@article {bnh-8318, title = {Improved predictions of severe weather to reduce community impact {\textendash} final project report}, number = {721}, year = {2022}, month = {02/2022}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

Extreme weather often occurs at relatively small scales.\  Accurate forecasts and understanding of such small-scale processes require high-resolution modelling.\  Forecasts are especially useful in severe weather events, since they play an essential role in allowing communities, industry and emergency services to prepare for and mitigate the impacts. Because forecasts are inherently uncertain in the severity, location and duration of an event, preparation needs to be more widespread than the eventual impact {\textendash} but this over-preparation comes at a cost.\  Detailed prediction of the probabilities of severe impacts would avoid the risk of failing to alert areas with the chance of an impact, while minimising the cost of over-warning.\ 

This project has studied the dynamics, predictability, and processes of severe weather, including fire weather, with the purpose of understanding phenomena with severe impact, improving forecasts of severe weather, and better depicting forecast uncertainty in these events. These goals help facilitate better risk management, improve user preparation, reduce adverse outcomes, and enable more effective mitigation.

The project has featured two main strands.

The first strand comprised case studies of severe weather events. For these, we have combined high-resolution numerical weather prediction (NWP) with a wide range of in situ and remotely sensed observations, to better understand both the high-impact event in question, and other events of that class. All except one of the studies used a version of the Bureau{\textquoteright}s operational NWP system, ACCESS. The events comprised two severe mid-latitude systems (an east coast low, and a severe thunderstorm and tornado outbreak), two severe fires, and a tropical cyclone, thereby covering the gamut of severe weather in Australia. Two of the studies featured ensemble simulation, with the east coast low case being the first time that ensemble ACCESS had been run at this resolution within the Bureau and foreshadowing the Bureau{\textquoteright}s new operational ensemble capability. Each case discovered important fine-scale features that contributed to the severity of the event, advancing our knowledge base and ability to respond.

The second strand studied two important phenomena associated with bushfire plumes: the formation of pyrocumulus clouds and ember transport. In each case, we began with idealised, high-resolution simulations of plumes using a large-eddy model. We used this technology, in which the model is run on a 50-m grid, to capture the most energetic size range of the turbulent eddies that are prominent in fire plumes. We found that the mean travel distance of firebrands depends mainly on wind speed and fire intensity, but the spread in the landing positions shifts from being substantially crosswind at light winds, to dominantly along-wind at high winds. This spread is greatly increased by the turbulence in the plume, and the maximum spotting distance can be more than double the mean for this reason.

We also used our plume modelling to study pyrocumulus clouds and analysed the processes that lead to pyrocumulus, with special attention on the relative importance of moisture from two sources, the atmosphere and combustion, and showed that the latter is negligible except in very dry environments. This somewhat controversial result has been confirmed by a conceptual study of the thermodynamics of pyrocumulus formation.

These initial studies laid a firm theoretical framework for our subsequent development of tools to provide predictions in a form, and at a speed, that is suitable for operational use.

For moist pyroconvection, we developed a paradigm to combine the necessary meteorological and fire information, the pyrocumulonimbus firepower threshold, or PFT. The PFT is defined as the power output from a fire at which pyroCb will begin to form and depends solely on meteorological parameters. Fires hotter than the PFT will initiate pyroCb, according to this paradigm, and cooler fires do not. We also developed a way of computing an approximate PFT from either NWP or observed data and tested this in real time during the extraordinarily severe 2019-20 fire season. The results of that trial were outstandingly successful, with nearly all the 30-odd events being captured, and the forecast guidance for non-events was also reported to have been extremely valuable. The Bushfire and Natural Hazards CRC have provided us with utilisation funding to take this tool another step closer to operational use. The tool has also attracted significant international interest, particularly given the recent spate of events in the USA. We have begun a utilisation project to take the PFT work further, detailed in this report.

