Wildfire Observations By Airborne and Truck-Based Mobile Doppler Lidars during the California Fire Dynamics Experiment (CalFiDE)

Wildfires and their threat to first responders and the general public have increased over the last decade. This has led to an increased need for fire-related products and forecasts, with observations being essential as “ground truth” for product development. New observations of fire behavior and dynamics such as fire-generated winds, plume rise processes, and topographical effects are important to advancing model performance. However, three-dimensional profiling of wind fields is particularly challenging around wildfires, especially in complex mountainous terrain where wildfires often occur. The California Fire Dynamics Experiment (CalFiDE) from 26 August to 26 September 2022 was designed to target this challenge with two state-of-the-art mobile Doppler lidars on airborne and mobile ground-based platforms supported by additional remote sensing and in-situ instrumentation. This presentation will focus on the two mobile Doppler lidars including the novel deployment strategies and a preliminary analysis of data from the immediate area of wildland fires. Observed features include updraft and plume characteristics, fire-affected winds such as inflow strength and extent, entrainment processes and finer scale dynamics embedded in wildfire plumes, and other non-linear fire-atmospheric dynamics. The aircraft also hosted high-resolution multispectral imaging to map out the fire and couple evolving fire behavior to the observed wind fields.

The system installed on the NOAA Twin Otter aircraft employed a multi-axis scanner to manipulate beam azimuth and elevation with varying scan rates, with 60 m range resolution and 2-10 Hz temporal resolution. Wind profiles were collected in the immediate vicinity of the wildfire to examine fire-affected winds and inflow characteristics. Vertical stares were performed over hotspots to capture cross-sections of the vertical motions. These vertical winds provide quantitative characterization of updrafts along with mixing or entrainment, including inference of horizontal vortices. The lidar attenuated aerosol backscatter return also provides information on plume structure and motion, and the lidar was able to track the plume well downwind of the fire.

The truck-based system is capable of making measurements while underway and while stationary, leveraging a motion-stabilization platform and inertial measurements for platform motion correction of the lidar measurements. Two channels are employed simultaneously to profile vertical winds (zenith stare) and horizontal winds (conical scan at 15 degrees off-zenith). The system design is rugged enough for dirt roads and other access challenges to achieve desired proximity to wildfires in remote and mountainous areas. During CalFiDE the truck-based system was deployed primarily to sample the upwind inflow regions of targeted wildfires, monitoring inflow velocity and structure over time. Where road access and science objectives allowed, the truck-based system sometimes stayed mobile to provide contextual winds throughout the region near the fire, which often featured complex terrain flows.