Upper atmosphere smoke penetration from mass fires simulated in WRF

In recent years, large wildland fires have been increasing in frequency (Flannigan et al., 2013). Accurately predicting how these fires will behave has become a critical aspect of weather forecasting, as they pose dangerous threats to both human and natural populations. It is particularly important to develop a good understanding of how fires and local atmospheric conditions can influence one another, because a blaze may behave differently under different meteorological circumstances.

Large fires can inject smoke into the stratosphere, as seen in 2017's British Columbia fire (Yu et al., 2019). In cases like this, the smoke can be advected over long distances where it may impact populations distant from the fire location. Atmospheric stability, wind speed, and humidity can all affect how high smoke will be lofted into the upper atmosphere. If enough smoke penetrates the upper troposphere and stratosphere following a series of mass urban fires, situations akin to nuclear autumn or nuclear winter can become a threat.

To quantify the role of such meteorological influences, we employ the fire simulation package in the Weather Research and Forecasting model (WRF-Fire), Version 4.0.1, to simulate a suite of idealized scenarios. In each of these scenarios of a 16 km² circular fire, a mass fire burns a homogenous, wildland fuel bed under varying meteorological conditions. We assess how each meteorological factor affects the depth of smoke rise and pattern of fire spread. Specifically, we examine how wind speed, relative humidity, and boundary-layer stability (induced by the diurnal cycle) influence the resulting vertical profile of smoke concentration.

Our results support the argument that meteorology will impact smoke distribution. First, consistent with observations that moisture enables the development of pyrocumulus clouds that can loft smoke higher into the atmosphere, the presence of moderate amounts of moisture does lead to deeper smoke penetration in the upper atmosphere. Further, we find that the stability of the boundary layer has a slight effect on the vertical smoke distribution. Finally, local winds can inhibit or enhance the depth of smoke rise, depending on their magnitude.