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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]

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]

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]

Presents information on the valley atmospheric transport diffusion and deposition model (VALDRIFT). Description of the model; Functions and operations of the model; Indepth look at the scientific foundations of VALDRIFT.

Develops a climatology of the Great Plains low-level jet (LLJ) from two years of research rawinsonde data, which was obtained up to eight times daily at a site in north-central Oklahoma. Methodology utilized in the study; Results and conclusion of the study.

Irregular oscillations of Equatorial Pacific sea surface temperatures known as El Niño-Southern Oscillation (ENSO) has been shown to affect air pollution in China by altering atmospheric circulations. In this study, we use hourly PM2.5 data from nearly 1500 monitoring stations across entire China to further explore the connection between ENSO and China's haze pollution by comparing PM2.5 concentrations for the two most recent ENSO events, the 2015–2016 El Niño and the 2017–2018 La Niña. The PM2.5 concentrations show significant differences between the two events and the differences are larger in winter and smaller in summer. In summer, the PM2.5 concentrations are higher across China during El Niño. However in winter, higher concentrations are found at most stations in northern China during El Niño, but at majority of stations in southern China during La Niña. Spring and autumn show a transitional pattern between summer and winter. Dividing the country into nine regions with similar geographical conditions, the relationship between the monthly PM2.5 concentration anomalies and the Niño3.4 indices, as revealed by linear regression, is generally positive at stations in North, Northeast and the northern parts of Central and East China, but generally negative in South, Southwest and the southern parts of Central and East China. No difference is found in the oscillation periods of the PM2.5 concentrations between the two events, except that the seasonal and intra-seasonal oscillatory signals appear to be slightly stronger during El Niño than La Niña. The regional and seasonal differences in PM2.5 concentrations between El Niño and La Niña can be partially explained by the differences in the anomalous atmospheric circulation patterns, particularly the low-level wind and surface precipitation patterns. • The influence of El Niño and La Niña on PM2.5 concentrations across China is explored. • The influence of El Niño-La Niña on PM2.5 in China differ by region and season. • The relationship between PM2.5 and Niño indices is opposite in north and south China. • PM2.5 seasonal and intra-seasonal oscillations are stronger with El Niño than La Niña. • The ENSO-PM2.5 connection is consistent with anomalous circulation and precipitation. [ABSTRACT FROM AUTHOR]

This study investigates the interannual variability of wind energy resources across China and how it changes with season by applying empirical orthogonal function (EOF) analyses to gridded wind data from the Climate Forecast System Reanalysis (CFSR) from January 1979 through December 2011. The first EOF mode (EOF1) represents between 22% variance for winter and 29% for summer. Spatially, the variation is largely consistent across China for summer and autumn and almost opposite between north and south for spring and winter, and the strongest variation in all seasons is found over Inner Mongolia and Tibet. The second EOF mode (EOF2) represents between 13% variance for autumn and 16% for spring, and is largely dominated by a sharp contrast between Inner Mongolia and Tibet for all seasons. The EOF1 appears to be linked statistically to the pacific decadal oscillation for summer and autumn and to the Pacific North American pattern for spring and winter, while the EOF2 seems to be connected to the Arctic Oscillation for spring and winter and to an interdecadal variability for summer and autumn. The anomalous wind fields associated with these large‐scale circulation patterns modify the climatological wind fields in different ways that lead to an increase or a decrease of the 80‐m winds in different regions of China. Understanding the low‐frequency variability of wind speeds, or how and why wind speed and wind power vary from 1 year to another, can be beneficial for seasonal outlook or long‐range forecasting of wind power generation. Using gridded wind data from a third‐generation global reanalysis, we statically identify and characterize the main modes of the interannual variability of wind speeds at the modern wind turbine level across entire China and the dominant climate forcing for these modes.Figure caption: The correlations between the 80‐m wind speed summer season anomalies across China and the Pacific Decadal Oscillation indices for 1979–2011. [ABSTRACT FROM AUTHOR]

