or southeast.
Occasionally a cyclone moves south and interacts with a mid-latitude trough and undergoes extra-tropical transition changing from a warm-cored tropical low to a cold-cored mid-latitude low. The structure, distribution of winds and rainfall changes significantly.
See also:Tropical Cyclones affecting Perth.
- 1. When is the cyclone season?
The Australian cyclone season officially runs from November to April, although very few have occurred in November.
The earliest cyclone to impact the northwest Australian coast in the season was on 19 November 1910 when the eye passed over Broome. One of the latest cyclones in a season was TC Herbie that formed near Cocos Islands and passed over Shark Bay on 21 May 1988.
Further west in the Indian Ocean, cyclones can occur all year around although the risk of one in the winter months is very low.
Monthly frequency of cyclones off northwest Australia 1988/89 - 2003/04 for all cyclones (red) and intense cyclones (category 4 and 5) (blue).
- 2. How many cyclones occur each year?
On average there are about thirteen cyclones that form in the Australian region (90-160° E) each cyclone season. This represents about 16 per cent of the global total. About half of these occur in the western region. About half of the total number of tropical cyclones become severe. Tropical cyclones in the western and eastern basins have around 25 per cent chance of making landfall, while those in the northern basin have an 80 per cent chance of making landfall.
The map of the incidence of cyclones in the Australian region shows an area north of Port Hedland has the maximum occurrence of cyclones.
The graph of cyclone activity in the Australian region shows the total number of severe and non-severe cyclones each year.
US Atlantic Oceanographic and Meteorological Laboratory's Hurricane FAQ: Comparison of the average, highest and least number of cyclones in different parts of the world.
- 3. What is the most cyclone-prone region in Australia?
The northwest of Western Australia between Broome and Exmouth is the most cyclone prone part of Australia's coastline. This is also the region most prone to severe cyclone impacts. Seventy-two of the total of 146 coastal crossings in Australia between 1970-71 and 2003-04 were in WA. Thirty-four of the total 42 severe cyclone crossings occurred in WA and most of these occurred between Broome and Exmouth. In WA this equates to 2.2 cyclone crossings per year, one of which is severe on average.
Map below: Tropical cyclone crossings in Australia 1970-71 to 2001-02. Black dots represent non-severe cyclone crossings and red dots severe cyclone crossings.
- 4. Do cyclones affect Perth and Brisbane or Sydney? What about other towns?
Although the considerable majority of cyclone impacts are located in the tropics, occasionally a cyclone affects areas further south down the east or west coast. It is not uncommon for a southward moving tropical cyclone to cause heavy rain over areas well to the south, even if it weakens below cyclone intensity. Indeed most of the widespread rain events over southern Australia in the warmer months are associated with a tropical low or cyclone.
Along the east coast strong easterly winds between the cyclone and a high to the south can cause heavy rain and strong easterly winds that may cause large waves.
Between 1910 and 2004 a total of 14 cyclones have impacted Perth, equating to one every seven years on average. Some of these such as the 1937 and 1978 (Alby) caused destructive winds in Perth while others such as 1955 caused heavy rain and some flooding.
See also: Tropical cyclones affecting WA towns.
- 5. What determines the movement of tropical cyclones?
One look at the tracks of cyclones in the Australian region (see map) shows that cyclones can move in many different directions. The path of each storm varies considerably in response to the weather patterns occurring at the time. Tropical cyclones can be thought of as being steered by the surrounding environmental flow particularly in the middle parts of the atmosphere (from about 2 to 6 km above the surface). Typically there is a preferred movement to the west to southwest at speeds of 5 - 20 km/h especially when they are in the northern tropics. As they reach further south they are more likely to take a more southerly track and then once south of about 25S they typically are moving to the south southeast and accelerating. This was the case with Vance in the image below. Cyclone Lena and Rewa however followed very different and more erratic paths.
The larger and more intense systems do influence the surrounding environment and in so doing affect their movement. Short-term fluctuations in the track are common for intense cyclones, for example Bobby (1995) made several loops and changed speed as it approached the Pilbara coast (see track of Bobby based on radar observations).
- 6. Are we getting stronger and more frequent tropical cyclones in the last several years? What about climate change?
Trends in tropical cyclone activity in the Australian region (south of equator; 105-160°E) show that the total number of cyclones has decreased in recent decades. However, the number of stronger cyclones (minimum central pressure less than 970 hPa) has not declined.
This overall decrease may partly be due to an improved discrimination between tropical cyclones and sub-cyclone intensity tropical lows. Tropical cyclone numbers in the Australian region are influenced by the El Niño-Southern Oscillation phenomenon and the decrease in total cyclone numbers may be associated with an increased frequency of El Niño events. A number of long-term trends and oscillations have been observed in other parts of the world, extending over many decades. It is difficult to sort these natural trends from those that may result from global warming.
Potential changes in tropical cyclone occurrence and intensity are discussed in detail in the 2007 report, Climate Change in Australia Technical Report - Chapter 5: Regional climate change projections (8.9MB) See: Chapter 5.9.1 Severe weather: Tropical cyclones. There have been three recent studies producing projections for tropical cyclone changes in the Australian region. Two suggest that there will be no significant change in tropical cyclone numbers off the east coast of Australia to the middle of the 21st century. The third study, based on the CSIRO simulations, shows a significant decrease in tropical cyclone numbers for the Australian region especially off the coastline of Western Australia. The simulations also show more long-lived eastern Australian tropical cyclones although one study showed a decrease in long-lived cyclones off the Western Australian coast.
Each of the above studies finds a marked increase in the severe Category 3-5 storms. Some also reported a poleward extension of tropical cyclone tracks.
