Abstract
Fire weather indices are commonly used by fire weather forecasters to predict when weather conditions will make a wildland fire difficult to manage. Complex interactions at multiple scales between fire, fuels, topography, and weather make these predictions extremely difficult. We define a new fire weather index called the Hot-Dry-Windy Index (HDW). HDW uses the basic science of how the atmosphere can affect a fire to define the meteorological variables that can be predicted at synoptic-and meso-alpha-scales that govern the potential for the atmosphere to affect a fire. The new index is formulated to account for meteorological conditions both at the Earth’s surface and in a 500-m layer just above the surface. HDW is defined and then compared with the Haines Index (HI) for four historical fires. The Climate Forecast System Reanalysis (CFSR) is used to provide the meteorological data for calculating the indices. Our results indicate that HDW can identify days on which synoptic-and meso-alpha-scale weather processes can contribute to especially dangerous fire behavior. HDW is shown to perform better than the HI for each of the four historical fires. Additionally, since HDW is based on the meteorological variables that govern the potential for the atmosphere to affect a fire, it is possible to speculate on why HDW would be more or less effective based on the conditions that prevail in a given fire case. The HI, in contrast, does not have a physical basis, which makes speculation on why it works or does not work difficult because the mechanisms are not clear.
Highlights
Predicting when weather conditions will make a wildland fire difficult to manage is important but extremely difficult
We acknowledge that multiplying U and vapor pressure deficit (VPD) is a limitation of Hot-Dry-Windy Index (HDW), but it appears to work at a basic level because our analyses have indicated that this simple formulation is able to discriminate the “worst” days on a given fire
The Haines Index variant that is recommended for this location is shaded in blue
Summary
Predicting when weather conditions will make a wildland fire difficult to manage is important but extremely difficult. The ultimate goal of HDW is to quantify the potential for a wildland fire to become difficult to manage due to the influence of predictable, large-scale components of weather forecasts at multiple-day lead times, while leaving the assessment of less-certain, smaller-scale details to experts on the ground. If a non-weather-only fire index fails to identify a fire event, it could be because of the aforementioned reasons and could be due to the fuels and/or topography information in the calculation, which makes determining attribution, efficacy, and failure modes for the index more difficult We want HDW to identify the potential for the worst fire conditions due to large-scale weather effects, so, for a given instant in time, we compute the surface-adjusted VPD that would result from adiabatically lowering air from each level in the lowest 500 m to the surface. To limit the potential for confusion among prospective users of the index, we recommend that units of hPa be used for the VPD and that units of m s−1 be used for U when calculating the value of HDW, but that the units of HDW (hPa m s−1 ) be ignored
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