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Turbulent fluxes of heat and momentum in the vicinity of wildland fires contribute to the redistribution of heat and momentum in the fire environment, which in turn can affect the heating of fuels, fire behavior, and smoke dispersion. As an extension of previous observational studies of turbulence regimes in the vicinity of wildland fires in forested environments, this study examines the effects of spreading surface fires and forest overstory vegetation on turbulent heat and momentum fluxes from near the surface to near the top of the overstory vegetation. Profiles of high-frequency (10 Hz) wind velocity and temperature measurements during two prescribed fire experiments are used to assess the relative contributions of horizontal and vertical turbulent fluxes of heat and momentum to the total heat and momentum flux fields. The frequency-dependent temporal variability of the turbulent heat and momentum fluxes before, during, and after fire-front passage is also examined using cospectral analyses. The study results highlight the effects that surface wildland fires and forest overstory vegetation collectively can have on the temporal and vertical variability of turbulent heat and momentum fluxes in the vicinity of the fires and the substantial departures of heat and momentum cospectra from typical atmospheric surface-layer cospectra that can occur before, during, and after fire-front passage. [ABSTRACT FROM AUTHOR]

In this study, the Flexible Particle (FLEXPART)-WRF, a Lagrangian particle dispersion model, is employed to simulate pollutant dispersion in and near the Lehigh Gap, a gap in a prominent ridgeline in eastern Pennsylvania. FLEXPART-WRF is used to evaluate the diagnostic value of the ventilation index (VI), an index that describes the potential for smoke or other pollutants to ventilate away from a source, for indicating dispersion potential in complex terrain. Little is known about the effectiveness of the ventilation index in diagnosing dispersion potential in complex terrain. The modeling approach used in this study is to release a dense cloud of particles across a portion of the model domain and evaluate particle behavior and VI diagnostic value in areas of the domain with differing terrain characteristics. Although both horizontal and vertical dispersion are examined, the study focuses primarily on horizontal dispersion, assessed quantitatively by calculating horizontal residence time (HRT) within a 1-km-radius circle surrounding the particle release point. Analysis of HRT across the domain reveals horizontal dispersion patterns that are influenced by the ridgeline and the Lehigh Gap. Comparison of VI and HRT in different areas of the domain reveals a robust relationship windward of the ridgeline and a weak relationship leeward of the ridgeline and in the vicinity of the Lehigh Gap. The results of this study suggest that VI users should consider whether they are windward or leeward of topographic features, and highlight the need for an alternative metric that better takes into account the influence of the terrain on dispersion. [ABSTRACT FROM AUTHOR]

Ambient and fire-induced atmospheric turbulence in the vicinity of wildland fires can affect the behavior of those fires and the dispersion of smoke. The presence of forest overstory vegetation can further complicate the evolution of local turbulence regimes and their interaction with spreading fires and smoke plumes. Previous observational studies of wildland fire events in forested environments have shown that turbulence energy and anisotropy in the vicinity of spreading line fires exhibit temporal and spatial variability influenced by the presence of overstory vegetation. This study builds on those previous observational studies to further examine turbulence regimes during two wildland fires in forested environments, with an emphasis on the effects of forest canopies on turbulence energy budgets, the skewness in turbulent velocity distributions, and stability- anisotropy variations before, during, and after fire-front-passage periods. Analyses indicate that turbulence anisotropy tends to persist throughout the vertical extent of overstory vegetation layers even during highly buoyant fire-front-passage periods, with horizontal velocity perturbations dominating over vertical velocity perturbations. The analyses also suggest that the periods before and after fire-front passage in forested environments can be very different with respect to how diffusion and shear production concurrently affect the evolution of turbulence energy within the canopy layer. In addition, horizontal and vertical velocity distribution analyses carried out in this study suggest that spreading line fires can have a substantial effect on the skewness of daytime velocity distributions typically observed inside forest vegetation layers. [ABSTRACT FROM AUTHOR]

