Southeast Australian foehn

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Fohn cloud over the Crackenback Range, near Jindabyne Australian foehn.jpg
Föhn cloud over the Crackenback Range, near Jindabyne

The southeast Australian foehn is a westerly foehn wind and a rain shadow effect that usually occurs on the coastal plain of southern New South Wales, and as well as in southeastern Victoria and eastern Tasmania, on the leeward side of the Great Dividing Range. [1] Ranging from cool to hot (depending on the season), the effect occurs when westerly winds descend steeply from the Great Dividing Range onto the coastal slopes, whereby causing major adiabatic compression (which is the rate at which temperature decreases with altitude) and a substantial loss of moisture. [2] [3] [4]

Contents

The effect is known by other names, such as the Australian chinook, the Great Dividing wind, the Great Dividing foehn or simply westerly foehn. The southeast Australian foehn is distinguished by three criteria; surface winds which blow from the mountains' direction, a sharp rise in air temperature in the leeward side of the mountains, and an accompanying diminution in atmospheric moisture. [2]

Typically occurring from late autumn to spring, though not completely unheard of in the summer (particularly in eastern Tasmania), [lower-alpha 1] the Great Dividing foehn mainly occurs when a westerly or southwesterly frontal system (including a general westerly flow) passes over the ranges, thereby providing clear to partly cloudy and relatively warmer conditions on the lee. [lower-alpha 2] [5]

Origins

Foehn winds usually occur when the westerly wind belt moves northwards. FMIB 36792 Coup de Vent d'Ouest, au Sud de l'Australie (4 September 1895).jpeg
Foehn winds usually occur when the westerly wind belt moves northwards.

The foehn effect on the coastal plains of southeastern Australia is mostly linked with the passage of a deep low pressure system or westerly cold fronts across the Great Australian Bight and southeastern Australia that cause strong winds to reorient virtually perpendicular to some parts of the Great Dividing Range, predominantly between late autumn into winter and spring, particularly during a negative SAM phase. Their occurrence is owed to the incomplete orographic blocking of comparatively moist low-level air and the subsidence of drier upper-level air in the lee of the mountains. [2] Foehn occurrence on the southeast coastal plains can also occur when hot, northwesterly winds blow from the interior (even when there is little moisture on the windward side), because the air heats up faster as it descends into the plains than it cooled as it ascended the ranges. [7]

Averaging between 60 km/h (37 mph) to 70 km/h (43 mph), sometimes they may be brought on by a large polar air mass from the southwest of the continent in the Southern Ocean which advances northwards across Victoria towards the east coast. [8] Moreover, temperatures on the lee of the Great Dividing Range tend to rise substantially (due to a katabatic effect) [9] when westerly cold fronts passing over southern Australia push warm and dry air from the desert across the country's eastern states and over the Range (this is generally followed by a southerly buster). [2] [10]

As such, the Great Dividing foehn is one the few reasons why Sydney, among other places on the coastal plain, registers high temperatures in the warm season but seldom attains cold maximum temperatures in the winter. [7] [11] [12] Furthermore, when the warm season northwesterly winds strike (such as the Brickfielder), the hottest and driest areas of southeastern Australia will generally be located along the southern coastal region of NSW in the lee of the Great Dividing range and coastal escarpment due to the foehn effect. Much lower relative humidity figures would also observed in these leeward stations. [13]

Formation

Fohn wind illustration (Left: NSW/VIC Western Slopes, Right: NSW/VIC Eastern Slopes). Foehn wind illustration.svg
Föhn wind illustration (Left: NSW/VIC Western Slopes, Right: NSW/VIC Eastern Slopes).

As the moist air rises over the windward side of the ranges, it cools and it would condense, thereby creating precipitation on the upwind slopes. The precipitation then gets rid of the moisture from the air mass on the lee side of the ranges, and the condensation raises the air temperature as it descends the lee slopes towards the coastal plains because of the adiabatic compression. [14]

During these conditions, an orographic cloud band, or the Föhn wall, builds up along the ridgelines of the southeastern highlands due to condensation of moisture as the air ascends the windward slopes. Meanwhile, the Föhn arch, with its broad layer of altostratus cloud, shapes downwind of the mountains in the ascending component of a standing lee mountain wave. In weather maps, a band of clear air called the Föhn gap, which is over the downwind of the Great Dividing region, can be seen between the wall and arched cloud cover. This foehn wind can be referred to as thermodynamically driven. [1]