For ember transport, we developed a simple model to predict plume-dominated transport, incorporating well-known plume modelling concepts, a model of plume turbulence, and a probabilistic model of ember transport within and beneath the plume. The results agree well with our large eddy-based simulations. This model was coupled to CSIRO{\textquoteright}s Spark fire-spread model and tested on the Kilmore East fire of Black Saturday with excellent results. The combination with Spark will make this tool suitable for future operational use, and the ember transport model could also, of course, be coupled to other fire spread simulators, including the coupled fire-atmosphere model ACCESS-Fire. Working with CSIRO and AFAC, we are putting the necessary things in place to be able to convert the prototype implementation of the ember transport parameterisation in Spark, into an operationally robust and supported system. This will both facilitate further research, and provide the framework to allow users to learn, understand and apply the system.

The value of case studies has been reinforced by our being asked to undertake five case studies of severe fires from the 2019-20 summer. This work will be done jointly with our colleagues from the CRC Coupled fire-atmosphere modelling project, who further developed the ACCESS-Fire model. We expect that this work will provide an impetus to further develop and streamline that capability.

We are also very excited about the potential of the PFT and ember transport tools. While both are relatively early in their development, with the PFT being the more mature, each is presently producing very encouraging results. We are also very conscious that each is raising further scientific questions as they address others, and that both are at the beginning of a steep development and refinement curve that we expect will further improve their accuracy and utility. We are confident that further scientific investment will yield substantial dividends.

}, keywords = {community, Impact, prediction, severe, weather}, issn = {721}, author = {Jeffrey Kepert and KJ Tory and Ching, Eng and Robert Fawcett and Serena Schroeter and W. Thurston and David Wilke and Dragana Zovko-Rajak} } @article {bnh-8253, title = {Coupled fire-atmosphere simulations of five Black Summer fires using the ACCESS-Fire model - Black Summer final report}, number = {705}, year = {2021}, month = {05/2021}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

Context

This research builds on previous work completed during the Coupled fire-atmosphere modelling ACCESS-Fire project, by conducting case studies of five major bushfires that occurred across Australia during the 2019-20 summer. The previous work provided valuable insights into the drivers of periods of extreme fire behaviour associated with the Waroona fire in WA, and the Sir Ivan fire in NSW.

The main objectives of this project are: to investigate the meteorological drivers of extreme fire behaviour at five further fire events; to further test the reliability and stability of the model in different environments across Australia; and to support and share learnings from the events of the 2019-20 summer with partner agencies.

The fire events nominated by state jurisdictions for investigation were: Badja Forest (New South Wales), Stanthorpe (Queensland), Green Valley Talmalmo/Corryong\ (NSW/Victoria), Kangaroo Island (South Australia); and Yanchep (Western Australia).

The case studies show that the local context for each fire is important. Each of the fires posed significant challenges in different ways. In the heavily forested areas of NSW and Victoria, the sheer size, intensity, and duration of the fires were distinctive. At other fires, the proximity of local communities and infrastructure posed serious challenges. All fires were active in the overnight period when conventional fire danger indices typically suggest a decrease in fire activity.

The marine boundary layer, local coastal effects and topography combined to generate complex wind flows that influenced the progression of the Kangaroo Island fire. We also studied the way in which the sea breeze circulation interacted with the Yanchep fire, and examined the impact of a prescribed burn on the rate and direction of spread of the Stanthorpe fire under extreme fire weather conditions.

This project has explored gaps in knowledge and understanding of the processes at play during these unusual events by using the Bureau{\textquoteright}s ACCESS-Fire model to examine the local three-dimensional interactions between the fires and the atmosphere. The learnings from this project may be applied to shape future fire weather products and services.

Method

Based on discussions with our partners in fire agencies, together with documents (including photographs and video) and information on fire spread, fuel types and fuel moisture and atypical fire behaviour, we have run a series of simulations and prepared case study chapters for each of the five fires.

The ACCESS-Fire model was initialised using information provided by fire agencies on ignition location or fire line perimeter and fuel type and availability. Several simulations were conducted for each of the fires, with a focus on different time periods of interest and using updated fire polygon boundaries based on available line scan imagery.