ABSTRACT Valleys can trap cold air to form Valley Cold Pools ( VCPs). Characterized by stable stratification and weak winds, VCPs, especially those that last over multiple days, can produce adverse effects such as poor visibility and severe air pollution. Using the gridded data set of the North American Regional Reanalysis ( NARR) for the period of 1979 through 2012, this study examines the climatology of VCPs in the western United States with a focus on spatial and temporal variability. The results reveal a widespread occurrence of short-lived (1-2 days) VCPs over both the mountainous areas of the West and the Northern Plains. Longer VCP episodes, however, tend to be limited to large basins/valleys in the Northwest and the Intermountain West. The leading mode of variability in the annual number of cold-season multi-day VCP event anomalies appears to be linked to a sea-surface temperature anomaly pattern typically found during the warm phase of Pacific Decadal Oscillation. Corresponding to the positive phase of the leading mode of variability is an anomalous 500- hPa ridge over the western United States induced by a Rossby wave train, which, by blocking cold-air intrusions and producing mid-level subsidence warming, leads to more persistent VCP episodes in the western United States. In addition to large-scale wintertime circulation anomalies, local surface temperature anomalies also contribute to the variability. [ABSTRACT FROM AUTHOR]

ABSTRACT The mean climatology, seasonal and interannual variability and trend of wind speeds at the hub height (80 m) of modern wind turbines over China and its surrounding regions are revisited using 33-year (1979-2011) wind data from the Climate Forecast System Reanalysis ( CFSR) that has many improvements including higher spatial resolution over previous global reanalysis products. Mean 80-m wind speeds are consistently higher over China Seas and the ocean areas than over land, and inside China high winds are found in areas of Inner Mongolia and the Tibetan Plateau. There is a considerable seasonal variability that reflects primarily the influence of East and Southeast Asia Monsoon with generally higher speeds in winter followed by summer, and weaker winds in autumn, followed by spring. There is also a strong interannual variability, and regions of larger amplitude of variability coincide with regions of higher mean winds. A decreasing trend, dominated by a sharp decline beginning in 2005, is seen across China and the surrounding seas in summer and autumn, and the summer trends over land and over ocean appear to be related respectively to the Pacific Decadal Oscillation ( PDO) and the East Asian summer monsoon. Trends are not consistent across the region in spring and winter, however, with positive trends over some areas in northeastern and northwestern China, Mongolia and tropical oceans whereas negative trends in other regions. Nearly all areas of China experience mean annual 80-m wind speed less than 6.9 m s−1 (wind power classes of 1-2) except for some areas of Inner Mongolia where mean annual 80-m wind speeds exceed 6.9 m s−1 (Classes 3 or higher, suitable for wind energy development). China Seas and ocean areas generally fall in Class 3 or above, with the Taiwan and Luzon Straights reaching the highest Class 7. [ABSTRACT FROM AUTHOR]

In this study, a North American Regional Reanalysis (NARR)‐based climate dataset of wind and Pasquill stability class is developed for the North American Great Lakes region. Motivated by the need to update the limited wind and Pasquill stability class climatology previously incorporated into a livestock odour nuisance‐mitigation application used in Michigan, this study is motivated more generally by the lack of a long‐term climate dataset suitable for studies of atmospheric dispersion in the Great Lakes region. As a preliminary step, NARR 10‐m wind speed, 10‐m wind direction, and 2‐m temperature are evaluated at eight weather stations in Michigan and northern Indiana, and the wind speed variable is subsequently bias‐corrected. To derive a Pasquill stability class dataset from the NARR variables, an existing empirical relationship between Pasquill stability class and Richardson number is applied to a previously developed NARR‐derived Richardson number dataset covering the period 1979–2008. Pasquill stability class frequency in NARR is subsequently evaluated using observations at two flux towers in Illinois. Although a tendency is noted for the NARR‐derived Pasquill stability class dataset to underestimate the frequency of stable and unstable conditions, and overestimate the frequency of neutral conditions, the analysis shows that the 10‐m wind speed bias‐correction procedure yields a frequency distribution that is in better agreement with the observations than the non‐bias‐corrected dataset. Finally, an analysis of the spatial and temporal variability of livestock odour dispersion potential in Michigan is provided as an example application of the NARR‐based wind and Pasquill stability class climate dataset. [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 spatial and temporal variability of wind speed at 80 m above ground (the average hub height of most modern wind turbines) in the contiguous United States using Climate Forecast System Reanalysis (CFSR) data from 1979 to 2011. The mean 80-m wind exhibits strong seasonality and large spatial variability, with higher (lower) wind speeds in the winter (summer), and higher (lower) speeds over much of the Midwest and U.S. Northeast (U.S. West and Southeast). Trends are also variable spatially, with more upward trends in areas of the Great Plains and Intermountain West of the United States and more downward trends elsewhere. The leading EOF mode, which accounts for 20% (summer) to 33% (winter) of the total variance and represents in-phase variations across the United States, responds mainly to the North Atlantic Oscillation (NAO) in summer and El Niño-Southern Oscillation (ENSO) in the other seasons. The dominant variation pattern can be explained by a southerly/southwesterly (westerly) anomaly over the U.S. East (U.S. West) as a result of the anomalous mean sea level pressure (MSLP) pattern. The second EOF mode, which explains about 15% of the total variance and shows a seesaw pattern, is mainly related to the springtime Arctic Oscillation (AO), the summertime recurrent circumglobal teleconnection (CGT), the autumn Pacific decadal oscillation (PDO), and the winter El Niño Modoki. The anomalous jet stream and MSLP patterns associated with these indices are responsible for the wind variation. [ABSTRACT FROM AUTHOR]