The existence of topographically induced atmospheric waves in connection with foehn occurrence has been indicated, which develop with the descent of upper-level air above of the ridgetop and pass into the lee of the ranges as broad-scale, vertically supporting gravity waves. The wind shears and the strength of the downslope motion manifested in the model examination also point that the onslaught of foehn conditions results in increased turbulence near the surface, evident in the gusty conditions observed at the lee stations. [2] In addition to the foehn winds, the same westerly winds also ward off the cooling sea breezes that arrive from the northeast, thereby preventing them from developing in the eastern seaboard. [15]

A vertically propagating gravity wave over the affected region exists. The descending motion over the coastal escarpment is stronger than that over the primary range and is connected with more powerful shear. The downslope winds tend to be strong, particularly near the lee's surface of the coastal escarpment. Smaller-scale, trapped lee waves over the affected region exist, and their incidence, together with the strong wind shears, signal significant turbulence throughout the boundary layer, which is concordant with the heavy gusty surface winds registered on the leeside. [2] At nighttime, the foehn effect subsides due to a mountain breeze – This is when denser cool air flows down the mountain slopes to settle in the downwind side, thereby providing relatively cold conditions in the night and, consequently, a high diurnal range of temperature.

Occurrence

Foehn effect coinciding with the 2009 Australian dust storm (notice the cloud streets forming on the slopes). SE Aust dust storm - MODIS Terra 250m - 22 Sept 2009.jpg
Foehn effect coinciding with the 2009 Australian dust storm (notice the cloud streets forming on the slopes).

The Great Dividing foehn is primarily observed in the southeast of New South Wales, east of the Great Dividing Range, in places such as the Sydney metropolitan area (Cumberland Plain), the Illawarra, some areas of the Southern Highlands, parts of the Monaro region, and the South Coast. It can also occur in the Central Coast, Hunter Valley and the Mid North Coast to the north. In many instances, it is observed in the East Gippsland region in Victoria [lower-alpha 3] as well as the eastern portion of Tasmania to the south.

Foehn winds may also impact other parts of Australia, such as east of the Great Dividing Range in southeast Queensland and northern New South Wales. [2] The Great Dividing foehn does not heavily impact areas northward from the Central Coast. The effect is gradient; being more common and efficacious towards the South Coast (due to the latter region being in the track of prevailing westerlies, which exponentially falters north of 35° S).

With leeward areas, or areas that receive foehn winds, precipitation is predominantly derived from the Tasman Sea to the east, since the Great Dividing Range blocks frontal westerlies off the Southern Ocean (which are most frequent between May and October). Therefore, due to the foehn effect, winters in leeward zones are drier with the summers being relatively wet, unlike those on the windward side which, conversely, have drier summers and damp winters. [2]

Areas that lie to the west of the Great Dividing Range are windward and therefore never experience a foehn effect under a westerly stream, with persistent cloud cover. On the contrary, the Great Dividing Range also blocks frontal systems originating in the southern Tasman as well as the eastern Bass Strait. When south/southeasterly frontal systems lift over the coastal slopes, the western edge of the Range would, conversely, experience foehn-like winds. [lower-alpha 4] [16]

Leeward zones

Snow covered farmland.jpg
Windward - Overcast, oftentimes foggy and/or snowy conditions resulting from uplift on the western slopes. Relative humidity largely in excess of 80% throughout the day. (near Orange, New South Wales)
Sunny Sydney Skyline.jpg
Leeward - Meanwhile, dry and mostly sunny conditions occur on the coastal plain due to compression of cold air as it descends the Ranges' leeward side. Relative humidity no greater than 50%. (Sydney CBD)
Transitional zones

Effects

The Great Dividing wind can be particularly damaging to homes and would affect flights, in addition to being uncomfortable, as the wind chill factor can paradoxically make the temperatures feel cooler than what they are. [18] [19] [20] The Australian foehn has also impacted international sporting events and as well as recreational aviation, such as in 2007, when a light aircraft crashed in the Central Highlands due to severe winds on a region that is prone to mountain-wind waves. [2] Much like the Santa Ana winds in California, they may elevate fire danger in the warmer months due to their dry, gusty nature. [21]

Foehn winds in general have been linked to headaches, depression and as well as suicide contemplation, although this study has not been proven. [22] Though recent studies regarding migraine attacks during Chinook winds suggest there may be some truth in it. [23]

Notable observations

In September, when the foehn effect is usually strong, green pastures on the windward side (left, Central Tablelands) can be contrasted from the dry landscape on the leeward (right, Greater Western Sydney). The Blue Mountains, Australia.jpg
In September, when the foehn effect is usually strong, green pastures on the windward side (left, Central Tablelands) can be contrasted from the dry landscape on the leeward (right, Greater Western Sydney).