The model configuration does not permit the simulations to account for suppression activities by fire agencies, changes in fuel types on the urban interface, or the impact of roads or other fire breaks in reducing the rate of spread of the fire. Another limitation is that the simulations are a single realisation, when in reality, consideration of forecast uncertainty (for example through running an ensemble scheme) would capture a range of potential outcomes.\ 

Learnings from the case studies reinforce and refine some previous learnings from historical fire events and uncover some new insights into the processes driving the periods of unusual fire behaviour that were observed during the 2019/20 season.

Key findings

The case study analyses have provided valuable insights into meteorological aspects of observed extreme fire behaviour during the 2019/20 summer. Key findings for each of the five case studies are included in the individual chapters for each fire and a synthesis is provided here.\ 

The ACCESS-Fire simulations run at 300 m spatial resolution show that some of the most dangerous effects of the fires, in particular damaging and destructive winds, are fire driven and therefore very localised at a much finer scale than current fire weather products.

The depth, elevation and structure of low-level jets over topography was a critical driver of overnight fire activity.\  Given that these jets are driven by larger scale processes (typically synoptic scale), there is an opportunity to develop new tools based on numerical weather prediction output that highlight the location and intensity of these features. Such tools would provide additional information for meteorologists and FBANs on the strength and height of representative winds for input to two-dimensional fire spread models and could be used to alert fire managers to the risk of atypical fire spread in the overnight period.

A key theme that emerged from the case studies and discussions with stakeholders was the consistent occurrence of heatwaves and their influence on boundary layer structure overnight and on inhibiting fuel moisture recovery during the overnight period. Further research quantifying the effects of overnight heatwaves is required to appropriately plan for the increased risk to communities and fire crews in a changing climate, as the frequency of elevated overnight temperatures has increased over the last few decades and is projected to increase further (along with other factors influencing fire activity).\ \ 

Deep moist pyroconvection producing pyrocumulus (pyroCu) or pyrocumulonimbus (pyroCb) clouds was a feature of the 2019-20 fire season and a record number of sustained outbreaks of pyroCbs occurred.\  However, the five fires examined here were not all associated with pyroCb, highlighting that it is not the sole weather phenomenon associated with extreme fire behaviour. In particular, no lightning was detected within the Green Valley Talmalmo fire (that subsequently became the Corryong fire when it burned into Victoria) as it burned through Green Valley, which shows that dangerous tornado-strength fire-generated winds can occur without pyroCb.

The simulations produced fire-generated extreme winds associated with both rotation and straight-line flow, which present a serious hazard. ACCESS-Fire simulated rotating winds in the updraft of the Green Valley Talmalmo/Corryong fire. More work is required to understand the mechanisms for these, they are likely related to the high intensity of the fire, coupled with topographic influences.

Spot fires were an important component in the observed rate of spread, particularly in the forest fires overnight. The extremely dry, drought-affected fuel beds meant that the ignition efficiency of spot fires was enhanced compared to an average season. Inclusion of a spotting parameterisation in ACCESS-Fire would accelerate the forwards fire spread and improve validation of the simulated fire perimeters.\ 

Coastal processes were evident in the simulations of the Kangaroo Island and Yanchep fires. Localised wind effects in the simulations showed interactions between the fire and maritime airmass including sea breeze circulations can modify the updrafts associated with the fire as well as produce rapid transitions in fire intensity around a fire perimeter. Cooler moister air associated with a maritime airmass may not necessarily reduce fire intensity, depending on its depth and penetration inland.

Utilisation - where to from here?

The high level of engagement in this work from a range of stakeholders has demonstrated the value of this research and the appetite for operational application.\ 

The learnings from this work should be incorporated into training packages tailored for fire meteorologists and FBANs that describe characteristics of the local meteorological environment favourable for extreme fire behaviour. Such training will complement other current research and training on fire behaviour aspects such as spotting and potential for pyroCb.

This work, which has been conducted in close collaboration with partners in fire agencies, has demonstrated that a comprehensive understanding of the mechanisms driving fire behaviour requires a multidisciplinary approach. Similarly, the successful application of fire behaviour and meteorology knowledge in operations requires locally connected specialised expertise, combined with tailored modelling tools to inform objective, evidence-based decisions.

