Brian E. Potter

Observing The Dynamics Of Wildland Grass Fires: FireFlux -A Field Validation Experiment

The first comprehensive set of in situ measurements of turbulence and dynamics in an experimental wildland grass fire should help improve fire models.

Evaluation of Real-Time High-Resolution MM5 Predictions over the Great Lakes Region

Real-time high-resolution mesoscale predictions using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) over the Great Lakes region are evaluated for the 2002/03 winter and 2003 summer seasons using surface and upper-air observations, with a focus on near-surface and boundary layer properties that are important for applications such as air quality and fire weather predictions. The summer season predictions produce a cold bias in maximum daily temperature and a warm bias in minimum temperature that together lead to a good prediction of daily mean temperature but a smallerthan-observed diurnal temperature cycle. In winter, the predicted near-surface temperatures are lower both day and night, yielding good agreement with the observed amplitude of the diurnal temperature cycle but relatively large cold bias in daily mean temperature. The predicted temperatures in the boundary layer are also systematically lower than the observed temperatures in the two seasons. The cold bias is consistent with the wetter-than-observed lower atmosphere in the model prediction, which in turn can be attributed to an inadequate specification of soil moisture. In both seasons, the model produced substantially more precipitation in all categories, especially in the heavy precipitation category, and the overprediction is primarily associated with more widespread area coverage in the model prediction. The chances of producing a false precipitation forecast are substantially higher than missing an observed precipitation event. Small systematic errors are found in the predictions of low-level winds, but above the boundary layer, the predicted winds are predominantly from the west, while the observed winds are from the west-northwest. The model is able to capture the general development and evolution of the lake–land breezes in areas surrounding Lake Michigan during summer, although errors exist in the strengths of the breezes and the timing of their transition. Predicted early morning inversions are slightly stronger than observed in winter and weaker than observed in summer. The weak summer morning inversion results in a rapid inversion breakup followed by an earlier growth of a mixed layer after sunrise. Despite the head start, the predicted mixed-layer heights in late afternoon are lower than those observed, suggesting that either the predicted surface sensible heat flux may be too low or the boundary layer flux divergence may be too high. Decreasing horizontal grid spacing from 12 to 4 km results in little improvement in the predictions of near-surface and boundary layer properties except for precipitation, for which the model bias is significantly reduced by the increase in horizontal resolution. The cold and wet biases and errors in inversion strengths and mixed-layer development call for extra caution when using products from mesoscale forecasts in applications such as air pollution and fire weather prediction.

Synthesis of Knowledge of Extreme Fire Behavior: Volume I for Fire Managers

The National Wildfire Coordinating Group definition of extreme fire behavior (EFB) indicates a level of fire behavior characteristics that ordinarily precludes methods of direct control action. One or more of the following is usually involved: high rate of spread, prolific crowning/spotting, presence of fire whirls, and strong convection column. Predictability is difficult because such fires often exercise some degree of influence on their environment and behave erratically, sometimes dangerously. Alternate terms include “blow up” and “fire storm.” Fire managers examining fires over the last 100 years have come to understand many of the factors necessary for EFB development. This work produced guidelines included in current firefighter training, which presents the current methods of predicting EFB by using the crown fire model, which is based on the environmental influences of weather, fuels, and topography. Current training does not include the full extent of scientific understanding. Material in current training programs is also not the most recent scientific knowledge. National Fire Plan funds have sponsored newer research related to wind profiles’ influence on fire behavior, plume growth, crown fires, fire dynamics in live fuels, and conditions associated with vortex development. Of significant concern is that characteristic features of EFB depend on conditions undetectable on the ground, relying fundamentally on invisible properties such as wind shear or atmospheric stability.Obviously no one completely understands all the factors contributing to EFB because of gaps in our knowledge. These gaps, as well as the limitations as to when various models or indices apply should be noted to avoid application where they are not appropriate or warranted. This synthesis will serve as a summary of existing extreme fire behavior knowledge for use by fire managers, firefighters, and fire researchers.The objective of this project is to synthesize existing EFB knowledge in a way that connects the weather, fuel, and topographic factors that contribute to development of EFB. This synthesis will focus on the state of the science, but will also consider how that science is currently presented to the fire management community, including incident commanders, fire behavior analysts, incident meteorologists, National Weather Service office forecasters, and firefighters. It will seek to clearly delineate the known, the unknown, and areas of research with the greatest potential impact on firefighter protection.

