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A first quantitative investigation into the causes of foehn warming in the lee of mountain ranges demonstrates the importance of three physical mechanisms, including one previously neglected.
Societal and environmental impacts of the warming experienced in the lee of mountains, known as the foehn warming effect, are significant and diverse. This warming can be spectacular (e.g., 25°C in an hour; Richner and Hächler 2013) and is typically accompanied by a decrease in humidity and accelerated downslope winds. The notoriety of these foehn winds has led to recognition by various local terms: among others, the Chinook and Santa Ana of North America and the Zonda of Argentina. The warmth brought by the foehn has implications for agriculture, ecosystems, and climate systems. It can increase the risk of avalanches or floods (Barry 2008), melt glaciers, and contribute to the disintegration of ice shelves (Cook et al. 2005; Kuipers Munneke et al. 2012). Foehn windstorms regularly cause damage to property and infrastructure (Whiteman and Whiteman 1974; Richner and Hächler 2013), and the combination of warm, dry air and high wind speeds promotes the ignition and rapid spread of wildfires (Westerling et al. 2004; Gedalof et al. 2005; Sharples et al. 2010). In California, Santa Ana winds are responsible for the majority of major wildfires, including 12 fires in October 2003 that burnt an area of over 300,000 ha, causing more than $1 billion (U.S. dollars) in property damage (Westerling et al. 2004; Ahrens 2012). Accurate forecasting of foehn events is a challenge for hazard assessment and management, one that is made significantly harder by a lack of quantitative understanding of the causes of foehn warming.
PARADIGMS OF THE FOEHN. Traditionally foehn winds are defined as any "warm, dry wind descending in the lee of a mountain range" (Brinkmann 1971, p. 230; Ahrens 2012; Barry 2008). However, this definition begs two critical questions: 1) what is the foehn warm and dry relative to and 2) why is the foehn warm and dry? While such imprecision is perhaps appropriate in describing something that is an everyday occurrence for many, it also ref lects the difficulty in concisely defining a phenomenon that is not fully understood. Indeed Brinkmann (1971, p. 238) challenges his own definition (above) by concluding, "Since the...