ABSTRACT The frequency and timing of frost events and the length of the growing season are critical limiting factors in many human and natural ecosystems. This study investigates the temporal and spatial variability of the date of last spring frost ( LSF), the date of first fall frost ( FFF), and the length of the frost-free season ( FFS) in the Great Lakes region of the United States using two gridded reanalysis datasets for the period of 1980-2010. Across most of the Great Lakes region, there has been a negative trend for the LSF and a positive trend for the FFF, leading to a lengthening of FFS. The three variables vary spatially across the region and exhibit large interannual variability. Empirical orthogonal function ( EOF) analyses indicate that the first mode for the three variables, which accounts for about 30% of the total variance, is in phase across most of the Great Lakes region and that it appears to be related to Pacific North American ( PNA) pattern in the case of LSF and to Pacific Decadal Oscillation ( PDO) in the case of FFF and FFS. The main reason for these connections is that the 200- hPa geopotential height anomaly over North America induced by a Rossby wave train influences the strength of the trough over North America, which in turn affects surface temperatures over the Great Lakes region. The second mode, explaining about 10% of the total variance, mainly shows the out-of-phase variability between the Great Lakes and the surrounding land and it appears to be related to the circumglobal teleconnection ( CGT) in the case of LSF and again to PDO in the case of FFF and FFS. [ABSTRACT FROM AUTHOR]

Mesoscale models are a class of atmospheric numerical model designed to simulate atmospheric phenomena with horizontal scales of about 2–200 km, although they are also applied to microscale phenomena with horizontal scales of less than about 2 km. Mesoscale models are capable of simulating wildland fire impacts on atmospheric flows if combustion byproducts (e.g., heat, smoke) are properly represented in the model. One of the primary challenges encountered in applying a mesoscale model to studies of fire-perturbed flows is the representation of the fire sensible heat source in the model. Two primary methods have been implemented previously: turbulent sensible heat flux, either in the form of an exponentially-decaying vertical heat flux profile or surface heat flux; and soil temperature perturbation. In this study, the ARPS-CANOPY model, a version of the Advanced Regional Prediction System (ARPS) model with a canopy submodel, is utilized to simulate the turbulent atmosphere during a low-intensity operational prescribed fire in the New Jersey Pine Barrens. The study takes place in two phases: model assessment and model sensitivity. In the model assessment phase, analysis is limited to a single control simulation in which the fire sensible heat source is represented as an exponentially decaying vertical profile of turbulent sensible heat flux. In the model sensitivity phase, a series of simulations are conducted to explore the sensitivity of model–observation agreement to (i) the method used to represent the fire sensible heat source in the model and (ii) parameters controlling the magnitude and vertical distribution of the sensible heat source. In both phases, momentum and scalar fields are compared between the model simulations and data obtained from six flux towers located within and adjacent to the burn unit. The multi-dimensional model assessment confirms that the model reproduces the background and fire-perturbed atmosphere as depicted by the tower observations, although the model underestimates the turbulent kinetic energy at the top of the canopy at several towers. The model sensitivity tests reveal that the best agreement with observations occurs when the fire sensible heat source is represented as a turbulent sensible heat flux profile, with surface heat flux magnitude corresponding to the peak 1 min mean observed heat flux averaged across the flux towers, and an e -folding extinction depth corresponding to the average canopy height in the burn unit. The study findings provide useful guidance for improving the representation of the sensible heat released from low-intensity prescribed fires in mesoscale models. [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]

