Seasons within the Wildfire Season: Marking Weather-Related Fire Occurrence Regimes

Abstract

Weather and climate contribute to the multidecadal, seasonal, and daily cycles of the potential for fire ignitions and for the severity of fires. We used a long-term dataset of weather parameters to characterize comparatively homogeneous periods, or subseasons, within the fire season. First, we conducted an exploratory analysis of weather conditions using the univariate t-test to determine if natural breaks in the weather conditions could be identified. Then, we used multivariate analysis of variance (MANOVA) of each calendar day, based on before and after periods of twelve days to identify the most distinct, natural breaks as expressed by the combination of weather variables as they change throughout the fire season. From this analysis, we identified six subseasons between March 1 and September 30 and explored the average weather conditions during each subseason. These results were partially validated against databases containing 29 years of historical fires and 16 years of historical Energy Release Component (ERC) data. From these results, we concluded that fire-weather can assume a uniform state for anywhere from two to six weeks, and then change into a considerably different regime. The quantitative establishment of these fire subseasons defines homogeneous periods of weather regimes that will improve the outputs of some fire models by controlling for seasonality. Our method for identifying subseasons could be applied by scientists using data from other regions to obtain subseason boundaries appropriate for their climatic regimes. The definition of subseasons also enhances our understanding of plant growth and development throughout the seasons, and provides managers with an objective tool to anticipate and adapt to the changing weather conditions.