The insights gathered from these five case studies have further demonstrated the value of this high resolution coupled modelling approach. In order to progress ACCESS-Fire to an effective operational tool, more robust testing of the model is required, in addition to the more subjective assessments that have been made through the case study work. This would include technical testing, additional sensitivity testing and routine verification.

This project has demonstrated that the Bureau{\textquoteright}s technology and simulation capability, scientific expertise and established relationships with fire agencies can meet the Australian community{\textquoteright}s need to understand the drivers of fire behaviour during the 2019-20 fire season. In doing so, we have further developed and validated our modelling systems and made progress towards their future operational use.

}, keywords = {ACCESS-Fire, black summer, coupled, fire-atmosphere, simulations}, issn = {705}, author = {Mika Peace and Barry Hanstrum and Jesse Greenslade and Dragana Zovko-Rajak and Abhik Santra and Jeffrey Kepert and Paul Fox-Hughes and Harvey Ye and Tasfia Shermin and Jeffrey Jones} } @article {bnh-6801, title = {Improved predictions of severe weather to reduce community impact}, number = {556}, year = {2020}, month = {03/2020}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

The project has completed the second year of its refresh and is progressing with some challenges. The first 3{\textonehalf} years featured a strong focus on developing underpinning science in some important areas, notably the transport of embers by bushfire plumes, and understanding the origin of the moisture when pyrocumulus clouds form {\textendash} is it from the atmosphere, or from the fuel? We also conducted significant case studies, including an ensemble-based study of a severe east coast low, an analysis of the meteorology of the State Mine Fire of October 2013, and a study into tropical cyclone secondary eyewall formation.

This second phase of the project pivots towards utilization. We will not neglect investigations into the underpinning science, or case studies, but we are looking to turn our previous discoveries and knowledge gained into useful products.

The first of these is a new thermodynamic tool for predicting the potential for pyrocumulonimbus (pyroCb) formation, the Pyrocumulonimbus Firepower Threshold, or PFT. This parameter depends on the atmospheric state, both the wind strength within the boundary layer and the full thermodynamic profile and aims to predict the minimum fire power (in GW) that is necessary to trigger deep convection. In favourable conditions, a fire of a few GW suffices, and at unfavourable times, 1000 GW would be insufficient. We have found that there is substantial variability in the atmospheric favourability in time and space, and that observed pyroCb often form during a narrow window of opportunity. We are presently preparing for a major utilization exercise, a near-real time trial of the tool over the coming summer. We expect that the results from this trial will help to finalize the development of the tool, and ready it for operations.

Our case study of the tornado outbreak in South Australia that triggered events which cut power to the entire state showed that the tornado cluster was predictable by modern high-resolution numerical weather prediction (NWP) {\textendash} indeed, the deterministic model predicted the precise location of one of the observed tornadoes. Encouraged by this success, we ran a 6-member ensemble simulation of the event, which showed that while the probability of tornadoes was well over 50\%, the level and location of the activity was less certain. To assist with analysis of this event, we also developed a novel diagnostic of tornadic precursors in NWP systems. A journal article is almost ready for submission on this work.

A second case study is analyzing the Tathra bushfire on the south coast of New South Wales, which led to the destruction of over 60 houses. Although occurring in strong northwesterly winds in the lee of the ranges, mountain wave activity appears to have been modest at the time of the greatest fire activity and probably not the major cause of the disaster. Rather, the extreme fire behavior appears to have been driven by strong, very unsteady and fluctuating winds, caused by roll-like circulations in the flow off the ranges, with the interaction between this flow and a later wind change also of interest. The weak role of mountain waves in this event illustrates the value of performing detailed case studies, as at first glance, several of the ingredients for a mountain wave event were present, and indeed active earlier in the day.

Unfortunately, our development of a parameterization of long-range ember transport has made little progress this year, due to the unanticipated difficulty of the task and the illness of a staff member. We plan to resume work on that shortly.

In recognition of the delays on the ember transport parameterization, and also of new opportunities that have arisen in pyrocumulus prediction, we have negotiated an altered project schedule with the BNHCRC.

