Two tropical cyclones occurred over the western North Pacific and the South China Sea in June 2009, both of which necessitated the issuance of tropical cyclone warning signals in Hong Kong.

Tropical Depression Linfa formed over the northern part of the South China Sea about 520 km south-southeast of Hong Kong on 17 June and moved slowly.  It intensified into a tropical storm the next day.  Linfa started to move northwards on 19 June and intensified into a severe tropical storm that night. Linfa moved north-northeastwards across the northeastern part of the South China Sea on 20 June.  It weakened into a tropical storm in the afternoon of 21 June and made landfall in Fujian in the evening.

After moving northeastwards across the coast of Fujian on 22 June, Linfa entered the East China Sea and weakened into a tropical depression that evening. It further weakened into an area of low pressure over the East China Sea on 23 June.

Tropical Depression Nangka formed over the western North Pacific about 760 km east-southeast of Manila on 23 June.  Moving west-northwestwards, it intensified into a tropical storm that evening.  Nangka crossed the central Philippines the next day and entered the South China Sea in the evening.  Nangka turned to move generally northwestwards across the South China Sea on 25 June.

It moved generally north-northwestwards on 26 June approaching the coast of eastern Guangdong and weakened into a tropical depression that evening.  Nangka made landfall over the coastal areas of Daya Bay in the small hours of 27 June.  Nangka moved further inland and weakened into an area of low pressure over Guangdong that morning.

Hurricane images are from the Defense Meteorological Satellite Program (DMSP) Operational Lines can System (OLS). DMSP operates day/night and dusk/dawn satellites. Daytime images are taken with sunlight striking perpendicular to the storm while dusk/dawn images have sunlight glancing across the top of the storm. Images from F12 and F14 are daytime images; those from the F13 satellite are dawn/dusk.

Each hurricane is shown in two separate images showing views in the OLS visible near-infrared band and the OLS thermal-infrared band. The spatial resolution of the imagery used is 2.7 km. The thermal infrared images show the temperatures of the cloud tops, and ocean and land surfaces, while the visible-near infrared images show reflected sunlight during the day and reflected moonlight or man-made lights during the night.

Tropical cyclones (also known as hurricane in North America and typhoons in Asia) are giant whirlwinds of air and dense cloud spiraling at over 120 km/hr around a central ‘eye’ of extreme low pressure.

Australia’s cyclone season is usually from December to April and affects most of the Queensland coast. The greatest threat lies north of the Tropic of Capricorn.

When a tropical depression develops and its associated winds reach gale force, it will be classified as a tropical cyclone and will be given a name.

Cyclones occur frequently in the Southern Hemisphere with an average of 10 cyclones per year tracked by the Bureau of Meteorology in the Australian region.

The ‘life-cycle’ of the average tropical cyclone is about seven days, but can extend to over three weeks.

Cyclone watch

A cyclone watch is issued by the Bureau of Meteorology when a cyclone or developing cyclone is likely to affect coastal or inland communities within 24 to 48 hours.

A cyclone watch will include an estimate of the cyclone’s position, its intensity, severity and movement.

Cyclone watches will be issued every three hours initially and hourly once the cyclone nears the coast.

Cyclone warnings

A cyclone warning is issued by the Bureau of Meteorology when a cyclone or developing cyclone is likely to affect coastal or inland communities within 24 hours.

Warnings will identify the communities likely to be affected, the name of the cyclone, its position, intensity, severity and movement.

Communities under threat will be advised to take certain precautions to safeguard life and property.

Cyclones often produce winds in excess of 200 km/h which can cause extensive damage to property and turn debris into dangerous missiles.

Cyclones can also bring flooding rains, which cause further damage to property, and increase the risk of drowning.

Cyclones can cause huge seas, putting vessels in danger both in harbor and out at sea.

Multiple vortex tornado

A multiple vortex tornado is a type of tornado in which two or more columns of spinning air rotate around a common center. Multivortex structure can occur in almost any circulation, but is very often observed in intense tornadoes. These vortices often create small areas of heavier damage along the main tornado path.