• The study offers insights into atmospheric turbulence during intense grass fires. • These findings are significant as they provide the first-ever in-situ turbulence data collected during Red Flag Warnings. • The data establishes an upper limit for turbulence in prescribed grass fires, offering valuable reference points. • These results have the potential to enhance turbulence schemes in fire behavior models, contributing to better fire predictions. • This study underscores the complexities of studying fire type effects in a real-world context. A wildland fire experiment was conducted on a 40-ha tall grass prairie in Texas to study fire-atmosphere interactions. The experiment coincided with a Red Flag Warning, signifying specific weather conditions associated with an elevated risk of wildfire ignition and rapid spread. This provided a rare opportunity to observe wildland fire behavior and its impact on atmospheric flows under conditions deliberately avoided in similar experiments. This study analyzed the high frequency meteorological data collected during the experiment using three-dimensional sonic anemometers mounted on a 43-m tower (at 5.8, 10 and 20 m) and on three 10-m towers (at 5.3 m) within the burn block. The tower placements were carefully chosen to capture the impact of both head and flank fires on the mean and turbulent flows. The hypothesis is that fire-induced perturbations in the lower atmosphere, quantified through parameters like turbulent kinetic energy (TKE) and vertical fluxes, would be substantially stronger than those observed in similar wildland fires under less fire-prone atmospheric conditions. While this hypothesis was validated for heat flux and TKE, it did not hold for momentum fluxes or friction velocity. Additionally, the observations failed to confirm the hypothesis that head fires would produce more intense perturbations than flank fires. Given the homogeneity of fuel, terrain and meteorological elements across the burn unit, this underscores the challenge of disentangling the effects of fire type from other natural factors in a real-world setting. Nevertheless, the finding holds promise for improving turbulence parameterizations in fire-behavior and smoke dispersion models. [ABSTRACT FROM AUTHOR]

The diurnal evolution of the three-dimensional summer-season mean wind and temperature structure in California's Sacramento and San Joaquin Valleys (collectively called the Central Valley) is investigated using data from 22 radar wind profiler/radio acoustic sounding systems (RASS) operated as part of the Central California Ozone Study in 2000. The profiler network revealed, for the first time, that the persistent summer-season flow pattern documented by surface observations extends 800–1000 m above the surface. At most locations, up-valley winds persist during both day and night except at the upper ends of the valleys and close to the valley sidewalls where diurnal wind reversals occur. Wind speeds exhibit pronounced diurnal oscillations, with amplitudes decreasing with height. A low-level wind maximum occurs in the lowest 300 m, with a sharp decrease in speed above the maximum. Especially well defined nocturnal low-level jets occur at sites in the southern San Joaquin Valley, where maximum speeds of 10 m s-1 or more occur 1–2 h before midnight at heights near 300 m. The afternoon mixed layer, generally deeper than 1000 m, increases in depth with up-valley distance in both valleys. At night, temperature inversions develop in the lowest several hundred meters with near-isothermal layers above. Mean temperatures in the lowest 500 m of the valleys are always warmer than at the same altitude over the coast, and temperature increases from the lower to upper valleys. The diurnal oscillations of the coast– valley and along-valley temperature and pressure difference reach a maximum in late afternoon and a minimum in early morning. These oscillations are in phase with the diurnal variation of westerly onshore flows. The along-valley wind maxima, however, occur 1–2 h before midnight, whereas the along-valley pressure gradient maxima are usually found just before sunset. [ABSTRACT FROM AUTHOR]