Projected changes in tropical cyclone characteristics are inherently tied to changes in large-scale teleconnection patterns such as ENSO, changes in sea surface temperature and changes in deep convection. As global climate models improve, their simulation of tropical cyclones is expected to improve, thus providing greater certainty in projections of tropical cyclone changes in a warmer world.
Tropical Cyclone Forecasting
- 1. Who is responsible for issuing tropical cyclone warnings?
The Bureau of Meteorology is responsible for issuing cyclone advisory messages between 90 and 160°E. Australia has three Tropical Cyclone Warning Centres, in Perth, Darwin and Brisbane.
Around the world there are five tropical cyclone Regional Specialized Meteorological Centres (RSMCs) together with six Tropical Cyclone Warning Centres (TCWCs) having regional responsibility to provide tropical cyclone advisories and bulletins.
- 2. How accurate are cyclone warnings from the Bureau of Meteorology?
Tropical cyclones (TC) have historically had a reputation for being unpredictable. Much effort has been dedicated to improving the forecasting skill in both location and intensity. The Bureau of Meteorology routinely issues forecasts of cyclone location and intensity to 5 days (120 h). All official forecasts are verified by comparison with the best track, the official estimate of the location and intensity of a tropical cyclone. A best track is prepared for every tropical cyclone, after the event using all available data.
Tropical cyclones vary considerably in their predictability. Some exhibit rapid changes in intensity or change course, speed up or slow down, primarily in response to changes in the surrounding environment.
Rewa (1993) was a TC for 23 days and moved very erratically and changed intensity many times making the forecasts very difficult especially for the longer lead times. Yasi (2011) by comparison maintained a steady movement towards the west southwest, thus increasing forecaster confidence in determining where it might cross the coast. The 24-hour forecast error for Yasi was 70 km, significantly less than the 5-year (2011–12 to 2015–16) average of 98 km.
Another factor influencing the forecast accuracy is how well we can determine the tropical low or cyclone centre location. The weaker TC's can be difficult to locate by satellite, compared to stronger cyclones that have a well-defined eye. Those cyclones that markedly change their course or intensity close to the coast present the greatest challenge to forecasters and decision-makers in the community. Community awareness is much higher when a cyclone develops well offshore prior to crossing compared to one that rapidly develops near the coast.
Figure 1(a): The track of Rewa in 1993–94
Figure 1(b): The steadier track of Yasi in 2011
Figure. 2 shows the yearly accuracy of position, year for the Australian region since 1985–86. The five-year average (2011–12 to 2015–16) accuracy is 20 km for the initial position, 69 km at 12 hours, 89 km at 24 hours, 154 km at 48 hours and 221 km at 72 hours. Today a 24 hour forecast is as accurate as those issued for a 12 hour prediction in the 1990s. Note that the errors were higher in 2015–16 partly because of the fewer number of forecasts issued as there were only three tropical cyclones in that season.
Figure 2: Yearly TC position accuracy for the Australian region, 1985–86 to 2015–16
Improvements in forecast position accuracy are due to a combination of more accurate computer model guidance, improved monitoring technology, and improved methods to combine computer model information. The use of multiple models has increased the forecast skill. The map below compared the actual track of TC Olwyn with forecast tracks from several different computer models sourced from around the world for a particular time. The spread of the model tracks provides an estimate of uncertainty in the forecast. Each model is different and no one particular model is always better than the others. The accuracy of each model also changes from one model run to the next. A forecast track made from the consensus of selected models provides the basis of the official forecast track as shown in the example for TC Olywn in Figure 3.
Figure 3: The official track of TC Olwyn shown with symbols and arrows, compared to the coloured lines of forecast tracks from selected computer models.
Understanding the historical skill and model variations, allows forecasters to present the official forecast track with the accompanying area of uncertainty (the grey area) on the Forecast Track Map as shown for TC Olwyn in 2015 in Figure. 4. This grey area represents the possible range of tracks in the 72 hour forecast.
Figure 4:The Forecast Track Map for TC Olwyn in 2015
Although the skill in track forecast has improved greatly, there has been much slower progress in intensity forecasting. This remains the focus of intensive study around the world. Forecasters can determine the general intensity changes but TCs can change their intensity very quickly. For example, Marcia (2015) rapidly intensified from a tropical low to Category 5 within 48 hours as it approached the coast. While Marcia was an extreme case, it is a reminder to be attentive to the latest information when a TC is in the area.
Go to the US Atlantic Oceanographic and Meteorological Laboratory's Hurricane FAQ page to see the accuracy of the US National Hurricane Center for tropical cyclones (hurricanes) in the north Atlantic Ocean.
- 3. What types of warning products are issued by the Bureau of Meteorology?
- Tropical cyclone seasonal outlook
- Tropical cyclone outlook
- Tropical cyclone information bulletin
- Tropical cyclone advice (watch/warning)
- Technical summary
- Tropical cyclone forecast track map
- Marine warnings
- 4. How can I access warnings?
Tropical cyclone advisory messages are available through a range of media including the Internet, automated telephone messages and weather by fax. Radio and television also broadcast details of tropical cyclones, especially when the cyclone nears the coast.
Bureau phone and fax numbers and contact details for emergency services agencies.
Details: Getting information during a cyclone
- 5. What should I do when a cyclone warning is issued?
See information provided by your state or territory emergency agency:
- WA: FESA
- NT: NTES
- Qld: EMQ
- NSW: SES
Contact details for emergency services agencies.
Details: Getting information during a cyclone
- 6. How does the Bureau estimate the intensity of a tropical cyclone?
For more information on tropical cyclones try the FAQ page from the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory (USA).