Computing the Low-Elevation Variant of the Haines Index for Fire Weather Forecasts

The Haines index is used in wildfire forecasting and monitoring to evaluate the potential contributions of atmospheric stability and humidity to the behavior of plume-dominated wildfires. The index has three variants (“low,” “mid,” and “high”) that accommodate differences in surface elevation. As originally formulated, the low variant is calculated from temperature observations at the 950- and 850-hPa levels and humidity observations at 850 hPa. In the early 1990s the National Weather Service implemented a new mandatory level for radiosonde observations at 925 hPa. Following this change, measurements at 950 hPa became less frequent. An informal survey of several forecast offices found no formalized adjustment to the calculation of the low Haines index to take into account the nonavailability of 950-hPa measurements. Some sources continue to use 950-hPa temperature, usually interpolated from 925-hPa and surface temperatures, to calculate the low Haines index. Others directly substitute the 925-hPa temperature for the originally specified 950-hPa value. This study employs soundings from the central United States when both 950- and 925-hPa levels were available to investigate the impact of different calculation approaches on the resulting values of the low variant of the Haines index. Results show that direct substitution of 925-hPa temperature for the 950-hPa temperature can dramatically underestimate the potential wildfire severity compared with the original formulation of the Haines index. On the other hand, a low-elevation variant of the Haines index calculated from the interpolated 950-hPa temperature is usually in close agreement with the original formulation of the index.

Regional Climate Change in the Southern United States: The Implications for Wildfire Occurrence

Fires have always been an important factor in determining the composition of forests worldwide, but particularly in the southern United States. Wildfires were a common occurrence in American forests in the early twentieth century. Before 1930, wildfires typically accounted for the burning of eight to twenty million hectares (ha) in the United States each year. By the early 1940s, wildfires were still responsible for the annual burning of over eight million ha. Over 90% of the area burned during this time was on privately owned lands, primarily in the southern United States (Fedkiw, 1989). Between 1950 and 1980, the hectares burned by wildfires steadily decreased as the area receiving organized protection increased and the intensity of the protection efforts increased (Peterson, 1982). In recent years, the total area burned by wildfires in the United States has diminished to about one to two million ha per year (USDA Forest Service, 1992). Although the decrease in the number of hectares burned by wildfires across the United States has been significant over the last seventy years, the relative importance of wildfires in the southern United States in relation to other regions of the United States is significant. More hectares are burned by wildfires in the southern United States than in any other region of the country.

Application of a Mini Unmanned Aircraft System for In Situ Monitoring of Fire Plume Thermodynamic Properties

AbstractDirect measurements of wildland fire plume properties are rare because of difficult access to regions near the fire front and plume. Moisture released from combustion, in addition to added heat, can enhance buoyancy and convection, influencing fire behavior. In this study, a mini unmanned aircraft system (miniUAS) was used to obtain in situ measurements of temperature and relative humidity during a prescribed fire. The miniUAS was successfully maneuvered through the plume and its associated turbulence and provided observations of temperature and humidity profiles from near the centerline of the plume. Within the plume, the water vapor mixing ratio increased by 0.5–3.5 g kg−1 above ambient and was caused by the combustion of fuels. Potential temperature perturbations were on the order of 2–5 K. These results indicate that significant moisture and temperature enhancement can occur and may potentially modify convection dynamics of fire plumes.

Biomass, thermal inertia, and radiative freeze occurrence in leafless forests

Using field measurements of air temperature, wind, and relative humidity from a clear-cut site and two wooded sites in northern Wisconsin, we used a radiative transfer model to simulate temperatures on seven calm, clear nights similar to those on which freezes typically occur. Each night was simulated twice for the wooded sites. One simulation ignored the presence of vegetation, the other approximated the vegetations heat storage capacity and its influence on air temperatures. The simulations including biomass heat storage showed smaller mean absolute temperature errors and decreased magnitude of systematic model error when compared with the simulations ignoring vegetation. The results suggest that the thermal inertia of forest biomass may play a significant role in controlling forest temperatures on calm, clear nights and, hence, in controlling freeze occurrence.

Climate and Atmospheric Deposition Patterns and Trends

One of the most important factors impacting terrestrial and aquatic ecosystems is the atmospheric environment. Climatic and weather events play a significant role in governing the natural processes that occur in these ecosystems. The current characteristics of the vast number of ecosystems that cover the northeast and north central United States are, in part, the result of climate, weather, disturbance, and atmospheric pollution patterns that exist in the northeast and north central United States. For example, basic ecosystem processes (e.g., heat and moisture exchanges with the atmosphere, photosynthesis, and respiration) along with species diversity and ecosystem health throughout the region all depend, to some degree, on these patterns. Furthermore, future characteristics of ecosystems in the region will depend on future climate, weather, disturbance, and pollution patterns that may develop in response to natural or human-caused changes in our atmospheric environment.