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. ©2013. American Geophysical Union. All Rights Reserved.