Synthesis of Knowledge of Extreme Fire Behavior

158 Pages·2011·4.83 MB·English

United States
Department of
Agriculture Synthesis of Knowledge of
Forest Service
Extreme Fire Behavior:
Pacifc Northwest
Research Station
Volume I for Fire Managers
General Technical
Report
PNW-GTR-854
November 2011 Paul A. Werth, Brian E. Potter, Craig B. Clements, Mark A. Finney,
Scott L. Goodrick, Martin E. Alexander, Miguel G. Cruz, Jason A.
Forthofer, and Sara S. McAllister
A SUMMARY OF
KNOWLEDGE FROM THE
The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of
multiple use management of the Nation’s forest resources for sustained yields of wood,
water, forage, wildlife, and recreation. Through forestry research, cooperation with the
States and private forest owners, and management of the national forests and national
grasslands, it strives—as directed by Congress—to provide increasingly greater service
to a growing Nation.
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disabilities who require alternative means for communication of program information (Braille,
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Room 1400 Independence Avenue, SW, Washington, DC 20250-9410 or call [phone]
(voice) or [phone] (TDD). USDA is an equal opportunity provider and employer.
Authors
Paul A. Werth is a fre weather meteorologist (retired), Northwest Interagency
Coordination Center, 150 SW Harrison Street, Suite 400, Portland, OR 97201. Brian E.
Potter is a research meteorologist, U.S. Department of Agriculture, Forest Service, Pacifc
th
Wildland Fire Sciences Laboratory, 400 N 34 St., Suite 201, Seattle, WA 98103. Craig
B. Clements is an assistant professor of meteorology, San Jose State University, San José,
CA 95192. Mark A. Finney is a research forester, Jason A. Forthofer is a mechanical
engineer, and Sara S. McAllister is a research mechanical engineer, U.S. Department of
Agriculture, Forest Service, Missoula Fire Sciences Laboratory, 5775 Highway 10 West,
Missoula, MT 59808. Scott L. Goodrick is a research meteorologist, U.S. Department
of Agriculture, Forest Service, Southern Research Station, 320 Green Street, Athens,
GA 30602. Martin E. Alexander is an adjunct professor of wildland fre science and
management, Department of Renewable Resources and Alberta School of Forest Science
and Management, University of Alberta, Edmonton, AB T6G 2H1, Canada. Miguel
G. Cruz is a senior research scientist, Bush Fire Dynamics and Applications, CSIRO
Ecosystems Sciences–Climate Adaptation Flagship, GPO Box 284, Canberra, ACT 2601,
Australia.
Cover Photos
Cover photos by Rick Trembath, USDA Forest Service (retired).
Synthesis of Knowledge
of Extreme Fire Behavior:
Volume I for Fire
Managers
Paul A. Werth, Brian E. Potter,
Craig B. Clements, Mark A. Finney,
Scott L. Goodrick, Martin E. Alexander,
Miguel G. Cruz, Jason A. Forthofer,
and Sara S. McAllister
U.S. Department of Agriculture, Forest Service
Pacifc Northwest Research Station
Portland, Oregon
General Technical Report, PNW-GTR-854
November 2011
Published in cooperation with:
U.S. Department of Agriculture, Forest Service
Southern Research Station
GENERAL TECHNICAL REPORT PNW-GTR-841
This page was intentionally left blank.
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Abstract
Werth, Paul A.; Potter, Brian E.; Clements, Craig B.; Finney, Mark A.; Goodrick,
Scott L.; Alexander, Martin E.; Cruz, Miguel G.; Forthofer, Jason A.; McAllister,
Sara S. 2011. Synthesis of knowledge of extreme fre behavior: volume I for fre
managers. Gen. Tech. Rep. PNW-GTR-854. Portland, OR: U.S. Department of
Agriculture, Forest Service, Pacifc Northwest Research Station. 144 p.
The National Wildfre Coordinating Group defnition of extreme fre behavior (EFB) indicates
a level of fre behavior characteristics that ordinarily precludes methods of direct control
action. One or more of the following is usually involved: high rate of spread, prolifc crown-
ing/spotting, presence of fre whirls, and strong convection column. Predictability is diffcult
because such fres often exercise some degree of infuence on their environment and behave
erratically, sometimes dangerously. Alternate terms include “blow up” and “fre storm.”
Fire managers examining fres over the last 100 years have come to understand many
of the factors necessary for EFB development. This work produced guidelines included in
current frefghter training, which presents the current methods of predicting EFB by using
the crown fre model, which is based on the environmental infuences of weather, fuels, and
topography.
