Observations of Turbulent Heat and Momentum Fluxes during Wildland Fires in Forested Environments

AbstractTurbulent 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.

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Observations of Sweep-Ejection Dynamics for Heat and Momentum Fluxes during Wildland Fires in Forested and Grassland Environments

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-20-0086.1 ◽

2020 ◽

Author(s):

Warren E. Heilman ◽

Tirtha Banerjee ◽

Craig B. Clements ◽

Kenneth L. Clark ◽

Shiyuan Zhong ◽

...

Keyword(s):

Boundary Layer ◽

High Frequency ◽

Three Dimensional ◽

Prescribed Fires ◽

Turbulent Heat ◽

Turbulent Transfer ◽

Wildland Fires ◽

Momentum Fluxes ◽

Turbulence Data ◽

The Mean

AbstractThe vertical turbulent transfer of heat and momentum in the lower atmospheric boundary layer is accomplished through intermittent sweep, ejection, outward interaction, and inward interaction events associated with turbulent updrafts and downdrafts. These events, collectively referred to as sweep-ejection dynamics, have been studied extensively in forested and non-forested environments and reported in the literature. However, little is known about the sweep-ejection dynamics that occur in response to turbulence regimes induced by wildland fires in forested and non-forested environments. This study attempts to fill some of that knowledge gap through analyses of turbulence data previously collected during three wildland (prescribed) fires that occurred in grassland and forested environments in Texas and New Jersey. Tower-based high-frequency (10 or 20 Hz) three-dimensional wind velocity and temperature measurements are used to examine frequencies of occurrence of sweep, ejection, outward interaction, and inward interaction events and their actual contributions to the mean vertical turbulent fluxes of heat and momentum before, during, and after the passage of fire fronts. The observational results suggest that wildland fires in these environments can substantially change the sweep-ejection dynamics for turbulent heat and momentum fluxes that typically occur when no fires are present, especially the relative contributions of sweeps compared to ejections in determining overall heat and momentum fluxes.

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Numerical simulation of the tree higro-thermal response in forest fire environment

WEENTECH Proceedings in Energy ◽

10.32438/wpe.0220 ◽

2020 ◽

pp. 57-65

Author(s):

Eusébio Conceiçã ◽

João Gomes ◽

Maria Manuela Lúcio ◽

Jorge Raposo ◽

Domingos Xavier Viegas ◽

...

Keyword(s):

Forest Fire ◽

Numerical Study ◽

Thermal Response ◽

View Factor ◽

Tree Model ◽

Pine Tree ◽

Surface Temperatures ◽

Fire Environment ◽

Fire Front ◽

Heat Exchanges

This paper refers to a numerical study of the hypo-thermal behaviour of a pine tree in a forest fire environment. The pine tree thermal response numerical model is based on energy balance integral equations for the tree elements and mass balance integral equation for the water in the tree. The simulation performed considers the heat conduction through the tree elements, heat exchanges by convection between the external tree surfaces and the environment, heat exchanges by radiation between the flame and the external tree surfaces and water heat loss by evaporation from the tree to the environment. The virtual three-dimensional tree model has a height of 7.5 m and is constituted by 8863 cylindrical elements representative of its trunks, branches and leaves. The fire front has 10 m long and a 2 m high. The study was conducted taking into account that the pine tree is located 5, 10 or 15 m from the fire front. For these three analyzed distances, the numerical results obtained regarding to the distribution of the view factors, mean radiant temperature and surface temperatures of the pine tree are presented. As main conclusion, it can be stated that the values of the view factor, MRT and surface temperatures of the pine tree decrease with increasing distance from the pine tree in front of fire.

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Using the Landsat Burned Area Products to Derive Fire History Relevant for Fire Management and Conservation in the State of Florida, Southeastern USA

Fire ◽

10.3390/fire4020026 ◽

2021 ◽

Vol 4 (2) ◽

pp. 26

Author(s):

Casey Teske ◽

Melanie K. Vanderhoof ◽

Todd J. Hawbaker ◽

Joe Noble ◽

John Kevin Hiers

Keyword(s):