}, keywords = {community impact, predictions, Severe Weather}, issn = {556}, author = {Jeffrey Kepert and KJ Tory and Dragana Zovko-Rajak and David Wilke and Serena Schroeter} } @conference {bnh-6511, title = {A case study of South Australia{\textquoteright}s severe thunderstorm and tornado outbreak }, booktitle = {AFAC19 powered by INTERSCHUTZ - Bushfire and Natural Hazards CRC Research Forum}, year = {2019}, month = {12/2019}, publisher = {Australian Institute for Disaster Resilience}, organization = {Australian Institute for Disaster Resilience}, address = {Melbourne}, abstract = {

On September 28, 2016 one of the most significant thunderstorm outbreaks recorded in South Australia impacted central and eastern parts of the state. Multiple supercell thunderstorms were embedded in a Quasi-Linear Convective System aligned with a strong cold front that was associated with an intense low-pressure system. The storms produced at least seven tornadoes, destructive wind gusts, large hail and intense rainfall. Transmission lines were brought down in four different locations, which contributed to a state-wide power outage.

Accurate prediction and understanding of tornadoes and other hazards associated with severe thunderstorms is very important, for timely preparation and announcement of warnings. By conducting high-resolution simulations, this study aims to offer a better understanding of the meteorology of the South Australian thunderstorm and tornado outbreak. It also contributes to improving knowledge of how to best predict similar severe weather events, which in turn enables better risk management and preparedness for such events. Updraft
helicity, a severe storm surrogate that indicates the potential for updraft rotation in simulated storms, is used to investigate the ability of the model to predict supercell and tornado likelihood.

Download the full non-peer reviewed research proceedings\ from the Bushfire and Natural Hazards CRC Research Forumhere.

}, keywords = {modelling, risk management, Severe Weather, thunderstorm, tornado}, url = {https://knowledge.aidr.org.au/resources/australian-journal-of-emergency-management-monograph-series/}, author = {Dragana Zovko-Rajak and KJ Tory and Jeffrey Kepert} } @article {bnh-4232, title = {Improved predictions of severe weather to reduce community impact: midterm report 2014-17}, number = {315}, year = {2017}, month = {09/2017}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

This report marks the end of the initially-approved 3{\textonehalf} years of the project. To\ mark that milestone, this annual report is much longer than previous annual\ reports, for it is also the final report of that initial part of the project, and as such\ contains detailed descriptions of the six main activities in the first 3{\textonehalf} years.

We have studied the dynamics, predictability and processes of severe weather, including fire weather, with the purpose of improving forecasts of severe weather and better depicting forecast uncertainty in these events, thereby facilitating better risk management and more cost-effective mitigation.

Two of the six main chapters of this report relate to our work with large-eddy modelling of turbulent plumes. We have used this technology, in which the model is run on a 50-m grid to capture the most energetic size range of the turbulent eddies, to both simulate ember transport and to model pyrocumulus formation. The mean travel distance of firebrands depends mainly on wind speed and fire intensity, but the spread in the landing positions shifts from being substantially cross-wind at light winds, to dominantly along-wind at high winds. This spread is greatly increased by the turbulence in the plume, and the maximum spotting distance can be more than double the mean for this reason.\ 

We have also used our plume modelling to study pyrocumulus clouds. We have analyzed the processes that lead to pyrocumulus, with special attention on the relative importance of moisture from two sources, the atmosphere and combustion, and shown that the latter is negligible except in very dry environments. This somewhat controversial result has been confirmed by a conceptual study of the thermodynamics of pyrocumulus formation.

We have prepared three detailed studies of severe weather events. East coast lows are intense low-pressure systems that form over the sea adjacent to the east coast of Australia, most commonly along the New South Wales coast. We analyzed the 20-23 April 2015 event using, for the first time, an ensemble of 24 simulations rather than just a single forecast. The use of an ensemble allows us to better discern the degree of risk, and to account for the inherent uncertainty in any forecast. It also enables insight into the processes that lead to the rapid intensification of these systems.\ 

The Blue Mountains fires of October 2013 were most damaging on the 17th. This was expected to be a day of high fire risk, but the extreme fire spread was not anticipated and the causes were unknown. Our high resolution simulations showed that the downward extension of high upper-level winds to the vicinity of the fire ground, caused by mountain wave activity, was a factor. A dry slot {\textendash} that is, a long, relatively narrow band of dry air {\textendash} which moved over the fire, further contributed to the conditions.