Current training does not include the full extent of scientifc understanding. Material
in current training programs is also not the most recent scientifc knowledge. National Fire
Plan funds have sponsored newer research related to wind profles’ infuence on fre behavior,
plume growth, crown fres, fre dynamics in live fuels, and conditions associated with vortex
development. Of signifcant concern is that characteristic features of EFB depend on condi-
tions undetectable on the ground, relying fundamentally on invisible properties such as wind
shear or atmospheric stability.
Obviously no one completely understands all the factors contributing to EFB because of
gaps in our knowledge. These gaps, as well as the limitations as to when various models or
indices apply should be noted to avoid application where they are not appropriate or war-
ranted. This synthesis will serve as a summary of existing extreme fre behavior knowledge
for use by fre managers, frefghters, and fre researchers.
The objective of this project is to synthesize existing EFB knowledge in a way that
connects the weather, fuel, and topographic factors that contribute to development of EFB.
This synthesis will focus on the state of the science, but will also consider how that science is
currently presented to the fre management community, including incident commanders, fre
behavior analysts, incident meteorologists, National Weather Service offce forecasters, and
frefghters. It will seek to clearly delineate the known, the unknown, and areas of research
with the greatest potential impact on frefghter protection.
Keywords: Extreme fre behavior, fuels, fre behavior.
Preface
In 2008, the National Wildfre Coordinating Group (NWCG) Fire Behavior Committee
(FBC) asked the Joint Fire Science Program (JFSP) to fund a synthesis and review of the
scientifc literature pertaining to extreme fre behavior (EFB). In September 2008, the JFSP
announced a call for proposals that included a request for “an examination of the state of
the science underlying predictions of extreme fre behavior, and an assessment of the appro-
priate uses and limits of this information.” This document is the result of that request.
In performing the review, it became progressively clearer that the concept of extreme
fre behavior (EFB) is vaguely defned and means something different to everyone. The
authors examined the offcial NWCG defnition and solicited input from the management
community to develop a defnition that was both operationally useful and scientifcally
tractable. This defnition and the initial stages of the review eventually led to the recogni-
tion that some relevant topics had not been included in the original outline. Other topics
from the original outline were expanded to include sections of their own.
The authors communicated these changes to both the JFSP and the FBC as they arose.
In those conversations, it became apparent that these two groups had different needs. The
JFSP needed something for fre managers and others without the technical background of
a fre behavior analyst. The FBC needed a document for fre behavior analysts that would
allow them to better understand the use and limitations of the tools they now have and
may have in the near future. To meet these two needs, this review has two parts. Volume
1 summarizes the state of the science for fre managers and frefghters with pertinent
references to scientifc papers. It is intended to be of use to anyone who works at or near the
freline. Volume 2 covers the same topics (with one exception) in more detail and includes
information necessary for fre behavior analysts to understand what is scientifcally known,
what science lies behind the tools they have, and what the limitations are on scientifc
knowledge and tools. It includes more references to scientifc literature. The one difference
in topical content between the volumes is that volume 2 includes a chapter on fuel dynamics
and volume 1 does not. As the study progressed, the scope of this topic led to the need to
include more experts, and the short time available precluded that section from publication
in volume 1.
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Summary
A working defnition of extreme fre behavior (EFB) was necessary for development of this
synthesis. Because the subjective nature of four of the fve properties of the EFB defnition
established by the National Wildfre Coordination Group makes the defnition intractable
for scientifc purposes, the lead authors asked the fre behavior community for input on
possible defnitions of EFB and examples of phenomena they considered EFB. The only
coherent theme was that EFB is not steady state. After discussing responses, the authors
agreed on the following working defnition for this project:
Fire spread other than steady surface spread, especially when it involves rapid
increases.
This defnition of EFB does not emphasize any one element of the behavior triangle.
Complexity
It is imperative that fre managers understand that much of what is referred to as “extreme
fre behavior” is happening where it cannot be seen. Multiple factors come into play and not
all factors need be present for EFB to occur. No one factor must be present in every case.
A number of interactions among the elements are noted, but the number of possible
interactions between elements is practically unlimited, making research and the resulting
tool development a key step in achieving successful forest management and safety.
Myths and Lore
There are many myths and lore with limited scientifc basis. Anecdotal evidence sometimes
takes the place of science and comes to be accepted as fact even when little scientifc infor-
mation exists to validate it. Extreme fre behavior can occur on any scale, great or small, in
any fuel type, and at any time of the day or night. There is no time or circumstance when
fre managers can safely assume EFB will not occur.
Over-Arching Gaps
The authors of this synthesis have identifed areas in each chapter where understanding of
the science is lacking and more research is needed. These knowledge gaps may pertain to
just one chapter’s topic, but they are nonetheless important areas in which further research
would be of value to the operational community. There are, however, certain over-arching
gaps where additional research of one element will advance the science for other elements
as well.