Prescribed Fire ◽

Spatial Data ◽

Fire History ◽

Fire Management ◽

Fire Behavior ◽

Fire Risk ◽

Behavior Modeling ◽

Burned Area ◽

Burned Areas ◽

Burn Probability

Development of comprehensive spatially explicit fire occurrence data remains one of the most critical needs for fire managers globally, and especially for conservation across the southeastern United States. Not only are many endangered species and ecosystems in that region reliant on frequent fire, but fire risk analysis, prescribed fire planning, and fire behavior modeling are sensitive to fire history due to the long growing season and high vegetation productivity. Spatial data that map burned areas over time provide critical information for evaluating management successes. However, existing fire data have undocumented shortcomings that limit their use when detailing the effectiveness of fire management at state and regional scales. Here, we assessed information in existing fire datasets for Florida and the Landsat Burned Area products based on input from the fire management community. We considered the potential of different datasets to track the spatial extents of fires and derive fire history metrics (e.g., time since last burn, fire frequency, and seasonality). We found that burned areas generated by applying a 90% threshold to the Landsat burn probability product matched patterns recorded and observed by fire managers at three pilot areas. We then created fire history metrics for the entire state from the modified Landsat Burned Area product. Finally, to show their potential application for conservation management, we compared fire history metrics across ownerships for natural pinelands, where prescribed fire is frequently applied. Implications of this effort include increased awareness around conservation and fire management planning efforts and an extension of derivative products regionally or globally.

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Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction

10.54206/102376/gieq2593 ◽

2010 ◽

Author(s):

Steve Kerber ◽

Keyword(s):

Homeland Security ◽

Fire Behavior ◽

The United States ◽

Fire Service ◽

United States Department ◽

Department Of Homeland Security ◽

Front Door ◽

Fire Environment ◽

Open Floor ◽

The Impact

Under the United States Department of Homeland Security (DHS) Assistance to Firefighter Grant Program, Underwriters Laboratories examined fire service ventilation practices as well as the impact of changes in modern house geometries. There has been a steady change in the residential fire environment over the past several decades. These changes include larger homes, more open floor plans and volumes and increased synthetic fuel loads. This series of experiments examine this change in fire behavior and the impact on firefighter ventilation tactics. This fire research project developed the empirical data that is needed to quantify the fire behavior associated with these scenarios and result in immediately developing the necessary firefighting ventilation practices to reduce firefighter death and injury. Two houses were constructed in the large fire facility of Underwriters Laboratories in Northbrook, IL. The first of two houses constructed was a one-story, 1200 ft, 3 bedroom,  bathroom house with 8 total rooms. The second house was a two-story 3200 ft, 4 bedroom, 2.5 bathroom house with 12 total rooms. The second house featured a modern open floor plan, two- story great room and open foyer. Fifteen experiments were conducted varying the ventilation locations and the number of ventilation openings. Ventilation scenarios included ventilating the front door only, opening the front door and a window near and remote from the seat of the fire, opening a window only and ventilating a higher opening in the two-story house. One scenario in each house was conducted in triplicate to examine repeatability. The results of these experiments provide knowledge for the fire service for them to examine their thought processes, standard operating procedures and training content. Several tactical considerations were developed utilizing the data from the experiments to provide specific examples of changes that can be adopted based on a departments current strategies and tactics.

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Challenges in Modeling Turbulent Heat Fluxes to Snowpacks in Forest Clearings

Journal of Hydrometeorology ◽

10.1175/jhm-d-18-0050.1 ◽

2018 ◽

Vol 19 (10) ◽

pp. 1599-1616 ◽

Cited By ~ 5

Author(s):

Jonathan P. Conway ◽

John W. Pomeroy ◽

Warren D. Helgason ◽

Nicholas J. Kinar

Keyword(s):

Heat Exchange ◽

Surface Temperature ◽

Latent Heat ◽

Turbulent Fluxes ◽

Heat Fluxes ◽

Turbulent Heat ◽

Snow Surface ◽

Turbulent Heat Exchange ◽

Uncertainty Estimates ◽

Model Tuning

Abstract Forest clearings are common features of evergreen forests and produce snowpack accumulation and melt differing from that in adjacent forests and open terrain. This study has investigated the challenges in specifying the turbulent fluxes of sensible and latent heat to snowpacks in forest clearings. The snowpack in two forest clearings in the Canadian Rockies was simulated using a one-dimensional (1D) snowpack model. A trade-off was found between optimizing against measured snow surface temperature or snowmelt when choosing how to specify the turbulent fluxes. Schemes using the Monin–Obukhov similarity theory tended to produce negatively biased surface temperature, while schemes that enhanced turbulent fluxes, to reduce the surface temperature bias, resulted in too much melt. Uncertainty estimates from Monte Carlo experiments showed that no realistic parameter set could successfully remove biases in both surface temperature and melt. A simple scheme that excludes atmospheric stability correction was required to successfully simulate surface temperature under low wind speed conditions. Nonturbulent advective fluxes and/or nonlocal sources of turbulence are thought to account for the maintenance of heat exchange in low-wind conditions. The simulation of snowmelt was improved by allowing enhanced latent heat fluxes during low-wind conditions. Caution is warranted when snowpack models are optimized on surface temperature, as model tuning may compensate for deficiencies in conceptual and numerical models of radiative, conductive, and turbulent heat exchange at the snow surface and within the snowpack. Such model tuning could have large impacts on the melt rate and timing of the snow-free transition in simulations of forest clearings within hydrological and meteorological models.

