The Dynamics of Wildfire-Generated Dry Convection: Fundamental Processes and Complicating Factors

Abstract

Wildfires are capable of inducing atmospheric circulations due predominately to the large temperature anomalies produced by the fire. The fundamental dynamics through which a forest fire and the atmosphere interact to yield different convective regimes is still not well understood. The work described in this dissertation is aimed at understanding, from the perspective of atmospheric dynamics, how different modes of convection (e.g. plumes and multicells) develop. This research is conducted through the use of a numerical model in which the fire is parametrized by a surface heat flux, and atmospheric variables (e.g. wind) and fire parameters (e.g. dimension, intensity) are varied independently. Although the focus of this work lies in the atmospheric processes, effort is also made to apply the findings to the problem of fire behavior by examining relevant atmospheric variables such as surface wind speed and temperature near the fire for different convective modes.

In the first set of experiments, two-dimensional simulations are performed wherein the upstream surface wind speed and mixed-layer mean wind speed are varied independently in order to better understand the fundamental processes governing the organizational mode and updraft strength. The result of these experiments is the identification of two primary classes of dry convection: plume and multicell. Simulated plume cases exhibit weak advection by the mean wind and are subdivided into intense plume and hybrid classes based on the degree of steadiness within the convection column. Hybrid cases contain columns of largely discrete updrafts versus the more continuous updraft column associated with the intense plume mode. Multicell cases develop with strong mixed layer advection and are subdivided into strong and weak classes based on the depth of convection. Intense plume and strong multicell (hybrid and weak multicell) cases occur when the surface buoyancy is large (small). The multicell (strong and weak) and intense plume modes are forced by a combination of buoyancy and dynamic pressure gradient forcing associated with the perturbation wind field, while the hybrid mode is forced by a combination of buoyancy and dynamic pressure gradient forcing associated with the strong background shear.

In the second set of experiments, the simulations of the first set are extended in several ways. First, the impact of surface wind speed and cross-fireline dimension on parcel potential temperature is examined. It is found that the buoyancy parameter fails as a control parameter when values of the parameter exceed unity. However, the parameter may be useful as a gauge of fire behavior predictability in such cases, since the first set of experiments indicated a relationship between parameter B and the potential for feedback to the atmosphere. A second way in which the first set of experiments are extended is by performing simulations in a 3D model. In general, salient results from the earlier 2D study are reproduced in the 3D model. In one case with strong vertical wind shear, new convection develops away from the main convective line as a result of local changes to parcel speed and heating. The third way in which results are extended is through 3D experiments wherein fireline shape and along-line inhomogeneity are varied. It is found that a sinusoidal-shaped fireline with along-line uniform intensity induces stronger parcel heating where the fireline bows into the wind (i.e. the back of the fire). In a separate simulation with a more realistic fire structure wherein surface heat fluxes are strongest where the fireline bows out in the direction of the wind (i.e. the head of the fire), parcel heating and convection are weaker than for a straight, uniformly heated fireline.

In the third set of experiments, the impact of Kelvin-Helmholtz (i.e. shear) instability and a critical level on dry convection above a prescribed heat source is examined. An analysis of the advection parameter indicates that prior to convection penetrating the critical level, multicell convection dominates. After convection reaches the critical level, overturning develops as a result of shear instability. The relationship between the advection parameter and organizational mode deduced from the first set of experiments does not apply after convection reaches the critical level. In addition to the intense plume and multicell modes, a third mode termed the deep wave mode is simulated. This mode consists of disturbances with wavelengths of 7-10 km, and results from the multicell convection perturbing the unstable layer centered at the critical level. The presence of an unstable wind profile at the critical level is shown to be crucial to development of both the deep wave and intense plume modes; in the absence of a critical level and shear instability, multicell convection dominates. For the third set of experiments, knowledge of the mean and surface wind speeds, and thus the values of parameters A and B, respectively, is not sufficient to understand what processes will dominate the generation of convection.