The dynamics of wildfire-generated dry convection: Fundamental processes and complicating factors.

摘要

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 parameterized by a surface heat flux, and atmospheric variables (e.g. wind) and fire parameters (e.g. dimension, intensity) are varied independently.;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. Two control parameters encapsulating the fundamental processes are developed: an advection parameter and a parcel heating parameter. It is found that organizational mode is most sensitive to the advection parameter, and updraft strength is most sensitive to the parcel heating parameter. In the second set of experiments, the impact on parcel processes of three-dimensional details such as fireline shape and along-line inhomogeneity is examined systematically. Experiments with more realistic sinusoidal-shaped firelines with heat fluxes strongest where the fireline bows out in the direction of the background wind, indicate that such 3D fireline structures can result, in weaker parcel heating and convection than that found in the earlier 2D experiments. 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. It is found that a combination of shear instability and a critical level can play an important role in the development of intense fire plumes in cases where multicell convection is otherwise preferred.