Southeast Australian foehn

The southeast Australian foehn is a westerly foehn wind that occurs on the coastal plain of southern New South Wales and, at times, eastern Victoria, Australia, on the leeward side of the Great Dividing Range.[1] Also known as the Australian foehn, the Great Dividing foehn, or simply westerly foehn, it is a (relatively)[a] warm and dry wind that occurs in the country's southeast on the leeward side of the Great Dividing Range.[2]

Föhn cloud over the Crackenback Range, near Jindabyne

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.[1] Typically occurring from late autumn to spring, though not completely unheard of in the summer, these winds occur when there is rainfall, snowfall and/or sleet in the windward side of the mountains that become a rain shadow foehn wind on the leeward side, thereby providing clear to partly cloudy and relatively warmer conditions.[3]

OriginsEdit

Foehn occurrence in southeastern Australia is generally 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, but with more frequency from late winter to spring. 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.[1]

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.[b][4] Moreover, temperatures on the lee of the Great Dividing Range tend to rise substantially (due to a katabatic effect)[5] 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).[1][6]

As such, the foehn effect is one the few reasons why Sydney, among other places in the coastal plain, register high temperatures in the warm season and seldom gets extremely cold in the winter; due to its protection by the ranges from the southwest winds that originate from the Southern Ocean, in addition to being on the lee of the ranges.[7][8][9] Furthermore, when the warm westerly winds strike, 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. Much lower relative humidity figures would also observed in these leeward stations.[10]

FormationEdit

 
Föhn wind illustration (Left: NSW/VIC Inland Slopes, Right: NSW/VIC Coastal 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.[11]

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.[1]

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.[1]

OccurrenceEdit

During a snow-bearing cold front, the frigid air is compressed as it descends the leeward side of the Ranges which melts snow at a rapid rate. (Perisher Valley)
As a result, the coastal plain would receive dry, gusty foehn with mostly clear to partly cloudy conditions. (Sydney CBD)

The Great Dividing foehn is primarily observed in southeast of New South Wales in the Hunter Valley, the Sydney metropolitan area, the Southern Highlands, the Illawarra, Canberra, the Monaro region, and the South Coast. In some instances, it is observed in the eastern Gippsland region in Victoria to the south.[c] It can also occur in the Central Coast and the Mid North Coast to the north. Foehn winds may also impact other parts of Australia, such as the east of the Flinders Ranges in South Australia, east of the Great Dividing Range in southeast Queensland and northern New South Wales, and the eastern part of Tasmania.[1]

The western portion of Blue Mountains and the eastern parts of Central Tablelands transitionally lie between the upwind and leeside of the ranges; Lithgow, Bathurst, Katoomba and as well as western Sydney (which straddles the mountains) are on the leeside, whereas Oberon and particularly Orange, to the west, are on the windward side, and would feature comparatively wetter winters.[12]

Being on the exposed western edge of the Great Dividing Range, Crookwell, Khancoban, Myrtleford, Batlow and Tumut to the south receive much of the precipitation that has not fallen farther west, chiefly in winter. Little or no precipitation is received from the Tasman Sea to the east,[d] due to the large distances and the Great Dividing Range; instead, precipitation derives from the Southern Ocean, advancing northwards across Victoria with south-westerly upwinds. In contrast, Cooma, Goulburn, Bowral, Bombala and Canberra are warmer, and as well as drier, in the winter, due to being on the leeward (eastern) side of the ranges (i.e. the Australian Alps); this is evident when compared to the windward towns on the western side of the ranges, which are few degrees cooler in the winter, in addition to being more rainier and snowier.[1]

In the cool season, these winds can be particularly damaging to homes and would affect flights, in addition to being uncomfortable, as the wind chill factor would usually make the temperatures feel cooler than what they are.[13][14][15] Much like the Santa Ana winds in California, they may elevate fire danger in the warmer months, due to their dry, gusty nature.[16]