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A Case Study of Clear-Air Turbulence at Upper Levels

10.5194/ems2021-182 ◽

2021 ◽

Author(s):

Léo Rogel ◽

Didier Ricard ◽

Eric Bazile ◽

Irina Sandu

Keyword(s):

Turbulent Flow ◽

Partial Information ◽

Weather Prediction ◽

Turbulent Fluxes ◽

Small Scale ◽

Mixing Length ◽

Vertical Resolution ◽

Stable Conditions ◽

Study Results

Because of the technical difficulties of achieving measurements at high altitudes, it is not clear how well turbulent phenomena are represented in the upper levels of current Numerical Weather Prediction (NWP) operational models. Indeed, turbulence in strongly stable conditions near the tropopause is known to be particularly difficult to correctly parameterize. The constraining buoyancy forces on the vertical lead to anisotropic turbulence, potentially inhibiting turbulent production in NWP models. Partial information for high altitude turbulence events is nonetheless available in the form of in-situ measurements from aircrafts. However, it only allows for qualitative comparisons with model outputs. This study focuses on a turbulent episode induced by a winter upper-level jet above east Belgium on January 27, 2018, for which in-situ EDR (Eddy Dissipation Rate) reports indicate moderate-or-greater turbulence levels. Numerical simulations are performed with the M&#233;t&#233;o-France operational model AROME, and with the mesoscale research model MesoNH (Laero/CNRM), at the same horizontal grid resolution (1.3km). These two models also use the eddy-diffusivity turbulence scheme of Cuxart et al (2000), a 1.5 order closure scheme based on a prognostic Turbulent Kinetic Energy (TKE) evolution equation, with a diagnostic computation of the mixing length. TKE budgets, as well as stability indices and gradient-based quantities (Richardson number, vertical wind shear) are computed from the model outputs, and qualitative comparison with in-situ data is presented. Time evolution of the turbulent event over Belgium is well captured by both models, agreeing with EDR data. Several sensitivity tests on the vertical resolution, on the mixing length formulation and on the parameters of the TKE equation are then performed. Most notably, the use of an increased vertical resolution near the tropopause greatly enhances the turbulent fluxes in both operational and research models. Secondly, comparison of various expressions of the mixing length shows that the Bougeault and Lacarrere (1989) formulation produces the higher amount of subgrid TKE and turbulent mixing. A decreased turbulent dissipation parameter also significantly increases the amount of subgrid TKE. On the contrary, the use of a 3D turbulence scheme appears to have very limited impacts on the turbulent flow at this kilometer-scale horizontal resolution. On a second part of this study, results from ongoing Large Eddy Simulations (LES) will be presented. These simulations aim at representing small-scale features of the turbulent flow. They will be used as a reference for the computation of turbulent fluxes at kilometer-scale resolution using a coarse-graining method, allowing for a comparison with the parameterized fluxes from the turbulence scheme. In particular, the dissipation term of the TKE equation will be examined. These results are expected to give insight on the leading turbulent mechanisms for which the current turbulence parameterization can be improved in stable conditions.

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Areal melt and discharge modelling of Storglaciären, Sweden

Annals of Glaciology ◽

10.3189/s0260305500012192 ◽

1997 ◽

Vol 24 ◽

pp. 211-216 ◽

Cited By ~ 22

Author(s):

Regine Hock ◽

Christian Noetzli

Keyword(s):