Mesoscale models are a class of atmospheric numerical model designed to simulate atmospheric phenomena with horizontal scales of about 2-200 km, although they are also applied to microscale phenomena, with horizontal scales less than about 2 km. Mesoscale models are capable of simulating wildland fire impacts on atmospheric flows if combustion by-products (e.g., heat, smoke) are properly represented in the model. One of the primary challenges encountered in applying a mesoscale model to studies of fire-perturbed flows is the representation of the fire sensible heat source in the model. Two primary methods have been implemented previously: turbulent sensible heat flux, either in the form of an exponentially-decaying vertical heat flux profile or surface heat flux; and soil temperature perturbation. In this study, the ARPS-CANOPY model, a version of the Advanced Regional Prediction System (ARPS) model with a canopy submodel, is utilized to simulate the turbulent atmosphere during a low-intensity operational prescribed fire in the New Jersey Pine Barrens. The study takes place in two phases: model assessment and model sensitivity. In the model assessment phase, analysis is limited to a single control simulation in which the fire sensible heat source is represented as an exponentially-decaying vertical profile of turbulent sensible heat flux. In the model sensitivity phase, a series of simulations are conducted to explore the sensitivity of model-observation agreement to (i) the method used to represent the fire sensible heat source in the model and (ii) parameters controlling the magnitude and vertical distribution of the sensible heat source. In both phases, momentum and scalar fields are compared between the model simulations and data obtained from six flux towers located within and adjacent to the burn unit. The multi-dimensional model assessment confirms that the model reproduces the background and fire-perturbed atmosphere as depicted by the tower observations, although the model underestimates the turbulent kinetic energy at the top of the canopy at several towers. The model sensitivity tests reveal that the best agreement with observations occurs when the fire sensible heat source is represented as a turbulent sensible heat flux profile, with surface heat flux magnitude corresponding to the peak 1-min mean observed heat flux averaged across the flux towers, and an e-folding extinction depth corresponding to the average canopy height in the burn unit. The study findings provide useful guidance for improving the representation of the sensible heat released from low-intensity prescribed fires in mesoscale models. [ABSTRACT FROM AUTHOR]

The regional climate of the western United States shows clear footprints of interaction between atmospheric circulation and orography. The unique features of this diverse climate regime challenges climate modeling. This paper provides detailed analyses of observations and regional climate simulations to improve our understanding and modeling of the climate of this region. The primary data used in this study are the 1/88 gridded temperature and precipitation based on station observations and the NCEP-NCAR global reanalyses. These data were used to evaluate a 20-yr regional climate simulation performed using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (Penn State-NCAR) Mesoscale Model (MM5) driven by large-scale conditions of the NCEP-NCAR reanalyses. Regional climate features examined include seasonal mean and extreme precipitation; distribution of precipitation rates; and precipitation intensity, frequency, and seasonality. The relationships between precipitation and surface temperature are also analyzed as a means to evaluate how well regional climate simulations can be used to simulate surface hydrology, and relationships between precipitation and elevation are analyzed as diagnostics of the impacts of surface topography and spatial resolution. The latter was performed at five east-west transects that cut across various topographic features in the western United States. These analyses suggest that the regional simulation realistically captures many regional climate features. The simulated seasonal mean and extreme precipitation are comparable to observations. The regional simulation produces precipitation over a wide range of precipitation rates comparable to observations. Obvious biases in the simulation include the oversimulation of precipitation in the basins and intermountain West during the cold season, and the undersimulation in the Southwest in the warm season. There is a tendency of reduced precipitation... [ABSTRACT FROM AUTHOR]

The hydroclimate of the western United States is influenced by strong interannual variability of atmospheric circulation, much of which is associated with the El Niño-Southern Oscillation (ENSO). Precipitation anomalies during ENSO often show opposite and spatially coherent dry and wet patterns in the Northwest and California or vice versa. The role of orography in establishing mesoscale ENSO anomalies in the western United States is examined based on observed precipitation and temperature data at 1/88 spatial resolution and a regional climate simulation at 40-km spatial resolution. Results show that during El Niño or La Niña winters, strong precipitation anomalies are found in northern California, along the southern California coast, and in the northwest mountains such as the Olympic Mountains, the Cascades, and the northern Rockies. These spatial features, which are strongly affected by topography, are surprisingly well reproduced by the regional climate simulation. A spatial feature investigated further is the positive-negative-positive precipitation anomaly found during El Niño years in the Olympic Mountains, and on the west side and east side of the Cascades in both observations and regional simulation. Observed streamflows of river basins located in those areas are found to be consistent with the precipitation anomalies. The spatial distribution of the precipitation anomalies is investigated by relating flow direction and moisture to the orientation of mountains and orographic precipitation. On the west side of the north-south-oriented Cascade Range, the increase in atmospheric moisture is not enough to compensate for the loss of orographic precipitation associated with a change in flow direction toward the southwest during El Niño years. In California, both the increase in atmospheric moisture and shift in wind direction toward the southwest enhance precipitation along the Sierra, which is oriented northwest to... [ABSTRACT FROM AUTHOR]

