Foehn-Like Winds and Elevated Fire Danger Conditions in Southeastern Australia

Jason J. Sharples School of Physical, Environmental and Mathematical Sciences, University of New South Wales at the Australian Defence Force Academy, Canberra, Australian Capital Territory, and Bushfire Cooperative Research Centre, Melbourne, Victoria, Australia

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Graham A. Mills Centre for Australian Weather and Climate Research, and Bushfire Cooperative Research Centre, Melbourne, Victoria, Australia

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Richard H. D. McRae Australian Capital Territory Emergency Services Agency, Canberra, Australian Capital Territory, and Bushfire Cooperative Research Centre, Melbourne, Victoria, Australia

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Rodney O. Weber School of Physical, Environmental and Mathematical Sciences, University of New South Wales at the Australian Defence Force Academy, Canberra, Australian Capital Territory, and Bushfire Cooperative Research Centre, Melbourne, Victoria, Australia

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Abstract

Bushfires in southeastern Australia are a serious environmental problem, and consistently cause loss of life and damage to property and other assets. Understanding synoptic processes that can lead to dangerous fire weather conditions throughout the region is therefore an important undertaking aimed at improving community safety, protection of assets, and fire suppression tactics and strategies. In southeastern Australia severe fire weather is often associated with dry cool changes or coastally modified cold fronts. Less well known, however, are synoptic events that can occur in connection with the topography of the region, such as cross-mountain flows and foehn-like winds, which can also lead to abrupt changes in fire weather variables that ultimately result in locally elevated fire danger. This paper focuses on foehn-like occurrences over the southeastern mainland, which are characterized by warm, dry winds on the lee side of the Australian Alps. The characteristics of a number of foehn-like occurrences are analyzed based on observational data and the predictions of a numerical weather model. The analyses confirm the existence of a foehn effect over parts of southeastern Australia and suggest that its occurrence is primarily due to the partial orographic blocking of relatively moist low-level air and the subsidence of drier upper-level air in the lee of the mountains. The regions prone to foehn occurrence, the influence of the foehn on fire weather variables, and the connection between the foehn and mountain waves are also discussed.

Corresponding author address: Jason J. Sharples, School of Physical, Environmental and Mathematical Sciences, University of New South Wales at the Australian Defence Force Academy, Canberra, ACT 2600, Australia. Email: j.sharples@adfa.edu.au

Abstract

Bushfires in southeastern Australia are a serious environmental problem, and consistently cause loss of life and damage to property and other assets. Understanding synoptic processes that can lead to dangerous fire weather conditions throughout the region is therefore an important undertaking aimed at improving community safety, protection of assets, and fire suppression tactics and strategies. In southeastern Australia severe fire weather is often associated with dry cool changes or coastally modified cold fronts. Less well known, however, are synoptic events that can occur in connection with the topography of the region, such as cross-mountain flows and foehn-like winds, which can also lead to abrupt changes in fire weather variables that ultimately result in locally elevated fire danger. This paper focuses on foehn-like occurrences over the southeastern mainland, which are characterized by warm, dry winds on the lee side of the Australian Alps. The characteristics of a number of foehn-like occurrences are analyzed based on observational data and the predictions of a numerical weather model. The analyses confirm the existence of a foehn effect over parts of southeastern Australia and suggest that its occurrence is primarily due to the partial orographic blocking of relatively moist low-level air and the subsidence of drier upper-level air in the lee of the mountains. The regions prone to foehn occurrence, the influence of the foehn on fire weather variables, and the connection between the foehn and mountain waves are also discussed.

Corresponding author address: Jason J. Sharples, School of Physical, Environmental and Mathematical Sciences, University of New South Wales at the Australian Defence Force Academy, Canberra, ACT 2600, Australia. Email: j.sharples@adfa.edu.au

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