Using WRF-LES Modeling Framework for Wildland Fire Plume Effects during the 2022 Mosquito Fire Event

Wildland fires (WF) interact dynamically with the ambient atmospheric boundary layer (ABL), and the state of the ABL significantly influences the transport and mixing of the released gases from the fire plumes. More importantly, the fire plume modifies the micro-climate in the region surrounding the fire. Understanding the two-way interaction between the ABL and the WF is an important and outstanding problem in atmospheric science research. With a growing number of wildland fires both in terms of intensity and number of incidences, improving the fundamental understanding of the two-way interactions between the WF and ABL has important contributions in progressing the science of Atmosphere dynamics, and with implications to climate change.

The Mosquito Wildland Fire was California's largest wildfire of the year which began on September 6, 2022, on the western slope of the Sierra Nevada. The Atmospheric conditions before and during the early days of the Mosquito Fire's spread included a combination of a heat wave with the all-time highest temperature of 116 degrees Fahrenheit and the vegetation moisture levels close to record lows. On September 7, the fire grew considerably, developing a massive pyro cumulus cloud, and September 8 was the largest day of growth as the fire activity intensified and became plume-dominated producing an enormous pyro cumulonimbus cloud that reached more than 12 km into the atmosphere.

The motivation of this study is to conduct high resolution model simulations using modified WRF-LES solver with addition of buoyancy forcing and additional turbulence production terms, to quantify the ABL dynamics during the time of the fire spread and correlate the ABL characteristics that influence the spread of the plume. Due to the release of a WF plume into the atmosphere, the boundary layer characteristics, including the wind speed, wind shear, turbulent kinetic energy (TKE), and the production, and temperature gradient significantly influence the scales of the horizontal and vertical plume transport. In addition, strong turbulent longitudinal vortices with updrafts and downdrafts originate near the ground. Thus, creating a vertical flux of momentum, buoyancy, and emissions from the WF. The horizontal transport scales of the plume are governed by the wind shear, wind strength, and the local atmospheric turbulence.

As the input data for the WRF model is available at a coarser resolution, the initial and boundary conditions to the actual domain are obtained by multiple nested WRF domains. The regional domain for all the cases is the Sierra Nevada region, California, United States. The fire plume was initialized in the center of the region ($39.006^o$ N, $120.745^o$ W), To study the release of the plume at 11 am, September 8th, 2022, high-resolution WRF-LES simulations of the ABL were conducted and the data gathered for the period from September $6^{th}$, 2022 to September $9^{th}$ 2022.The initial and lateral boundary conditions needed for WRF are provided by the HRRR model with a spatial resolution of 3 $km$.

The results of the wind profiles and correlations with the turbulence kinetic energy transport terms will be presented in this presentation. The mixing and entrainment have been quantified as the instantaneous change in volume of the plume scaled by the plume mean velocity and source diameter.