@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-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} } @article {bnh-5432, title = {Impact-Based Forecasting for the Coastal Zone: East Coast Lows- Annual Report 2017/2018}, number = {463}, year = {2019}, month = {03/2019}, pages = {1-28}, type = {Report}, keywords = {Built Environment, Forecasting, Severe Weather}, issn = {463}, author = {Harald Richter and Craig Arthur and Serena Schroeter and Martin Wehner and Jane Sexton and Beth Ebert and Mark Dunford and Jeffrey Kepert and Shoni Maguire and Russell Hay and Mark Edwards} } @conference {bnh-4792, title = {Impact-based forecasting for the coastal zone}, booktitle = {AFAC18}, year = {2018}, month = {09/2018}, publisher = {Bushfire and Natural Hazards CRC}, organization = {Bushfire and Natural Hazards CRC}, address = {Perth}, abstract = {

Strong surface wind gusts and heavy rain are meteorological hazards that are predominantly produced by storms such as east coast lows, tropical cyclones or thunderstorms. Interest in these hazards from a response agency point of view lies in their impact on the natural and built environment. At present, weather forecast models still predict mostly {\textquoteright}raw{\textquoteright} meteorological output such as surface wind speeds at certain times, or rain accumulations over a specified period. This model output needs to be combined with exposure and vulnerability information to translate the forecast hazard into predicted impact.

The Bushfire and Natural Hazards CRC project Impact-based forecasting for the coastal zone: East-Coast Lows attempts to demonstrate a pilot capability to deliver impact forecasts for residential housing from an ensemble of weather prediction models runs. The project is a collaborative effort between the Australian Bureau of Meteorology and Geoscience Australia.

The project is initially focusing on the wind and rainfall impact from the 20-22 April 2015 east coast low event in NSW. The wind and rainfall hazard data are provided by a 24-member ensemble of the ACCESS model on a 1.3 km grid, with damage data acquired from NSW State Emergency Services (SES) and the Emergency Information Coordination Unit (EICU) for the 2015 event.

We will show that the multi-hazard nature of an east coast low event makes attributing the observed building damage to a single hazard difficult. Wind damage to residential housing in this case is largely due to tree fall. This {\textquoteright}damage-byintermediary{\textquoteright} mechanism requires not just the knowledge of building properties in an exposed area, but also additional knowledge of the surrounding vegetation and its response to strong winds. We will discuss enhancements to the SES/EICU damage survey templates that would lead to improvements in the development of the hazard-damage relationships.

}, author = {Harald Richter and Craig Arthur and Serena Schroeter and Martin Wehner and Jane Sexton and Beth Ebert and Mark Dunford and Jeffrey Kepert and Shoni Maguire and Russell Hay and Mark Edwards} }