Meteorological Data ◽

Grid Point ◽

Digital Terrain Model ◽

Turbulent Fluxes ◽

Heat Fluxes ◽

Balance Model ◽

Turbulent Heat ◽

Terrain Model ◽

Digital Terrain ◽

One Year

A grid-based glacier melt-and-discharge model was applied to Storglaciären, a small valley glacier (3 km2) in northern Sweden, for the melt seasons of 1993 and 1994. The energy available for melt was estimated from a surface energy-balance model using meteorological data collected by automatic weather stations on the glacier. Net radiation and the turbulent heat fluxes were calculated hourly for every grid point of a 30 m resolution digital terrain model, using the measurements of temperature, humidity, wind speed and radiative fluxes on the glacier. Two different bulk approaches were used to calculate the turbulent fluxes and compared with respect to their impact on discharge simulations. Discharge of Storglaciären was simulated from calculated meltwater production and precipitation by three parallel linear reservoirs corresponding to the different storage properties of firn, snow and ice. The performance of the model was validated by comparing simulated discharge to measured discharge at the glacier snout. Depending on which parameterization of the turbulent fluxes was used, the timing and magnitude of simulated discharge was in good agreement with observed discharge, or simulated discharge was considerably underestimated in one year.

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The efficacy of fire and fuels reduction treatments in a Sierra Nevada pine plantation

International Journal of Wildland Fire ◽

10.1071/wf06097 ◽

2009 ◽

Vol 18 (7) ◽

pp. 791 ◽

Cited By ~ 39

Author(s):

Leda N. Kobziar ◽

Joe R. McBride ◽

Scott L. Stephens

Keyword(s):

Sierra Nevada ◽

Prescribed Fire ◽

Large Scale ◽

Structural Characteristics ◽

Fire Behavior ◽

Pine Plantation ◽

Understorey Vegetation ◽

Fuels Reduction ◽

Reduction Techniques ◽

Reducing Potential

Plantations are the most common means of reforestation following stand-replacing wildfires. As wildfires continue to increase in size and severity as a result of fire suppression or climate change, establishment of plantations will likely also increase. Plantations’ structural characteristics, including dense, uniform spacing and abundant ladder fuels, present significant wildfire hazards. Large-scale fuels reduction techniques may be necessary to reduce potential fire behavior in plantations and to protect surrounding forests. In the present study, four different manipulations aimed at reducing potential fire behavior in a Sierra Nevada pine plantation are compared. The treatments include: mechanical shredding, or mastication, of understorey vegetation and small trees; mastication followed by prescribed fire; fire alone; and controls. Fire behavior modeling shows that mastication is detrimental whereas prescribed fire is effective in reducing potential fire behavior at moderate to extreme weather conditions. Predicted fire behavior was compared with actual values from the prescribed burns in an effort to explore the limitations of fire modeling. Fire behavior predictions were similar to field observations in the more structurally homogeneous stands, but differed greatly where mastication created forest openings and patchy fuels distributions. In contrast to natural stands, the homogeneity of pine plantations make the results of the present work applicable to other regions such as the south-eastern US, where similar fuels reduction techniques are used to increase fire-resistance and stand resilience.

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Seasonal Variations In The Surface Energy Exchanges Over Antarctic Sea Ice and Coastal Waters

Annals of Glaciology ◽

10.1017/s0260305500002445 ◽

1982 ◽

Vol 3 ◽

pp. 12-16 ◽

Cited By ~ 20

Author(s):

I. Allison ◽

C.M. Tivendale ◽

G.J. Akerman ◽

J.M. Tann ◽

R.H. Wills

Keyword(s):

Heat Transfer ◽

Sea Ice ◽

Seasonal Variations ◽

Lower Boundary ◽

Open Water ◽

Ice Cover ◽

Turbulent Fluxes ◽

Turbulent Heat ◽

Radiative Heat Loss ◽

Antarctic Sea Ice

Seasonal variations in radiative and turbulent fluxes at the surface of, and in the heat transfer within, sea ice are discussed from results of energy balance studies at a site of annual ice cover near Mawson, Antarctica. In mid-summer, the open water gains heat mostly by radiation but by early February the ocean is cooling predominantly by strong turbulent losses, with some radiative heat loss occurring also by March. When an ice cover forms, turbulent fluxes decrease from several 100 W m−2over open water to only 40 w m−2over ice less than 0.2 m thick and even less over thicker ice.Net radiative losses over mature ice in mid-winter are balanced mostly by conduction through the ice cover but with some turbulent heat gain at the surface. By mid-spring, there is a net radiative gain, the turbulent fluxes are again outgoing, and there is little total heat transfer through the ice. At break-out, the albedo increase from ice to open water causes a large increase in the net radiative gain.At the lower boundary of the ice, the oceanic heat flux provides an important contribution. A net advection of heat into the region is shown from temperature profiles in the water under the ice. Salinity changes in the water during the period of ice melt are also discussed.

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