The Mean and Turbulent Properties of a Wildfire Convective Plume

Abstract

The time-mean and time-varying smoke and velocity structure of a wildfire convective plume is examined using a high-resolution scanning Doppler lidar. The mean plume is shown to exhibit the archetypal form of a bent-over plume in a crosswind, matching the well-established Briggs plume-rise equation. The plume cross section is approximately Gaussian and the plume radius increases linearly with height, consistent with plume-rise theory. The Briggs plume-rise equation is subsequently inverted to estimate the mean fire-generated sensible heat flux, which is found to be 87 kW m−2. The mean radial velocity structure of the plume indicates flow convergence into the plume base and regions of both convective overshoot and sinking flow in the upper plume. The updraft speed in the lower plume is estimated to be 13.5 m s−1 by tracking the leading edge of a convective element ascending through the plume. The lidar data also reveal aspects of entrainment processes during the plume rise. For example, the covariation of the radial velocity and smoke perturbations are shown to dilute the smoke concentration with height.

Simulation of a Large Wildfire in a Coupled Fire-Atmosphere Model

“…The largest simulated downward motion occurs in the upper plume behind the fire front (maximum of −7 m s −1 ), consistent with previous observations [47] and simulations [48]. (Figure 10d), which fall within the range of sensible heat fluxes as reported by [49]. They induce maxima of the 2-m temperature difference over the active fire area up to 70 K (Figure 10c).…”

“…However, above in the plume, the dynamics mainly controls the TKE production, probably partly due to the initial shear environment and partly due to the shear induced by the updraft itself. In agreement with observational studies [47,49], the simulation shows increases in both horizontal and vertical velocity variances at the fire front and above, up to a 5000-m height ( Figure 13). Simulated values are in the same order of magnitude as the LiDAR observation of [49] reporting maximum variance values from 7 to 14 m 2 s −2 in a plume overshooting with comparable height.…”

“…In agreement with observational studies [47,49], the simulation shows increases in both horizontal and vertical velocity variances at the fire front and above, up to a 5000-m height ( Figure 13). Simulated values are in the same order of magnitude as the LiDAR observation of [49] reporting maximum variance values from 7 to 14 m 2 s −2 in a plume overshooting with comparable height. The simulated turbulence associated with the fire front is characterized by quasi-isotropic behavior with both high horizontal and vertical variances associated with fire-induced winds at the fire-front (Figure 14).…”

Wildfire Impact on Environmental Thermodynamics and Severe Convective Storms

“…This type of deep convective cloud is unique in its microphysical structure for the following reasons: (1) There exist high concentrations of small cloud droplets that result from a large number of cloud condensation nuclei due to the aerosol particles emitted by fire that compete for limited condensable water vapor (Andreae et al, ), and (2) these clouds initiate and develop under the vigorous dynamics and thermodynamics induced by the fire itself. The buoyancy generated by the sensible heat of wildfires can produce very strong updrafts (Lareau & Clements, ; Tory et al, ), which can further enhance the aerosol activation at the cloud base by increasing the supersaturation (Kablick et al, ; Luderer et al, ; Trentmann et al, ). Note that pyroCb often produces severe weather events that include large hail (>25 mm), enhanced lightning activity, extreme low‐level winds, and in some cases even tornadoes (Fromm et al, ; Lareau et al, ; Rosenfeld et al, ).…”

A review of a new generation of wildfire–atmosphere modeling

“…Heating from the fire produces buoyancy forces in the atmosphere and modifies near-surface winds, which can eventually increase the rate of fire spread and intensity. The buoyancy forces in a wildfire can increase the updraft by 1 to 2 orders of magnitude of an average thunderstorm (Lareau and Clements 2017). Using a scanning Doppler lidar and mobile radiosonde system during two large wildfires in northern California, Lareau and Clements (2016) found that compared with the ambient wind, the flow within the plume is characterized by much stronger velocities when outbound speeds exceeded 15 m·s -1 and the wind profiles showed significant shear over the lowest 2 km of the atmosphere.…”