Atmospheric turbulent circulations in the vicinity of wildland fire fronts play an important role in the transfer of momentum into and out of combustion zones, which in turn can potentially affect the behavior and spread of wildland fires. The vertical turbulent transfer of momentum is accomplished via individual sweep, ejection, outward interaction, and inward interaction events, collectively known as sweep-ejection dynamics. This study examined the sweep-ejection dynamics that occurred before, during, and after the passage of a surface fire front during a prescribed fire experiment conducted in an open-canopied forest in the New Jersey Pine Barrens. High-frequency (10 Hz), tower-based, sonic anemometer measurements of horizontal and vertical wind velocity components in the vicinity of the fire front were used to assess the relative frequencies of occurrence of the different types of momentum-flux events, their contributions to the overall momentum fluxes, and their periodicity patterns. The observational results suggest that the presence of surface fire fronts in open-canopied forests can substantially change the sweep-ejection dynamics that typically occur when fires are not present. In particular, sweep events resulting in the downward transport of high horizontal momentum air from above were found to be more prominent during fire-front-passage periods. [ABSTRACT FROM AUTHOR]

The increase in wildfire risk in the United States in recent decades has been linked to rapid growth of the wildland‐urban interface and to changing climate. While there have been numerous studies on wildfires and climate change, few have separately assessed the impact of climate response to land‐use‐land‐cover change (LULCC) on wildfires. In this study, we analyse two 10‐year regional climate simulations driven by the current (2011) and future (2100) land‐use‐land‐cover patterns to assess modifications by the projected LULCC to the frequency and severity of fire‐prone atmospheric conditions described by two fire weather indices, the Canadian Forest Fire Weather Index and the Hot‐Dry‐Windy Index. The simulation corresponding to future land‐use‐land‐cover pattern yields higher surface temperature and vapour pressure deficit and lower precipitation compared to the simulation with the current pattern in areas where urbanized landscapes replace forests and grasslands, such as along the Piedmont and outside the Chicagoland region, while in areas where croplands replace forests, such as the southeast Coastal Plains, the results are reversed. These changes to local and regional atmospheric conditions lead to longer fire seasons and more extreme fire‐weather conditions in much of the eastern United States, specifically in the Southeast and Ohio River Valley where significant urban expansion is projected by the end of the century. Whereas in Southern California where some highly flammable shrublands will be replaced by urban or crop lands, fire‐prone atmospheric conditions are likely to be less frequent and less extreme in the future. However, much of California moves towards a year‐round fire season under the projected LULCC. The results suggest that by altering atmospheric conditions, LULCC may play an important role in determining fire regime, but the effects are highly heterogeneous and regionalized. [ABSTRACT FROM AUTHOR]

Wildfire regimes respond to atmospheric variability on multiple time scales from interannual variations of drought to daily fluctuations of humidity and wind. Synoptic weather patterns effectively link both short- and long-term atmospheric variability, and thus, understanding fire-prone synoptic patterns is an integral part of fire management programs. Different analysis procedures of varying complexity have been used to identify the most common synoptic weather patterns conducive to wildfires. In this study, we examine the utility of three synoptic-pattern classification methodologies, namely Composite, Empirical Orthogonal Function (EOF), and Self Organizing Map (SOM). We focused on the synoptic weather patterns associated with 203 wildfires that burned 50,000 acres (20,250 ha) or more and 80 wildfires that burned 100,000 acres (40,500 ha) or more in the Northwestern United States between 1984 and 2014. The basic 500-hPa geopotential height patterns identified by the three analysis methods were found to be similar for the two fire-size categories, but the strengths of the geopotential height anomalies were larger for the larger size fires. All three methods were able to consistently identify the dominant pattern with only small differences in strength and position of the anomalies, but the EOF and SOM methods yielded other less dominant, but still important, patterns. Particularly, the SOM method captured transitional synoptic patterns that may account for 20–30% of these large wildfires. [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]

We used 35-year (1979-2013) hourly rainfall data from the North American Land Data Assimilation System (NLDAS-2) to examine the relationships of the symmetric and asymmetric components of two types of El Niño-Southern Oscillation (ENSO) (El Niño and El Niño Modoki) episodes with occurrences of extreme precipitation events across the United States. During the cold season, the symmetric impacts of El Niño and El Niño Modoki are more significant than during the warm season. The asymmetric components associated with El Niño during the two seasons are comparable, while the asymmetric components associated with El Niño Modoki during the warm season are more significant than those during the cold season. The connections between the symmetric and asymmetric components of the two types of ENSO and extreme precipitation occurrences are related to atmospheric moisture convergence patterns that tend to occur in response to the anomalous large-scale atmospheric circulations during the ENSO episodes. [ABSTRACT FROM AUTHOR]

