Wind tunnel generation of horizontal roll vortices over a differentially heated surface
Wildland crown fires represent an extreme type of forest fire that advances from top to top of trees or shrubs. Some such fires generate energy comparable to several nuclear explosions. They occur most frequently in coniferous forests found in the United States and Canada, and in broad-leafed eucalyptus forests of Australia. Here we report visualization of wind tunnel experiments in which airflow along the perimeter of a simulated forest fire produced horizontal vortex pairs with axes in the downstream direction. These vortex pairs formed above a differentially heated surface simulating the thermal energy flux from a crown fire. The thermal energy was produced by an electrically heated surface ribbon at a temperature of ∼250°C and also, in a separate experiment, by an electrically heated embedded wire coil at a temperature of ∼825°C. Wind velocities ranged from 0.2 to 3.6 m s−1. Similar vortex action may cause the unburned tree-crown strips that are observed when crown fires, driven by a significant wind, burn over flat terrane.
Simulations of buoyancy-generated horizontal roll vortices over multiple heating lines
A two-dimensional nonhydrostatic atmospheric model is used to simulate the boundary-layer circulations that develop from multiple lines of extremely high surface temperatures. Numerical simulations are carried out to investigate the role of buoyancy and ambient crossflow effects in generating horizontal roll vortices in the vicinity of adjacent wildland fire perimeters. Numerical results suggest that the downdraft segments of buoyancy-generated horizontal roll vortices can be reinforced when multiple heating lines are present, with the degree of reinforcement dependent on the heating-line spacing. Significant low-level horizontal flow directly over the bounding heating lines is observed in some simulations, suggesting a mechanism for low-level flame migration in actual wildland fires. Multiple heating lines also affect the development and behavior of buoyancy-generated horizontal roll vortices when ambient crossflows are present. Simulations indicate that vortices are more likely to persist for multiple heating lines than for a single heating line when light ambient crossflows are introduced. Although simulation results should be viewed qualitatively, they suggest the potential importance of adjacent fire perimeters in actual wildland fire episodes for the development and behavior of buoyancy-generated horizontal roll vortices.
Patterns of fish and wildlife use for subsistence in Fort Yukon, Alaska
Fort Yukon, located along the Yukon River in northeastern interior Alaska, is the largest Athabaskan community in the state with an estimated population of 584 residents comprising 212 households in 1987. This study was undertaken to gather quantitative information on subsistence harvest and use of ftih and wildlife resources by Fort Yukon residents. These data, along with updated land use maps and socioeconomic information, were intended to complement a previous description of Fort Yukon subsistence activities published by the Division of Subsistence in 1983. Data collection took place from August 1987 through October 1988 using a household survey administered to a stratified sample of 72 (34 percent) of Fort Yukon households. Serving as the hub for administration and services in the Yukon Flats region and surrounding communities, 65 percent of the wage employment opportunities in Fort Yukon were in the federal, state, and local government sector. Jobs were predominantly seasonal or part-time in nature with only about 25 percent of jobs offering permanent, full-time employment. The median annual household income from all sources in 1987 was estimated to be $17,856. Among all households, the Alaska permanent fund dividend was the largest single source of non-wage income, averaging almost $1,529 per household. Among the estimated 43 trapping households, trapping was the largest source of nonwage income, contributing an average of $5,149 per trapping household. Fort Yukon residents displayed a high degree of involvement in the harvest, use, and sharing of fish and wildlife resources. All households used some wild resources during the survey year and an estimated 91.5 percent of all households made direct attempts at harvesting resources. Examining the use of major resource groups, mammals (excluding furbearers) were used by 100 percent of the households, salmon by 97.2 percent, birds by 90.4 percent, non-salmon fish by 89.2 percent, furbearers by 43.8 percent, and plants by 37.6 percent. The estimated total edible weight of resources harvested by Fort Yukon residents during the survey year was 625,725 pounds. This provided an average household harvest of 2,951 pounds and an average per capita harvest of 1,071 pounds. Almost two thirds (61 percent) of the total harvest consisted of salmon. Differences between the percentage of households harvesting and using specific resources point to patterns of resource sharing that remain an important aspect of contemporary subsistence production. Land use data showed that Fort Yukon residents utilized a large geographic area of the surrounding Yukon Flats to support subsistence activities. While some activities such as salmon fishing were found to be concentrated within 10 or 20 miles of Fort Yukon, the overall community use area encompassed a 150 mile stretch of the Yukon River and tributary streams between the communities of Beaver and Circle, as well as the Alaska portion of the Porcupine River drainage and its tributaries. Dogs continued to play an important role in supporting subsistence activities during the study year, especially as a mode of winter transportation for trappers. This study also found that more than one third of the total harvest, composed largely of chum salmon, was utilized as dog food. The per capita harvest of wild resources in Fort Yukon was estimated to be 679.0 pounds when resources fed to dogs were excluded. These data indicate that subsistence harvest and use of local resources for human consumption and as food for dogs was an integral part of the mixed economy of Fort Yukon during the study period.