The North American Regional Reanalysis (NARR) was used to develop an expanded, long-term (1979-2009) climatology of meridional (southerly and northerly) low-level jets over North America and surrounding coastal environs. NARR has greater spatial coverage and finer temporal (3 hourly) and horizontal (32 km) resolutions than do routine rawinsonde wind measurements. The NARR climatology focuses on jet frequency and average speed and elevation by month and 3-hourly time step. To evaluate the plausibility of the climatology, jet characteristics were compared with those obtained from prior climatological analyses, case studies, field campaigns, and numerical simulations. Strong agreement was found with many of the previously documented characteristics of well-known jets, including the northerly Pacific coast jet and southerly Great Plains jet. The NARR climatology provides additional insights into the spatial extent and seasonal shifts of large jet frequencies and into diurnal fluctuations in frequency, speed, and elevation. Weaker and/or less spatially extensive jets are also well depicted in the NARR climatology, including the southerly Gulf of California jet, summertime southerly jets and autumn northerly jets off the mid-Atlantic coast, and northerly jets in the high plains. Furthermore, several new areas of relatively frequent jet occurrence, most of which align with shallow thermal gradients, are seen in the NARR climatology. The NARR climatology supplements and enhances our understanding of North American low-level jets and points to the need for additional research on both the climatological characteristics of these jets and on the processes contributing to their formation. [ABSTRACT FROM AUTHOR]

This study examines the sensitivity of mean and turbulent flow in the planetary boundary layer and roughness sublayer to a low-intensity fire and evaluates whether the sensitivity is dependent on canopy and background atmospheric properties. The ARPS-CANOPY model, a modified version of the Advanced Regional Prediction System (ARPS) model with a canopy parameterization, is utilized for this purpose. A series of numerical experiments are conducted to evaluate whether the ability of the fire to alter downstream wind, temperature, turbulent kinetic energy (TKE), and vertical heat flux differs between forested and open areas, sparse and dense forests, weak and strong background flow, and neutral and convective background stability. Analysis of all experiments shows that, in general, mean and turbulent flow both prior to and during a low-intensity fire is damped in the presence of a canopy. Greater sensitivity to the fire is found in cases with strong ambient wind speed than in cases with quiescent or weak wind speed. Furthermore, sensitivity of downstream atmospheric conditions to the fire is shown to be strongest with a neutrally stratified background. An analysis of the TKE budget reveals that both buoyancy and wind shear contribute to TKE production during the period of time in which the fire conditions are applied to the model. On the basis of the results of the ARPS simulations, caution is advised when applying ARPS-simulation results to predictions of smoke transport and dispersion: smoke-model users should consider whether canopy impacts on the atmosphere are accounted for and whether neutral stratification is assumed. [ABSTRACT FROM AUTHOR]

Smoke prediction products are one of the tools used by land management personnel for decision making regarding prescribed fires. This study documents the application to a prescribed fire of a smoke prediction system that employs ARPS-CANOPY, a modified version of the Advanced Regional Prediction System (ARPS) model containing a canopy submodel, as the meteorological driver. In this paper, the performance of ARPS-CANOPY in simulating meteorological fields in the vicinity of a low-intensity fire is assessed using flux-tower data collected prior to and during a low-intensity prescribed fire in the New Jersey Pine Barrens in March 2011. A three-dimensional high-resolution plant area density dataset is utilized to define the characteristics of the canopy, and the fire is represented in ARPS-CANOPY as a heat flux to the atmosphere. The standard ARPS model is compared with reanalysis and upper-air data to establish that the model can simulate the observed synoptic-mesoscale and planetary boundary layer features that are salient to this study. ARPS-CANOPY profiles of mean turbulent kinetic energy, wind speed/direction, and temperature exhibit patterns that appear in the flux-tower observations during both the preburn phase of the experiment and the period of time the flux tower experienced perturbed atmospheric conditions due to the impinging fire. Last, the character and source of turbulence in and around the fire line are examined. These results are encouraging for smoke prediction efforts since transport of smoke from low-intensity fires is highly sensitive to the near-surface meteorological conditions and, in particular, turbulent flows. [ABSTRACT FROM AUTHOR]

