Abstract
Abstract. Forest fire danger rating based on sparse meteorological stations is known to be potentially misleading when assigned to larger areas of complex topography. This case study examines several fire danger indices based on data from two meteorological stations at different elevations during a major drought period. This drought was caused by a persistent high pressure system, inducing a pronounced temperature inversion and its associated thermal belt with much warmer, dryer conditions in intermediate elevations. Thus, a massive drying of fuels, leading to higher fire danger levels, and multiple fire occurrences at mid-slope positions were contrasted by moderate fire danger especially in the valleys. The ability of fire danger indices to resolve this situation was studied based on a comparison with the actual fire danger as determined from expert observations, fire occurrences and fuel moisture measurements. The results revealed that, during temperature inversion, differences in daily cycles of meteorological parameters influence fire danger and that these are not resolved by standard meteorological stations and fire danger indices (calculated on a once-a-day basis). Additional stations in higher locations or high-resolution meteorological models combined with fire danger indices accepting at least hourly input data may allow reasonable fire danger calculations under these circumstances.
Highlights
IencmolaongyycaonudntmrieasnaagroDemunyednnttahaesmwwoiecrlllsd.asInpororddeurcttoivfitaysociated thermal belt with much warmer, dryer conditions cilitate fire danger assessment on a given day and location, in intermediate elevations
The results revealed that, during temperature inversion, differences in daily cycles of meteorological parameters influence fire danger and that these are not resolved by standard (Van Wagner, 1987) – areGalseooavsacilaieblne.tDifeipcending on local ethnevsierocnamnebnetaluMsceodnodadistieoanlmsDaanneadgvteehmeleodneptstimoreodel ean.pgtp.lifcoartiporne,paalrledofmeteorological stations and fire danger indices
Further mountain wind effects are dynamic channelling, low-level jets, and deflections and perturbations of airflows by mountainous terrain (mountain wind waves linked to induced instability, strong gusty winds potentially leading to blow-ups, and thunderstorms with lightning as an ignition source) (Sharples, 2009, and the references therein)
Summary
IencmolaongyycaonudntmrieasnaagroDemunyednnttahaesmwwoiecrlllsd.asInpororddeurcttoivfitaysociated thermal belt with much warmer, dryer conditions cilitate fire danger assessment on a given day and location, in intermediate elevations. Additional stations in higher locations sistance and scheduling of prescribed fires (Baumgartner et or high-resolution meteorological models combined with fire danger indices accepting at least hourly input data may allow reasonable fire danger calculations under these circumal., 1967; Camia et al.,H20y0d6;rPoylnoegeyt aal.,n1d996). Key phenomena include changes of temperature and relative humidity associated with exposition and elevation, leading to a distinctive spatial distribution of fuel types, effects on snowmelt and fuel moisture patterns, as well as diurnal mountain wind systems (e.g. thermally-caused along-slope, alongvalley, cross-valley and mountain–plain wind systems, all affecting fire behaviour and especially the direction of fire spread). More complex features comprise temperature inversions and associated thermal belt formation (conditions more favourable to wildfires at mid-slope than at lower and higher elevations, McRae and Sharples, 2011) and thermodynamic foehn winds (warm and dry katabatic lee slope winds causing high fire danger and intensive fire behaviour). Further mountain wind effects are dynamic channelling (a combination of upper winds and complex topography potentially leading to unexpected extreme fire behaviour and spotting), low-level jets (narrow currents of fast moving air influencing fire behaviour), and deflections and perturbations of airflows by mountainous terrain (mountain wind waves (e.g. gravity waves) linked to induced instability, strong gusty winds potentially leading to blow-ups, and thunderstorms with lightning as an ignition source) (Sharples, 2009, and the references therein)
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