A Case Study Featuring the Time Evolution of a Fire-Induced Plume Jet Over the Rum Creek Fire: Mechanisms, Processes, and Dynamical Interplay

The California Fire Dynamics Experiment (CalFiDE) of 2022 was a multi-institutional collaboration between academia and government to address a series of scientific objectives that included mapping active portions of the fire perimeter to quantify the rate of spread; the characterization of fire-atmosphere coupling within the boundary layer and associated 3D dynamics; and the characterization, transport, and chemical make-up and age of plumes generated over active burn areas. Six wildland fires were sampled through a combined measurement strategy to target the evolution of fire behavior and the subsequent impacts on the governing dynamics and wildfire chemistry. In this presentation, we focus on one of two flights dedicated to the Rum Creek fire in Oregon with a particular interest on a persistent updraft centered over the southwest corner of the fire perimeter on 09/01/2022.

The NOAA Twin Otter was deployed to the Rum Creek fire and was fitted with a scanning Doppler lidar (DL) capable of measuring horizontal and vertical winds, in situ chemistry packages, and a multi-spectral infrared camera for mapping heat generated from the fire across the surface. In addition, a ground-based mobile DL system downwind of the fire simultaneously measured the vertical and horizontal wind profile and served as a reference point when examining spatial differences in the wind structure. The flight strategy over Rum Creek featured box patterns to estimate the inflow winds and spatial mapping across the fire perimeter to identify wildfire hotspots and fire-induced dynamics. On several occasions, the TO would resample a dynamically active area of the fire perimeter to understand the time evolution of the fireline and associated dynamics. One of the more active burn areas of Rum Creek that was revisited multiple times along the same flight path was located on the southwest corner of the fire, which also featured an updraft that eventually intensified into a buoyant plume jet (BPJ).

The time evolution of the BPJ was addressed by employing an approach that was previously developed to isolate updrafts, but with a modification that accounts for the Gaussian core-like structure spanning the width of the updraft. The separation of the updraft from the surrounding atmosphere allowed for an analysis of the intrinsic properties of the updraft, plume top entrainment, lateral entrainment, and the interaction between the updraft and the environment. The four flight legs conducted over the updraft not only allowed for a time evolution analysis of the BPJ, but also the identification of dynamical consistencies and differences of both the updraft and neighboring environment. A persistent cross-valley flow leading up to the updraft positioned on a ridge and the downward transport of aerosol following smoke reaching the injection height were observed during all four passes. The cross-valley flow merged into the updraft that occasionally led to heat and ozone transport above the fire boundary layer (BL). By the second overpass, the cross-valley flow began to intensity which was followed by the intensification of the BPJ by the third overpass. Length scales of lateral and plume top entrainment were estimated following the separation of the overall structure from perturbations within the column of the updraft. An analytical form of the updraft profile above the vertical velocity maximum combining plume-top entrainment along with an assumed thermal structure was exploited following a partial budget analysis. For the assumed thermal structure, we tested different inversion strengths to understand the sensitivity to the analytical form of the updraft near the top of the BL. Velocity core strength and characteristics were also analyzed away from the updraft with similar relationships between depth to widths of velocity cores as found during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign.