@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} } @conference {bnh-4114, title = {Mesoscale features related to the Blue Mountains fires of 17 October 2013 revealed by high resolution Numerical Weather Prediction (NWP) modelling}, booktitle = {5th International Fire Behaviour and Fuels Conference}, year = {2016}, month = {04/2016}, publisher = {International Association of Wildland Fire}, organization = {International Association of Wildland Fire}, address = {Melbourne}, author = {Simon Ching and Robert Fawcett and W. