Downdraft outflows: climatological potential to influence fire behaviour

Sudden wind shifts caused by atmospheric gust fronts can lead to firefighter entrapments and fatalities. In this study, we describe the physical processes involved in the related phenomena of convective downdrafts, gust fronts and downbursts. We focus on the dominant process, evaporative cooling in a dry surface layer, as characterised by the measure known as downdraft maximum available potential energy (DMAPE). We present a climatological analysis of DMAPE for the coterminous United States, developed from the Climate Forecast System Reanalysis data for the period 1979-2008. Diurnal and seasonal patterns are described. We conclude with a discussion of the implications and limitations of the analysis and DMAPE as an indicator of gust front strength or downburst occurrence.