Evaluating Outflow Boundary Characteristics in Areas of Complex Terrain

Outflow boundaries from thunderstorms interact with wildland fires to cause rapid and severe changes to fire behavior, resulting in potential threats to firefighter safety. During the 2013 Arizona Yarnell Hill Fire, 19 firefighters were killed due to such changes in fire behavior, driven by outflow boundary interactions. Tragic events such as these have the wildland fire community interested in understanding how outflow boundaries evolve in areas of complex terrain for the purposes of ensuring the safety of firefighters.
This study investigates thunderstorm outflow boundary characteristics using two datasets from Colorado field campaigns in complex terrain. First, 27 thunderstorm outflow boundary events were detected and analyzed using wind-profiling lidars, sodars, and temperature-profiling microwave radiometers to contextualize the passage and evolution of outflow boundaries in and near areas of complex terrain. The second dataset consists of velocity and reflectivity observations from a scanning X-band mobile Doppler on Wheels (DOW) radar situated at Bristol Head, CO, a 3875 m mountain peak in the San Juan Mountains during August, 2016. Outflow boundary characteristics related to depth, propagation speed and direction, temperature and humidity change, turbulence and maximum velocity at the surface were analyzed and clustered by the parent thunderstorm type, terrain, and ambient atmospheric conditions.
Initial results from this study suggest that propagation speed depends on distance from the parent thunderstorm. Slower boundaries tend to be further from their parent thunderstorms, while faster boundaries are closer. Additionally, the strength of the gust front, defined by the sharpest shifts in atmospheric conditions during passage, is related to the overall depth of the outflow boundary. The deeper boundaries resulted in sharper shifts in temperature, wind speed and direction, and relative humidity at the observational sites when compared to the shallower boundaries. This study provides critical information as to which remote-sensing instruments can be useful in detecting outflow boundaries in mountainous terrain. Additionally, results from this analysis provide a framework of criteria that numerical modeling studies can focus on when studying outflow boundaries in mountainous terrain. As thunderstorms pop up in the vicinity of ongoing mountain wildfires, knowledge from this study could help forecasters and emergency responders better understand how outflow boundaries may cause rapid shifts in temperature, wind speed and direction, and moisture content in and around the immediate fire area.