@article {bnh-7773, title = {Pyrocumulonimbus Firepower Threshold: Assessing the atmospheric potential for pyroCb}, journal = {Weather and Forecasting}, year = {2020}, month = {12/2020}, abstract = {

Pyrocumulonimbus (pyroCb) clouds are difficult to predict and can produce extreme and unexpected wildfire behaviour that can be very hazardous to fire crews. Many forecasters modify conventional thunderstorm diagnostics to predict pyroCb potential, by adding temperature (Δθ) and moisture increments (Δq) to represent smoke plume thermodynamics near the expected plume condensation level. However, estimating these Δθ\ and Δq\ increments is a highly subjective process that requires expert knowledge of all factors that might influence future fire size and intensity. In this paper, instead of trying to anticipate these Δθ\ and Δq\ increments for a particular fire, the minimum firepower required to generate pyroCb for a given atmospheric environment is considered. This concept, termed the PyroCb Firepower Threshold (PFT) requires only atmospheric information, removing the need for subjective estimates of the fire contribution.

A simple approach to calculating PFT is presented that incorporates only basic plume-rise physics, yielding an analytic solution that offers important insight into plume behaviour and pyroCb formation. Minimum increments of Δθ\ and Δq\ required for deep, moist convection, plus a minimum cloud-base height (Zfc), are diagnosed on a thermodynamic diagram. Briggs plume rise equations are used to convert Δθ,\ Zfc, and a mean horizontal windspeed (U) to a measure of the PFT: the minimum heat flux entering the base of the plume. This PFT is proportional to the product of\ U, Δθ, and the square of\ Zfc. Plume behaviour insights provided by the Briggs equations are discussed, and a selection of PFT examples presented.

}, keywords = {atmosphere, fire power, Fire weather, meteorology, pyrocumulonimbus}, doi = {https://doi.org/10.1175/WAF-D-20-0027.1}, url = {https://journals.ametsoc.org/view/journals/wefo/aop/WAF-D-20-0027.1/WAF-D-20-0027.1.xml}, author = {KJ Tory and Jeffrey Kepert} } @article {bnh-8134, title = {The real-time trial of the Pyrocumulonimbus Firepower Threshold}, number = {694}, year = {2020}, month = {03/2020}, institution = {Bushfire and Natural Hazards CRC}, address = {MELBOURNE}, abstract = {

This document reviews the real-time trial of the Pyrocumulonimbus Firepower Threshold (PFT) diagnostic during the 2019/2020 southern Australian fire season. The PFT was developed with support from the Bushfire and Natural Hazards Cooperative Research Centre, through the {\textquotedblleft}Improved prediction of severe weather to reduce community impact{\textquotedblright} project. While this report was primarily prepared to meet project requirements, it contains useful information and insight into the PFT and deep moist pyro-convection prediction more generally, that should be of value to fire-weather forecasters, fire-behaviour analysts and their parent agencies.

The report introduces the PFT in Section 1 and shows examples of the forecast products from the 2019/2020 season in Section 2. Throughout the trial users provided feedback that ranged from technical issues to broader forecasting challenges. This feedback identified confusion surrounding the definition of pyroCb (Section 3), and difficulties in forecasting this not-well-defined and not- well-understood phenomenon (Section 4). Many examples are presented in Section 5 to illustrate some of the more pertinent issues that arose throughout the trial. These include the limitations of using parcel theory to predict moist- plume growth, and the need to include more factors in lightning prediction. Examples are also provided that illustrate the limitations of various PFT products. The application of the PFT to climate studies is discussed in Section 6, where the minimising of false positives is of higher importance than for forecasting applications where expert users can separate real from false threats. Methods to address two of the issues raised in the report were developed with support from the Earth System and Climate Change Hub of the Australian Government{\textquoteright}s National Environmental Science Programme. These are the reduction of false positives caused by unstable conditions during cold- outbreaks, and a procedure to bypass the parcel-theory assumption (Section 7). The report is summarized in Section 8, which includes a set of dot points that highlight areas for future investigation and lessons learned.

