Kinematic structure of a wildland fire plume observed by Doppler lidar

Abstract

Wildland fires present a challenging environment to make meteorological measurements. Observations in the vicinity of wildland fires are needed to better understand fire‐atmosphere interactions and to provide data for the evaluation of coupled fire‐atmosphere models. An observational study was conducted during a low‐intensity prescribed fire in an area of complex terrain with grass fuels east of San José, California. A ground‐based scanning Doppler lidar acquired radial wind velocities and backscatter intensity in and around the fire plume from multiple horizontal and vertical scans. The development of a convergence zone was consistently observed to exist downwind of the plume and was indicated by a decrease in radial velocity of 3–5 m s−1. Divergence calculations made from the lidar radial velocities showed that the magnitude of convergence ranged between −0.06 and −0.08 s−1 downwind of the plumes, while a maximum of −0.14 s−1 occurred within the plume near the fire front. Increased radial velocities were observed at the plume boundary, indicating fire‐induced acceleration of the wind into the base of the convection column above the fire front. Thermodynamic measurements made with radiosondes showed the smoke plume had a potential temperature perturbation of 3.0 to 4.4 K and an increase in water vapor mixing ratio of 0.5 to 1.0 g kg−1. Plume heights determined from sequential range height indicator scans provided estimates of vertical velocity between 0.4 and 0.6 m s−1, representing the ambient background vertical velocity as the top of the plume likely reached equilibrium.