Fire whirls present a powerful intensification of combustion, long studied in the fire research community because of the dangers they present during large urban and wildland fires. However, their destructive power has hidden many features of their formation, growth, and propagation. Therefore, most of what is known about fire whirls comes from scale modeling experiments in the laboratory. Both the methods of formation, which are dominated by wind and geometry, and the inner structure of the whirl, including velocity and temperature fields, have been studied at this scale. Quasi-steady fire whirls directly over a fuel source form the bulk of current experimental knowledge, although many other cases exist in nature. The structure of fire whirls has yet to be reliably measured at large scales; however, scaling laws have been relatively successful in modeling the conditions for formation from small to large scales. This review surveys the state of knowledge concerning the fluid dynamics of fire whirls, including the conditions for their formation, their structure, and the mechanisms that control their unique state. We highlight recent discoveries and survey potential avenues for future research, including using the properties of fire whirls for efficient remediation and energy generation.

5Non-specialist summary The flight of firebrands and resulting spot fires is a major hazard 6 for firefighters. This paper presents a statistical description of the size and shape of firebrands 7 generated from coniferous trees. This characterization of firebrands' size will improve modeling 8 credibility of their flight. 9 Abstract 13The process of ember/firebrand formation, lofting, wind driven transport, and resulting 14 spot fire ignition during a wildfire is still poorly understood. Lack of a tractable firebrand 15 formation model along with a detailed statistical description of the size and shape distribution 16 of typical firebrand that could be used in simulations of firebrand flight and combustion may 17 result in unrealistic outcomes. In this regard, a simple, yet quite informative, mechanical 18 failure model of the firebrand break-off process is proposed. This model suggests that the 19 previous laboratory scale firebrand generation experiments would likely provide a reasonable 20 analogue for the formation process in a full scale wildfire. In addition, geometric scaling 21 analysis is conducted and shows that the firebrand surface area scales with the firebrand 22 1 © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ mass raised to the 2/3 rds power. This is in close agreement with measurements of firebrand 23 from previously published data in the literature that are obtained under controlled laboratory 24 combustion of coniferous tress of different sizes. Also, a detailed statistical characterization 25 of the size and shape of these firebrands are presented. A nonlinear regression model on the 26 firebrands' data led to the generation of a set of virtual firebrands. The resulting data could be 27 used as inputs to a Monte-Carlo simulation of firebrands' transport through the velocity field 28 induced by the interaction of a fire plume and the atmospheric boundary layer. Moreover, 29 it is shown that the size distribution of firebrands is more dependent on the mechanics of 30 combustion and limb failure than on a simple geometric relationship with the tree height. 31 regression 33 34 1 Introduction 35Wildfires are a major threat to people and property. In 2010 there were over 70,000 wildfires recorded in 36 the US by the National Interagency Fire Center NIFC [2011], which burned over 3.4 million acres. In an 37 average year over 1,000 homes, 1,000 outbuildings and 40 businesses are destroyed by wildfires in the US. 38In 2014 only San Diego County wildfires burned over 29,300 acres of land which resulted in damage or 39 destruction of more than 55 properties. So far, this has caused the total cost of $60 million (2014 USD) 40 and damage estimate still continues [Repard, 2014]. Also on June 2012 Waldo Canyon fire, the most 41 destructive fire in Colorado fire history in terms of consumed homes after Black Forest fire [Parker et al., 42 2013], burned a total of 18,247 acres in Colorado and Manitou Springs a...

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Highly buoyant plumes, such as wildfire plumes, in low to moderate wind speeds have initial trajectories that are steeper than many industrial waste plumes. They will rise further into the atmosphere before bending significantly. In such cases the plume's trajectory will be influenced by the vertical variation in horizontal velocity of the atmospheric boundary layer. This paper examined the behavior of a plume in an unstratified environment with a power-law ambient velocity profile. Examination of previously published experimental measurements of plume trajectory show that inclusion of the boundary layer velocity profile in the plume model often provides better predictions of the plume trajectory compared to algebraic expressions developed for uniform flow plumes. However, there are many cases in which uniform velocity profile algebraic expressions are as good as boundary layer models. It is shown that it is only important to model the role of the atmospheric boundary layer velocity profile in cases where either the momentum length (square root of source momentum flux divided by the reference wind speed) or buoyancy length (buoyancy flux divided by the reference wind speed cubed) is significantly greater than the plume release height within the boundary layer. This criteria is rarely met with industrial waste plumes, but it is important in modeling wildfire plumes.

Fire is an integral component of ecosystems globally and a tool that humans have harnessed for millennia. Altered fire regimes are a fundamental cause and consequence of global change, impacting people and the biophysical systems on which they depend. As part of the newly emerging Anthropocene, marked by human-caused climate change and radical changes to ecosystems, fire danger is increasing, and fires are having increasingly devastating impacts on human health, infrastructure, and ecosystem services. Increasing fire danger is a vexing problem that requires deep transdisciplinary, trans-sector, and inclusive partnerships to address. Here, we outline barriers and opportunities in the next generation of fire science and provide guidance for investment in future research. We synthesize insights needed to better address the long-standing challenges of innovation across disciplines to (i) promote coordinated research efforts; (ii) embrace different ways of knowing and knowledge generation; (iii) promote exploration of fundamental science; (iv) capitalize on the “firehose” of data for societal benefit; and (v) integrate human and natural systems into models across multiple scales. Fire science is thus at a critical transitional moment. We need to shift from observation and modeled representations of varying components of climate, people, vegetation, and fire to more integrative and predictive approaches that support pathways towards mitigating and adapting to our increasingly flammable world, including the utilization of fire for human safety and benefit. Only through overcoming institutional silos and accessing knowledge across diverse communities can we effectively undertake research that improves outcomes in our more fiery future.