Southeast Australian foehn

The southeast Australian foehn is a westerly foehn wind and a rain shadow effect that occurs on the coastal plain of eastern New South Wales, and as well as in southeastern Victoria and eastern Tasmania, on the eastern side of the Great Dividing Range. 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, causing major adiabatic compression (the rate at which temperature decreases with altitude) and a substantial loss of moisture. The effect is known by other names, such as the Australian chinook, the Great Dividing wind, the Great Dividing foehn or simply westerly foehn. Typically occurring from late autumn to spring, though not completely unheard of in the summer (particularly in eastern Tasmania), the foehn effect mainly occurs when a westerly or south-westerly frontal system (which brings rainy and windy weather to southern capitals like Melbourne, Perth and Adelaide) passes over the Great Dividing Range and thereby results in clear to partly cloudy, relatively warmer conditions on the lee.

Formation and characteristics

This foehn effect on the east coast of Australia is linked with the passage of a deep low pressure system across the Great Australian Bight and Bass Strait that cause strong winds to reorient virtually perpendicular to some parts of the Great Dividing Range, predominantly between late autumn into winter and spring (being most common during a negative AAO 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. Foehn occurrence on the east coast can also occur when hot, north-westerly winds blow from the interior (even when there is little moisture on the windward side), because the air heats up faster as it descends onto the plains than it cooled as it ascended the ranges. Typically 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 south-west of the continent in the Southern Ocean which moves east or north-eastward across Victoria towards the east coast. Moreover, temperatures on the lee of the Great Dividing Range tend to rise substantially (due to a katabatic effect) when cold fronts 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). 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. Furthermore, when the warm season north-westerly 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.

Mechanism

The southeast Australian foehn is characterised by three principal features: surface winds blowing from the direction of the mountains, a marked increase in air temperature on the leeward side of the ranges, and an accompanying decrease in atmospheric moisture. As moist air ascends the windward slopes of the ranges, it cools and condenses, enhancing orographic lifting. Precipitation removes moisture from the air mass, while the release of latent heat during condensation contributes to warming. As the now drier air descends the lee slopes towards the coastal plain, it undergoes adiabatic compression and warms further. During these conditions, an orographic cloud band known as the Föhn wall develops along the ridgelines of the southeastern highlands as moisture condenses on the windward slopes. Downwind of the mountains, a Föhn arch, consisting of a broad layer of altostratus cloud, forms within the ascending branch of a standing lee mountain wave. On weather maps, a band of clear air known as the Föhn gap can often be observed between the cloud wall and the arched cloud cover on the lee side of the Great Dividing Range. This type of foehn event is considered thermodynamically driven. Studies have identified topographically induced atmospheric waves associated with foehn development. These form as upper-level air descends over the mountain crest and propagates into the lee as large-scale, vertically propagating gravity waves. Model simulations indicate that strong downslope motion and enhanced wind shear contribute to increased turbulence near the surface, consistent with the gusty conditions frequently observed at lee-side stations. In addition to generating foehn winds, the prevailing westerlies can suppress the development of cooling sea breezes from the northeast, preventing them from penetrating inland across parts of the eastern seaboard. Descending motion over the coastal escarpment is typically stronger than over the main range and is associated with more pronounced wind shear. As a result, downslope winds are often strongest near the surface on the lee side of the escarpment. Smaller-scale trapped lee waves may also develop, and their occurrence, combined with strong wind shear, can generate substantial turbulence throughout the boundary layer, contributing to the intense gustiness observed at the surface. At night, the foehn effect generally weakens as a mountain breeze develops. During this process, cooler, denser air flows downslope and accumulates on the lee side of the ranges, promoting significant night-time cooling and a large diurnal temperature range.

Distribution and regional effects

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