Assessment of the vertical exchange of heat, moisture, and momentum above a wildland fire using observations and mesoscale simulations

A wildland fire is capable of producing vertical exchanges of heat, moisture, and momentum over a broad range of spatial and temporal scales. At very fine scales (about 0.1–1 m), in the portion of the atmosphere directly above the fire, this exchange is dominated by products of the combustion process: extremely high air temperatures, moisture evaporated from burning fuels, superheated gases emitted by the fuels, and solid material released in the form of smoke. At larger scales (about 10–1000 m), the atmospheric response to these combustion products depends upon the meteorological conditions in which the fire develops. Under certain meteorological conditions, the atmosphere around and above a wildland fire can allow feedback mechanisms to develop that enable these larger-scale atmospheric circulations to alter the combustion process and profoundly affect the evolution of the fire.

This study will present observations, idealized mesoscale simulations, and real-data mesoscale simulations of the meteorological conditions associated with wildland fires. We will address situations wherein the vertical distribution of heat, moisture, and momentum associated with mesoscale features has the potential to impact the evolution of a fire. Routine surface, upper air, wind profiler, and satellite observations will be employed to describe the meteorological conditions observed in association with wildland fires that exhibited a response to vertical exchanges of heat, moisture, and momentum. Idealized numerical simulations using the ARPS model will detail how atmospheric conditions contribute to the development of different convective modes in response to the heat and moisture released by a fire. MM5 and WRF simulations of the meteorological conditions associated with wildland fires will show how vertical profiles of heat, moisture, and momentum in the vicinity of the fire can be linked to the evolution of mesoscale features.

The observations, idealized simulations, and mesoscale simulations of weather associated with wildland fires will be synthesized into a conceptual framework for understanding the environment in which vertical exchanges of heat, moisture, and momentum can influence the evolution of a fire. Specifically, we will discuss how conditions can develop wherein a fire-generated convective plume interacts with planetary boundary layer turbulence and deep boundary layer eddies such that dry, high-momentum air above a fire can be transported and mixed downward toward the ground in the vicinity of the fire. Since dry air and high winds are well-established fire-weather ingredients, short-term fluctuations in these critical fire-weather ingredients are important to users of fire-weather and fire-behavior forecasts. This study is designed to address the inherent difficulties in observing and forecasting these ingredients on the fine spatial and temporal scales that are important for wildland fire management. We will demonstrate how the potential for vertical exchanges of heat, moisture, and momentum can be diagnosed from observations and predicted using mesoscale model simulations.