Addressing Plume Rise and the In-Plume Velocity Core Structure from Three Wildfires During FIREX-AQ

The representation of wildland fire plumes and the interaction that a plume has with the atmosphere as it is advected downwind from the fire source is extremely important when addressing pollution transport and the local impacts that the plume has on the surrounding meteorology. Direct observations of wildfire plume evolution at a high spatiotemporal resolution and the impact of the background conditions on the structure of the plume are still rare. Furthermore, techniques to isolate the characteristics and structure of the plume within the vicinity of active burn areas are far and few between.

The NOAA Chemical Sciences Laboratory (CSL) participated in the Fire Influence on Global to Regional Environments and Air Quality (FIREX-AQ) to address key questions related to air quality and fire behavior. The study presented herein focuses on the latter and was accomplished using Doppler lidar (DL) data collected aboard the NOAA Twin Otter (TO) aircraft. This study highlights three fires of different sizes (i.e., the Nevada Creek, Granite Gulch, and 163 HK Complex fires) that burned similar vegetation types and attempts to quantify the rise of the plume as well as the dynamical evolution of the plume at different distances downwind from the source.

In the study presented, we estimate sensible heat fluxes using the Brigg’s formula for bent-over plumes in the presence of a background wind along with estimates of plume entrainment. While a range of sensible heat flux values were estimated using different definitions of the source radius and bulk wind, the plume entrainment was largest for actively ascending plumes and smallest for plumes that no longer experienced active ascent. The expansion of the plume, which led to an increase in plume area, shared an inverse relationship with the depth over width (DOW) of the plume, such that an increase in area was found to be coincident with plume widening.

For each of the plumes investigated, we isolated the velocity core structures within cross-sectional areas positioned at different distances away from the fire source to characterize the intensity, width and depth, and area of updrafts and downdrafts. Velocity core intensities were found to generally decrease with distance. A distance independent scaling relationship in the form of a power law was found between the width and depth of velocity cores, with exponents typically ranging between ½ and 2/3. The significance of this finding is believed to be related to limitations of vertical growth of velocity cores due to the boundary layer (BL) depth, while less limiting factors exist in the horizontal that would lead to continued widening. Employing methods to examine how velocity core DOW and area changed with distance was statistically significant for certain cases. For instance, downdrafts typically exhibited a more robust statistical relationship compared to updrafts, especially for the 163 HK Complex fire, which featured a plume that rose over a short distance before advecting downwind at a nearly constant height. We surmise that the increase in statistical robustness for the 163 HK Complex fire was related to sampling the plume at a more dissipative stage where added energy into the plume was no longer factor downwind of the injection height. The lack of additional forcing could support the energy cascade to smaller scales that would lead to a reduction in the velocity core area of downdrafts with distance as well as a widening of velocity core structures as vertical velocities reduced and lateral dissipation ensued. Other cases considered statistically significant, but not to the degree observed for the 163 HK Complex fire, also featured a reduction in velocity core area and a decrease in DOW. For all cases, the number of velocity cores increased linearly with plume area, adding an additional 3 velocity cores for every 1 km² increase of the plume area.