Heat flux assumptions contribute to overestimation of wildfire smoke injection into the free troposphere

Abstract

Injections of wildfire smoke plumes into the free troposphere impact air quality, yet model forecasts of injections are poor. Here, we use aircraft observations obtained during the 2019 western US wildfires (FIREX-AQ) to evaluate a commonly used smoke plume rise parameterization in two atmospheric chemistry-transport models (WRF-Chem and HRRR-Smoke). Observations show that smoke injections into the free troposphere occur in 35% of plumes, whereas the models forecast 59–95% indicating false injections in the simulations. False injections were associated with both models overestimating fire heat flux and terrain height, and with WRF-Chem underestimating planetary boundary layer height. We estimate that the radiant fraction of heat flux is 0.5 to 25 times larger in models than in observations, depending on fuel type. Model performance was substantially improved by using observed heat flux and boundary layer heights, confirming that models need accurate heat fluxes and boundary layer heights to correctly forecast plume injections.

Fire Influence on Regional to Global Environments and Air Quality (FIREX‐AQ)

“…Taking four observed cases of smoke reaching into the free troposphere identified by Thapa et al. (2022), the vertical distribution of fire emissions was constrained with an inversion system used to assimilate HSRL observations. These constrained runs were compared to base runs using the Freitas et al.…”

“…Thapa et al. (2022) took a closer look at the commonly used plume rise parameterization in WRF‐Chem and HRRR‐Smoke from Freitas et al. (2007).…”

“…The impact of FRP-based plume rise was more subtle without clear evidence for improved performance. Thapa et al (2022) took a closer look at the commonly used plume rise parameterization in WRF-Chem and HRRR-Smoke from Freitas et al (2007). Using Active Fire Area and FRP measured by the MASTER instrument during overflights by the DC-8, Fire Radiative Energy (FRE) was calculated and compared to the model estimates.…”

“…Adjusting model calculations based on observed FRE resulted in fewer injections to the free troposphere but did not entirely eliminate the bias. Taking four observed cases of smoke reaching into the free troposphere identified by Thapa et al (2022), the vertical distribution of fire emissions was constrained with an inversion system used to assimilate HSRL observations. These constrained runs were compared to base runs using the Freitas et al (2007) parameterization.…”

Assessing Vertical Allocation of Wildfire Smoke Emissions Using Observational Constraints From Airborne Lidar in the Western U.S.

“…However, the errors in modeled thermodynamic structure and PBL heights can contribute to uncertainties in the inversely estimated injection fraction. In the recent study for all fire events sampled by FIREX-AQ in the western U.S. (Thapa et al, 2022), the underestimated PBL heights are found to be one of the key reasons of the more frequent free-troposphere injection cases modeled by WRF-Chem than observed. Taking the errors in modeled PBL heights into consideration, we examine the adjusted injection fractions (f >PBL,adj ) by using the corrected PBL heights based on DIAL-HSRL data.…”