See also

Notes

  1. They occur throughout the year in Tasmania as the island sits in the path of the Roaring Forties and/or the prevailing westerlies.
  2. Temperatures on the coastal plain are relative and therefore variable, ranging from 15 °C (59 °F) at the coolest (which is usual during polar blasts) to as high as 45 °C (113 °F) – All depending on the conditions on the windward side.
  3. Victoria is mostly exposed to westerly fronts due to its south-facing location and western longitude. Therefore, Victoria's east can still be windward in some occasions, especially when westerly fronts are vigorous.
  4. Windward areas are namely the Riverina, South West Slopes and North West Slopes regions, as well as the grand majority of Victoria and the entirety of South Australia.
  5. When southwesterly frontal systems are powerful, their accompanying clouds and precipitation may occasionally 'spillover' the NSW coastal plain for a short period of time, although no more than 2 mm (0.079 in) of rain will be recorded.

Related Research Articles

<span class="mw-page-title-main">Great Dividing Range</span> Mountain range in Australia

The Great Dividing Range, also known as the East Australian Cordillera or the Eastern Highlands, is a cordillera system in eastern Australia consisting of an expansive collection of mountain ranges, plateaus and rolling hills. It runs roughly parallel to the east coast of Australia and forms the fifth-longest land-based mountain chain in the world, and the longest entirely within a single country. It is mainland Australia's most substantial topographic feature and serves as the definitive watershed for the river systems in eastern Australia, hence the name.

<span class="mw-page-title-main">Lapse rate</span> Vertical rate of change of temperature in atmosphere

The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. Lapse rate arises from the word lapse, in the sense of a gradual fall. In dry air, the adiabatic lapse rate is 9.8 °C/km. The saturated adiabatic lapse rate (SALR), or moist adiabatic lapse rate (MALR), is the decrease in temperature of a parcel of water-saturated air that rises in the atmosphere. It varies with the temperature and pressure of the parcel and is often in the range 3.6 to 9.2 °C/km, as obtained from the International Civil Aviation Organization (ICAO). The environmental lapse rate is the decrease in temperature of air with altitude for a specific time and place. It can be highly variable between circumstances.

<span class="mw-page-title-main">Windward and leeward</span> Position relative to wind direction: upwind and downwind respectively

In geography and seamanship, windward and leeward are directions relative to the wind. Windward is upwind from the point of reference, i.e., towards the direction from which the wind is coming; leeward is downwind from the point of reference, i.e., along the direction towards which the wind is going.

<span class="mw-page-title-main">Foehn wind</span> Type of dry down-slope wind occurring near mountains

A Foehn or Föhn, is a type of dry, relatively warm, downslope wind that occurs in the lee of a mountain range. It is a rain shadow wind that results from the subsequent adiabatic warming of air that has dropped most of its moisture on windward slopes. As a consequence of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes.

<span class="mw-page-title-main">Orographic lift</span> Air mass forced upwards as it moves over rising terrain

Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.

<span class="mw-page-title-main">Rain shadow</span> Leeward side of a mountain range

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<span class="mw-page-title-main">Chinook wind</span> Warm, dry wind in North America

Chinook winds, or simply Chinooks, are two types of prevailing warm, generally westerly winds in western North America: Coastal Chinooks and interior Chinooks. The coastal Chinooks are persistent seasonal, wet, southwesterly winds blowing in from the ocean. The interior Chinooks are occasional warm, dry föhn winds blowing down the eastern sides of interior mountain ranges. The coastal Chinooks were the original term, used along the northwest coast, and the term in the interior of North America is later and derives from the coastal term.

<span class="mw-page-title-main">Lee wave</span> Atmospheric stationary oscillations

In meteorology, lee waves are atmospheric stationary waves. The most common form is mountain waves, which are atmospheric internal gravity waves. These were discovered in 1933 by two German glider pilots, Hans Deutschmann and Wolf Hirth, above the Giant Mountains. They are periodic changes of atmospheric pressure, temperature and orthometric height in a current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range. They can also be caused by the surface wind blowing over an escarpment or plateau, or even by upper winds deflected over a thermal updraft or cloud street.

<span class="mw-page-title-main">Prevailing winds</span> Strongest direction of wind on a region of Earths surface

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<span class="mw-page-title-main">Southerly buster</span>

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The Brickfielder is a hot and dry wind in Southern Australia that develops in the country's deserts in late spring and summer, which heavily raises temperatures in the southeast coast.

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<span class="mw-page-title-main">Berg wind</span> Wind in South Africa

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<span class="mw-page-title-main">Climate of Sydney</span> Climate of the city in Australia

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<span class="mw-page-title-main">Precipitation types</span> Characters, formations, and phases of water condensed in the atmosphere

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