}, keywords = {extreme, firepower, PFT, pyrocumulonimbus, real-time, threshold, trial}, issn = {694}, author = {KJ Tory} } @article {bnh-7478, title = {Climate Change Increases the Potential for Extreme Wildfires}, journal = {Geophysical Research Letters}, volume = {46}, year = {2019}, month = {07/2019}, pages = {8517-8526}, abstract = {


Pyrocumulonimbus (pyroCb) wildfires cause devastation in many regions globally. Given that fire-atmosphere coupling is associated with pyroCbs, future changes in coincident high index values of atmospheric instability and dryness (C-Haines) and near-surface fire weather are assessed for southeastern Australia using a regional climate projection ensemble. We show that observed pyroCb events occur predominantly on forested, rugged landscapes during extreme C-Haines conditions, but over a wide range of surface fire weather conditions. Statistically significant increases in the number of days where both C-Haines and near-surface fire weather values are conducive to pyroCb development are projected across southeastern Australia, predominantly for November (spring), and less strongly for December (summer) in 2060-2079 versus 1990-2009, with future C-Haines increases linked to increased 850-hPa dewpoint depression. The increased future occurrence of conditions conducive to pyroCb development and their extension into spring have implications for mitigating these dangerous wildfires and urbanizing fire-prone landscapes.

}, keywords = {atmospheric instability, dewpoint depression, Fire weather, Natural hazards, pyrocumulonimbus, regional climate modeling}, doi = {https://doi.org/10.1029/2019GL083699}, url = {https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL083699}, author = {Giovanni Virgilio and Jason P. Evans and Stephanie Blake and Matthew Armstrong and Dowdy, Andrew J and Jason J. Sharples and McRae} } @article {bnh-6392, title = {Evolution of an extreme Pyrocumulonimbus-driven wildfire event in Tasmania, Australia}, journal = {Natural Hazards and Earth System Sciences}, year = {2019}, month = {12/2019}, abstract = {

Extreme fires have substantial adverse effects on society and natural ecosystems. Such events can be associated with intense coupling of fire behaviour with the atmosphere, resulting in extreme fire characteristics such as pyrocumulonimbuscloud (pyroCb) development. Concern that anthropogenic climate change is increasing the occurrence of pyroCbs globally is driving more focused research into these meteorological phenomena. Using 6-minute scans from a nearby weather radar, we describe the development of a pyroCb during the afternoon of 4 January 2013 above the Forcett-Dunalley fire in south-eastern Tasmania. We relate storm development to: (1) near-surface weather using the McArthur Forest Fire Danger Index (FFDI), and the C-Haines Index, a measure of the vertical atmospheric stability and dryness both derived from gridded weather reanalysis for Tasmania (BARRA-TA), and (2) a chronosequence of fire severity derived from remote sensing. We show that the pyroCb rapidly developed over a 24-minute period in the afternoon of 4 January, with the cloud top reaching a height of 15 km. The pyroCb was associated with a highly unstable atmosphere (C-Haines 10-11) and Severe-marginally Extreme (FFDI 60-75) near-surface fire weather, and formed over an area of forest that was severely burned (total crown defoliation). We use spatial patterns of elevated fire weather in Tasmania, and fire weather during major runs of large wildfires in Tasmania for the period 2007-2016 to geographically and historically contextualise this pyroCb event. Although the Forcett-Dunalley fire is the only known record of a pyroCb in Tasmania, our results show that eastern and south-eastern Tasmania are prone to the conjunction of high FFDI and C-Haines values that have been associated with pyroCb development. Our findings have implications for fire weather forecasting and wildfire management, and highlight the vulnerability of southeast Tasmania to extreme fire events.

}, keywords = {C-Haines, extreme fire, fire severity, Fire weather, McArthur Forest Fire Danger Index, pyrocumulonimbus, smoke plume injection, Tasmania, wildland fire}, doi = {https://doi.org/10.5194/nhess-2019-354}, url = {https://www.nat-hazards-earth-syst-sci-discuss.net/nhess-2019-354/}, author = {Mercy Ndalila and Grant Williamson and Paul Fox-Hughes and Jason J. Sharples and David Bowman} }