Wildfires that occurred over the western United States during August 2012 were fewer in number but larger in size when compared with all other Augusts in the twenty-first century. This unique characteristic, along with the tremendous property damage and potential loss of life that occur with large wildfires with erratic behavior, raised the question of whether future climate will favor rapid wildfire growth so that similar wildfire activity may become more frequent as climate changes. This study addresses this question by examining differences in the climatological distribution of the Haines index (HI) between the current and projected future climate over the western United States. The HI, ranging from 2 to 6, was designed to characterize dry, unstable air in the lower atmosphere that may contribute to erratic or extreme fire behavior. A shift in HI distribution from low values (2 and 3) to higher values (5 and 6) would indicate an increased risk for rapid wildfire growth and spread. Distributions of Haines index are calculated from simulations of current (1971-2000) and future (2041-70) climate using multiple regional climate models in the North American Regional Climate Change Assessment Program. Despite some differences among the projections, the simulations indicate that there may be not only more days but also more consecutive days with HI ≥ 5 during August in the future. This result suggests that future atmospheric environments will be more conducive to erratic wildfires in the mountainous regions of the western United States. [ABSTRACT FROM AUTHOR]

The Haines index (HI) is a fire-weather index that is widely used as an indicator of the potential for dry, low-static-stability air in the lower atmosphere to contribute to erratic fire behavior or large fire growth. This study examines the interannual variability of HI over North America and its relationship to indicators of large-scale circulation anomalies. The results show that the first three HI empirical orthogonal function modes are related respectively to El Niño-Southern Oscillation (ENSO), the Arctic Oscillation (AO), and the interdecadal sea surface temperature variation over the tropical Pacific Ocean. During the negative ENSO phase, an anomalous ridge (trough) is evident over the western (eastern) United States, with warm/dry weather and more days with high HI values in the western and southeastern United States. During the negative phase of the AO, an anomalous trough is found over the western United States, with wet/cool weather and fewer days with high HI, while an anomalous ridge occurs over the southern United States-northern Mexico, with an increase in the number of days with high HI. After the early 1990s, the subtropical high over the eastern Pacific Ocean and the Bermuda high were strengthened by a wave train that was excited over the tropical western Pacific Ocean and resulted in warm/dry conditions over the southwestern United States and western Mexico and wet weather in the southeastern United States. The above conditions are reversed during the positive phase of ENSO and AO and before the early 1990s. [ABSTRACT FROM AUTHOR]

Recent research suggests that high levels of ambient near-surface atmospheric turbulence are often associated with rapid and sometimes erratic wildland fire spread that may eventually lead to large burn areas. Previous research has also examined the feasibility of using near-surface atmospheric turbulent kinetic energy (TKE s) alone or in combination with the Haines index (HI) as an additional indicator of anomalous atmospheric conditions conducive to erratic or extreme fire behavior. However, the application of TKE s-based indices for operational fire-weather predictions in the United States on a regional or national basis first requires a climatic assessment of the spatial and temporal patterns of the indices that can then be used for testing their operational effectiveness. This study provides an initial examination of some of the spatial and temporal variability patterns across the United States of TKE s and the product of HI and TKE s (HITKE s) using data from the North American Regional Reanalysis dataset covering the 1979-2008 period. The analyses suggest that there are regional differences in the behavior of these indices and that regionally dependent threshold values for TKE s and HITKE s may be needed for their potential use as operational indicators of anomalous atmospheric turbulence conditions conducive to erratic fire behavior. The analyses also indicate that broad areas within the northeastern, southeastern, and southwestern regions of the United States have experienced statistically significant positive trends in TKE s and HITKE s values over the 1979-2008 period, with the most substantial increases in values occurring over the 1994-2008 period. [ABSTRACT FROM AUTHOR]