Notable observationsEdit

  • On 29 May 2007, it was observed that temperature at Sale (leeward side) was around 4–9 °C (7–16 °F) higher than the corresponding temperatures at Melbourne and Wangaratta (which lie on the upwind side). In this foehn event, Sale had a high above 24 °C (75 °F), whereas the latter cities struggled to reach higher than 12 °C (54 °F). Furthermore, the relative humidity was 31% at Sale and as high as 80%–90% at Melbourne and Wangaratta.[1] Unusually warm and dry conditions were also registered at other stations in the downwind side of the ranges – Bairnsdale, Orbost, Latrobe Valley, and Nowa Nowa, which recorded temperatures of 24 °C (75 °F), 24.2 °C (76 °F), 22.9 °C (73 °F), and 22.6 °C (73 °F), respectively, making this location in the lee of the ranges consistent with the position of the foehn gap and foehn arch.[1]
  • On 27 October 2008, foehn wind dynamics were observed over the Gippsland region to the southeast of the Australian Capital Territory on the lee of the ranges, associated with northwesterly winds over southern New South Wales. These downwind regions experienced lower humidity levels and higher than average temperatures. The temperature at Orbost reached 32 °C (90 °F); the temperature at Mount Nowa Nowa rose to 26 °C (79 °F); Bega reached 36 °C (97 °F), which is approximately 14 °C (57 °F) above the average maximum temperature for October–November. At Moruya, the temperature rose to a maximum of 35.4 °C (96 °F). Similar effects were also observed at Green Cape, Narooma, Merimbula, Braidwood, Canberra, Bombala, and Cooma. In contrast, Albury, which is on the windward side of the ranges, only reached a maximum of 27.7 °C (82 °F).[1]
  • On 18 July 2016, Mallacoota reached an unseasonable high of 23.5 °C (74 °F) due to the foehn effect, a record warm winter day in the state of Victoria.[17]

See alsoEdit

NotesEdit

  1. ^ When it snows on the windward side in winter, the foehn winds on the leeward side raise the temperature to around 16 °C (61 °F), which is cool for the average person, but still very warm relative to the freezing temperatures on the upwind side.
  2. ^ Such winds may reach the South Island of New Zealand, where they produce a similar foehn effect on the leeward side of the Southern Alps.
  3. ^ Victoria is mostly exposed to westerly fronts due to its south-facing location. Therefore, Victoria's east can still be windward in some occasions, especially when westerly fronts are vigorous.
  4. ^ The ranges usually block Tasman lows, such as East coast lows, which originate from Tasman Sea. Therefore, when these lows make landfall, the western edge of the Range would, conversely, experience foehn-like winds.

ReferencesEdit

  1. ^ a b c d e f g h i j k l m Jason J. Sharples, Graham A. Mills, Richard H. D. McRae, and Rodney O. Weber. "Foehn-Like Winds and Elevated Fire Danger Conditions in Southeastern Australia". Journal of Applied Meteorology and Climatology. American Meteorological Society.CS1 maint: multiple names: authors list (link)
  2. ^ Rain Shadows by Don White. Australian Weather News. Willy Weather. Retrieved 24 May 2021.
  3. ^ Rain one side, heat the other in NSW by Joel Pippard. Weatherzone. 16 April 2020. Retrieved 6 October 2021
  4. ^ Wilder winds, less rain, as Roaring Forties become Furious Fifties By Peter Hannam and Environment Editor, Sydney Morning Herald, 11 May 2014. Retrieved 6 August 2020
  5. ^ The climate of Sydney, Australia The Department of Atmospheric Science. University of Wyoming. E. Linacre and B. Geerts, November 1998
  6. ^ Early taste of spring in eastern Australia Ben Domensino from Weatherzone. Thursday August 19, 2021
  7. ^ Weather Glossary - F Farmonline Weather
  8. ^ Was Penrith the hottest place on Earth on Sunday? by Ben Domensino, 8 January 2018. Retrieved 7 October 2021.
  9. ^ Local climate processes in the Illawarra by Edward A. Bryant, Department of Geography, University of Wollongong, 1982
  10. ^ Urban Heat Island Mitigation Technologies. Edited by Rohinton Emmanuel. Glasgow Caledonian University. 2021.
  11. ^ Sharples, J.J., McRae, R.H.D., Weber, R.O., Mills, G.A. (2009) Foehn-like winds and fire danger anomalies in southeastern Australia. Proceedings of the 18th IMACS World Congress and MODSIM09. 13–17 July, Cairns.
  12. ^ And the outlook for winter is … wet by Kate Doyle from The New Daily. Retrieved 24 May 2021.
  13. ^ Cold, damaging winds blast Sydney by The Leader, 9 August 2019. Retrieved 22 April 2020
  14. ^ Sydney weather: Flights cancelled as wild winds set to batter NSW throughout weekend by Seven News, Saturday, 10 August 2019. Retrieved 22 April 2020
  15. ^ BOM warns NSW to brace for worse weather as strong winds tear roof off Newcastle nursing home by ABC News Australia, 9 August 2019. Retrieved 22 April 2020
  16. ^ Sharples, J.J. (2009) An overview of mountain meteorological effects relevant to fire behaviour and bushfire risk. International Journal of Wildland Fire, 18, 737-754.
  17. ^ Weather map explainer: What are cold fronts, synoptic charts, isobars? by Debra Killalea from News.com.au. July 22, 2016. Retrieved November 15, 2021