Lastly, we have analyzed a simulation of a secondary eyewall formation and eyewall replacement cycle in a tropical cyclone, yielding better understanding of the underlying processes and the important factors in predicting these developments. Eyewall replacement cycles are associated with marked expansions of the tropical cyclone wind field, leading to a wider damage swath, earlier onset of damaging winds, and increased storm surge and wave damage.\ 

We are pleased that the project will continue for another three years. Our focus will shift to a greater emphasis on utilization activities during this period. In particular, we aim to develop simple methods of calculating ember transport and pyrocumulus development, so as to transfer the knowledge we have developed in these areas into operations. We will also continue to study severe weather events in detail.

}, issn = {315}, author = {Jeffrey Kepert and KJ Tory and W. Thurston and Dragana Zovko-Rajak and Ching, Eng and Robert Fawcett} } @article {bnh-2974, title = {Improved predictions of severe weather to reduce community impact: Annual project report 2015-2016}, number = {169}, year = {2016}, month = {05/09/2016}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

We aim to study the dynamics, predictability and processes of severe weather, including fire weather. We seek also to improve forecasts of severe weather, and to better depict forecast uncertainty in these events, thereby facilitating better risk management and more cost-effective mitigation. Our research into ember transport in smoke plumes and the meteorology of the Blue Mountains bushfires of October 2013 has reached maturity, and journal articles have been written. We continue to develop our work on pyrocumulus clouds, and the east coast low event of April 2015, and have commenced a study of an eyewall replacement cycle in a tropical cyclone. These studies span a wide range of time and space scales and require a range of different methods.

Our ember transport work has confirmed that the mean travel distance of firebrands for a given fire intensity depends mainly on wind speed. However, the spread in the landing positions shifts from being substantially cross-wind at light winds, to dominantly along-wind at high winds. This spread is greatly increased by the turbulence in the plume, and the maximum spotting distance can be more than double the mean for this reason. These sophisticated and computationally intensive calculations can be used to inform the development of physically realistic and computationally cheap parameterizations of ember transport for use in fire models.

We have also used our plume modelling to study pyrocumulus clouds. Intense fire plumes in suitably moist environments can lead to cloud development, with the possibility of strong downbursts {\textendash} one of our simulations is shown on the cover of this report. We have analyzed the processes that lead to pyrocumulus, with special attention on the relative importance of moisture from two sources, the atmosphere and combustion, and shown that the latter is close to negligible. We aim to use the knowledge gained to develop a forecast tool for pyrocumulus formation.

Although the Blue Mountains fires of October 2013 persisted for several weeks, much of the spread occurred on the 17th. While this was expected to be a day of high fire risk, the extreme fire spread was not anticipated and the causes were unknown. Our high resolution simulations showed that the downward extension of high upper-level winds to the vicinity of the fire ground, caused by mountain wave activity, was a factor. In addition, the marked wind change on that day was associated with a dry slot, but the underlying cause of that dry slot seems to be different to previously documented cases.

East coast lows are intense low-pressure systems that form over the sea adjacent to the east coast of Australia, most commonly along the New South Wales coast. We continue our analysis of the event of 20-23 April 2015, using, for the first time, an ensemble of 24 simulations rather than just a single forecast. Collectively, these simulations accurately predict the position and intensity of the low, the strong winds and the rainfall. The differences between them give insight as to the forecast uncertainty, the overall envelope of areas at some risk, and those at highest risk. The ensemble also enables insight into the processes that lead to the rapid intensification of these systems. The risk these systems pose was further illustrated by the two events in June of 2016.

}, issn = {169}, author = {Jeffrey Kepert and KJ Tory and W. Thurston and Simon Ching and Dragana Zovko-Rajak and Robert Fawcett} }