ABSTRACT The inter-annual variability of southerly low-level jets ( SLLJs) over North America during the warm (April-September) and cool (October-March) seasons is investigated. SLLJ occurrences over a 31-year period (1979-2009) were identified from the North American Regional Reanalysis ( NARR) vertical wind profiles. The first empirical orthogonal function ( EOF) modes of the SLLJ frequency during the warm and cool seasons account for about 30 and 20% of the total variance, respectively. Both modes can be interpreted as a strengthening or weakening of the core area of SLLJ anomalies. The principal component ( PC) time series display significant positive trends, suggesting an increase in SLLJ activity during both seasons on inter-decadal time scales and are significantly correlated to the summertime Pacific Decadal Oscillation ( PDO) and Atlantic Multidecadal Oscillation ( AMO) for the warm season and the wintertime PDO, AMO and El Niño Modoki for the cool season. The second modes account for about 20 and 15% of the total variance for the warm and cool seasons, respectively, and are interpreted as primarily a subseasonal latitudinal shift in SLLJ activity between the central Great Plains and the western Gulf of Mexico and southern Texas during the warm season and a longitudinal shift between the western Gulf of Mexico and the Caribbean during the cool season. The second mode appears to be significantly correlated to El Niño Modoki for the warm season and to Niño 3.4 for the cool season. [ABSTRACT FROM AUTHOR]

Low-intensity wildland fires occurring beneath forest canopies can result in particularly adverse local air-quality conditions. Ambient and fire-induced turbulent circulations play a substantial role in the transport and dispersion of smoke during these fire events. Recent in situ measurements of fire-atmosphere interactions during low-intensity wildland fires have provided new insight into the structure of fire-induced turbulence regimes and how forest overstory vegetation can affect the horizontal and vertical dispersion of smoke. In this paper, we provide a summary of the key turbulence observations made during two low-intensity wildland fire events that occurred in the New Jersey Pine Barrens. [ABSTRACT FROM AUTHOR]

Climate change is expected to alter the frequency and severity of atmospheric conditions conducive for wildfires. In this study, we assess potential changes in fire weather conditions for the contiguous United States using the Haines Index (HI), a fire weather index that has been employed operationally to detect atmospheric conditions favorable for large and erratic fire behavior. The index summarizes lower atmosphere stability and dryness into an integer value with higher values indicting more fire-prone conditions. We use simulations produced by the North American Regional Climate Change Assessment Program (NARCCAP) from multiple regional climate models (RCMs) driven by multiple general circulation models (GCMs) to examine changes by midcentury in the seasonal percentage of days and the consecutive number of days with high (values ≥ 5) HI across the United States. Despite differences among the six RCM–GCM combinations in the magnitude and location of the projected changes, the results consistently suggest an increase in the number of days with high HI values over most of the United States during the summer season, with the dryness factor of the HI contributing more than the stability parameter to the projected changes. In addition, the consecutive number of days with high HI is projected to increase in summer. Together, these results suggest that future summers might be more conducive to large and dangerous fires. The projections for other seasons are inconsistent among the model combinations. [ABSTRACT FROM PUBLISHER]

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 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. [ABSTRACT FROM AUTHOR]

This paper investigates the diurnal evolution of thermally driven plain-mountain winds, up- and down-valley winds, up- and downslope winds, and land-lake breezes for summer fair weather conditions in four regions of the Intermountain West where dense wind networks have been operated. Because of the diverse topography in these regions, the results are expected to be broadly representative of thermally driven wind climates in the Intermountain West. The regions include the Wasatch Front Valleys of northern Utah, the Snake River Plain of Idaho, the southern Nevada basin and range province, and central Arizona. The analysis examines wind characteristics, including the regularity of the winds and interactionsof the four types of thermally driven winds, using meteorological data from the University of Utah's MesoWest network. In general, on fair weather days, winds in all four regions exhibit a consistent direction from day to day at a given hour. A measure of this wind consistency is defined. The nighttime hours exhibit a generally higher consistency than thedaytime hours. Lower consistency during the day-night and night-day transition periods reflects day-to-day variations in the timing of wind system reversals. Thermally driven circulations are similar in the four regions, but the WasatchFront Valleys are influenced by lake breezes from the adjacent Great Salt Lake, the Snake River Plain is influenced by along-plain circulations and localized outflow from the Central Idaho Mountains, and winds in both southern Nevada and central Arizona are influenced by plain-mountain circulations associated with regionalscale contrasts in elevation andsurface heating. [ABSTRACT FROM AUTHOR]

