The influence of an upper‐level frontal zone on the Mack Lake Wildfire environment

Abstract

AbstractMeteorological assessment of wildland fire danger has traditionally involved the identification of several synoptic weather types empirically determined to influence wildfire spread. Specifically, in the Great Lakes Region, high wildfire danger is often witnessed in association with northwesterly synoptic‐scale flow aloft. Such synoptic‐scale flow is regularly associated with the development of upper‐level frontal zones also known as upper‐level jet/front systems, which are often characterised by intrusions of stratospheric air into the troposphere. The notion that upper‐frontal development can play an important role in promoting wildfire spread is advanced through interrogation of the output from a fine‐scale numerical simulation of a documented explosive wildfire case; the Mack Lake Fire of May 1980.The Mack Lake case was characterised by a developing upper‐level front embedded within a shortwave trough in the vicinity of the fire location. The upper‐level front originated in northwesterly flow in central Canada as an upper‐tropospheric ridge amplified over western North America. A thermally indirect circulation at the jet exit region both contributed to the intensification of the front and was associated with a maximum in quasi‐geostrophic descent at mid‐levels upstream of the fire region. The subsidence ushered dry air from the middle and upper‐troposphere downward along sloping isentropes adiabatically warming and drying it along the way. A well‐developed dry air intrusion associated with the operation of these processes extended to nearly the 750 hPa level far downstream from the actual upper‐frontal zone supplying the fire environment with dry air that originated in the upper‐troposphere/lower stratosphere. The organised subsidence was also responsible for downward advection of high momentum air from within the frontal zone into the fire environment, further influencing the wildfire spread. We conclude that upper‐frontal processes, characteristic of northwesterly synoptic‐scale flow, are likely a contributing factor to the prevalence of wildfire spread under such synoptic‐scale conditions. Copyright © 2007 Royal Meteorological Society