• A greater recognition of the importance of plume dynamics to EFB and spotting.
• Advances in the understanding of fuel structure, especially as it relates to ember
production and crown fre.
• Better high-resolution observations on windfow in complex terrain to improve wind
models used in fre behavior and spotting tools, and to identify fre whirl potential. For
example, upper air soundings on project-size fres.
• The infuence of ambient winds or topography on fre interactions.
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• More research beyond the Haines Index to quantify the effects of atmospheric stability
on fre behavior.
New and expanded research into these areas will increase the understanding of the
science on which they are based and are a necessary starting point for enhanced wildland
fre management and advances in frefghter training and safety.
Operational implications
Even the most advanced tools and models are limited by their design and assumptions.
They can never, nor should they be expected to, take the place of direct observations one
makes on the freline, such as the “L” in LCES (Lookouts-Communications-Escape Routes-
Safety Zones) and the concept of ”situational awareness.” Scientifcally sound application of
tools and models requires that the tools/models be used within their design limitations and
in accordance with the tool assumptions.
Current training identifes circumstances that can result in extreme fre behavior,
where increased awareness of multiple factors can guide fre managers to make decisions.
Research can lead to development of additional or improved tools to help fre managers
better identify those situations where extreme fre behavior may occur. The lack of a tool
or model for a situation seen in the feld does not mean EFB cannot occur. Knowing what
conditions can lead to EFB, and knowing that you do not know whether those conditions
exist, can be more important than any tools or models. Extreme fre behavior can occur on
any fre.
The state of the science at present can
be summed up as follows:
• Fire is three dimensional and is not
steady state.
• The tools available to us today are
two dimensional and are predomi-
nantly steady state.
• Additional research into EFB may
one day result in development of
three-dimensional tools.
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Contents
1 Chapter 1: Introduction
Brian E. Potter and Paul A. Werth
1 Defnition
2 Methods
3 Liturature Cited
5 Chapter 2: Effects of Complex Terrain on Extreme Fire Behavior
Craig B. Clements
5 Introduction
5 Wind Systems in Mountainous Terrain
6 Dynamically Driven Winds
10 Thermally Driven Winds
16 Wind Modeling Tools: WindNinja
17 Summary
18 Future Needs
19 Literature Cited
25 Chapter 3: Critical Fire Weather Patterns
Paul A. Werth

25 Introduction
25 Weather Elements That Promote Extreme Fire Behavior
27 Critical Fire Weather Patterns
29 Regional Critical Fire Weather Patterns
30 Northern Plains, Great Lakes, and the Northeastern United States
31 Southeastern United States
32 Southwestern United States
34 Rocky Mountain and Intermountain Regions
37 Pacifc Northwest Region
39 California Region
42 Alaska
43 Models and Predictive Tools
43 The National Wildland Signifcant Fire Potential Outlook
43 GACC 7-Day Signifcant Fire Potential
45 Fuel and Fire Behavior Advisories
45 Other GACC Products and Services
45 Fire Weather Watches and Red Flag Warnings
45 Spot Weather Forcasts/Digital Web Services
45 Summary/Knowledge Gaps
46 References
49 Chapter 4: Fire Interactions and Mass Fires
Mark A. Finney and Sara S. McAllister
49 Introduction
50 Background: Time-Dependent Fire Behaviors
50 Fire Acceleration
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GENERAL TECHNICAL REPORT PNW-GTR-854
51 Length of Fire Front
52 Flame Tilt
53 Spread Thresholds
53 Conditions Where Fire Interactions Occur
54 Specifc Effects of Fire Interation
54 Burning Rate
56 Flame Dimensions
58 Flame Temperatures and Pollutants
59 Indraft Velocity
61 Pulsation
62 Convection Column
63 Summary of Interaction Effects
63 Needs for Further Research and Application
64 Literature Cited
73 Chapter 5: Column/Plume Dynamics
Brian E. Potter
73 Introduction
74 Plume-Dominated and Wind-Driven Fires
74 Adverse Wind Profles and Low-Level Jets
75 Stability and Instability
76 Downbursts and Plume Collapse
78 Summary
78 Literature Cited
81 Chapter 6: Spot Fires
Brian E. Potter
81 Introduction
81 The Spotting Process
83 Management Tools
85 Knowledge Gaps
86 Literature Cited
89 Chapter 7: Vortices and Wildland Fire
Jason A. Forthofer and Scott L. Goodrick
89 Introduction
89 Vorticity Basics
90 Fire Whirls
91 Fire Whirl Physics
91 Vorticity Sources
92 Vortex Stretching
93 Increased Combustion Rates
93 Scaling Fire Whirls
93 Fire Whirls in the Real World: Common Features
93 Whirl Shedding on the Lee Side of a Plume
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