Satellite tornado

A satellite tornado is a term for a weaker tornado which forms very near a large, strong tornado contained within the same mesocyclone. The satellite tornado may appear to “orbit” the larger tornado (hence the name), giving the appearance of one, large multi-vortex tornado. However, a satellite tornado is a distinct funnel, and is much smaller than the main funnel.

Waterspout

A waterspout is defined by the National Weather Service simply as a tornado over water. However, researchers typically distinguish “fair weather” waterspouts from tornadic waterspouts.

• Fair weather waterspouts are less severe but far more common, and are similar in dynamics to dust devils and landspouts. They form at the bases of cumulus congestus cloud towers in tropical and semitropical waters. They have relatively weak winds, smooth laminar walls, and typically travel very slowly, if at all. They occur most commonly in the Florida Keys and in the northern Adriatic Sea.
• Tornadic waterspouts are more literally “tornadoes over water”. They can form over water like mesocyclonic tornadoes, or be a land tornado which crosses onto water. Since they form from severe thunderstorms and can be far more intense, faster, and longer-lived than fair weather waterspouts, they are considered far more dangerous.

Landspout

A landspout (officially known as a dust-tube tornado) is a tornado not associated with a mesocyclone. The name stems from their characterization as essentially a “fair weather waterspout on land”. Waterspouts and landspouts share many defining characteristics, including relative weakness, short lifespan, and a small, smooth condensation funnel which often does not reach the ground. Landspouts also create a distinctively laminar cloud of dust when they make contact with the ground, due to their differing mechanics from true mesoform tornadoes. Though usually weaker than classic tornadoes, they still produce strong winds and may cause serious damage.

Tornado-like circulations

Gustnado

A gustnado (gust front tornado) is a small, vertical swirl associated with a gust front or downburst. Because they are technically not associated with the cloud base, there is some debate as to whether or not gustnadoes are actually tornadoes. They are formed when fast moving cold, dry outflow air from a thunderstorm is blown through a mass of stationary, warm, moist air near the outflow boundary, resulting in a “rolling” effect . If low level wind shear is strong enough, the rotation can be turned horizontally (or diagonally) and make contact with the ground. The result is a gustnado. They usually cause small areas of heavier rotational wind damage among areas of straight-line wind damage. It is also worth noting that since they are absent of any Coriolis influence from a mesocyclone, they seem to be alternately cyclonic and anticyclonic without preference.

Dust devil

A dust devil resembles a tornado in that it is a vertical swirling column of air. However, they form under clear skies and are rarely as strong as even the weakest tornadoes. They form when a strong convective updraft is formed near the ground on a hot day. If there is enough low level wind shear, the column of hot, rising air can develop a small cyclonic motion that can be seen near the ground. They are not considered tornadoes because they form during fair weather and are not associated with any actual cloud. However, they can, on occasion, result in major damage, especially in arid areas.

Fire whirl

Tornado-like circulations occasionally occur near large, intense wildfires and are called fire whirls. They are not considered tornadoes except in the rare case where they connect to a pyrocumulus or other cumuliform cloud above. Fire whirls usually are not as strong as tornadoes associated with thunderstorms. However, they can produce significant damage.

Steam devil

A steam devil is a term describing a rotating updraft that involves steam or smoke. A steam devil is very rare, but they mainly form from smoke emitting from a power plant smokestack. Hot springs and deserts may also be suitable locations for a steam devil to form. There have also been reports of cold air steam devils as well.

Potentially the most dangerous hazard associated with tropical cyclones which make landfall is storm surge. Storm surge has been responsible for more deaths than any other feature of tropical cyclones. Storm surge is a raised dome of water about 60 to 80 kilometres across and typically about two to five metres higher than the normal tide level. It is caused by a combination of strong winds driving water onshore and the lower atmospheric pressure in a tropical cyclone. In the southern hemisphere the onshore winds occur to the left of the tropical cyclone’s path. In Australia, this is the east side on the north west and north coasts and the south side on the east coast.

The largest surge usually extends between 30 and 60 kilometres from the crossing point of the tropical cyclone centre, or eye. Its influence also depends on the local topography of the seafloor and the angle at which the cyclone crosses the coast. If the surge occurs at the same time as a high astronomical tide the area inundated can be extensive, particularly along low-lying coastlines.