The ventilation index (VI) is an index that describes the potential for smoke or other pollutants to disperse from a source. In this study, a Lagrangian particle dispersion model was utilized to examine smoke dispersion and the diagnostic value of VI during a September 2018 prescribed fire in southwestern Colorado. Smoke dispersion in the vicinity of the fire was simulated using the FLEXPART-WRF particle dispersion model, driven by meteorological outputs from Advanced Regional Prediction System (ARPS) simulations of the background (non-fire) conditions. Two research questions are posed: (1) Is a horizontal grid spacing of 4 km comparable to the finest grid spacing currently used in operational weather models and sufficient to capture the spatiotemporal variability in wind and planetary boundary layer (PBL) structure during the fire? (2) What is the relationship between VI and smoke dispersion during the prescribed fire event, as measured by particle residence time within a given horizontal or vertical distance from each particle's release point? The ARPS no-fire simulations are shown to generally reproduce the observed variability in weather variables, with greatest fidelity to observations found with horizontal grid spacing of approximately 1 km or less. It is noted that there are considerable differences in particle residence time (i.e., dispersion) at different elevations, with VI exhibiting greater diagnostic value in the southern half of the domain, farthest from the higher terrain across the north. VI diagnostic value is also found to vary temporally, with diagnostic value greatest during the mid-morning to mid-afternoon period, and lowest during thunderstorm outflow passage in the late afternoon. Results from this study are expected to help guide the application of VI in complex terrain, and possibly inform development of new dispersion potential metrics. [ABSTRACT FROM AUTHOR]

The impact of a forest canopy on smoke concentration is assessed by applying a numerical weather prediction model coupled with a Lagrangian particle dispersion model to two low-intensity wildland (prescribed) fires in the New Jersey Pine Barrens. A comparison with observations indicates that the coupled numerical model can reproduce some of the observed variations in surface smoke concentrations and plume heights. Model sensitivity analyses highlight the effect of the forest canopy on simulated meteorological conditions, smoke concentrations, and plume heights. The forest canopy decreases near-surface wind speed, increases buoyancy, and increases turbulent mixing. Sensitivities to the time of day, plant area density profiles, and fire heat fluxes are documented. Analyses of temporal variations in smoke concentrations indicate that the effect of the transition from a daytime to a nocturnal planetary boundary layer is weaker when sensible heat fluxes from the fires are stronger. The results illustrate the challenges in simulating meteorological conditions and smoke concentrations at scales where interactions between the fire, fuels, and atmosphere are critically important. The study demonstrates the potential for predictive tools to be developed and implemented that could help fire and air-quality managers assess local air-quality impacts during low-intensity wildland fires in forested environments. [ABSTRACT FROM AUTHOR]

Lower-tropospheric wind maxima, known as low-level jets (LLJs), play a vital role in weather and climate around the world. In this study, two 10-year (2006–2015) regional climate simulations using current (2011) and future (2100) land-use/land-cover (LULC) patterns over the continental United States (CONUS) are used to assess the sensitivity of LLJ properties, including jet occurrence, maximum speed, and the elevation of the maximum, to changes in LULC. The three simulated LLJ properties exhibit greater sensitivity in summer than in winter. Summertime jets are projected to increase in frequency in the central CONUS, where cropland replaces grassland, and decrease in parts of the Ohio-River Valley and the Southeast, particularly Florida, where urban expansion occurs. Little change is projected for wintertime jet frequency. Larger modifications to jet speed and elevations are projected in parts of the Ohio River Valley, the upper Southeast, and the Intermountain West. While there is some evidence of weaker, more elevated jets with urban expansion, the connection between changes in jet speed and elevation and changes in LULC patterns at a given location is weak. This result suggests that LULC will primarily affect the large-scale atmospheric conditions that contribute to the formation of LLJs, particularly in winter. [ABSTRACT FROM AUTHOR]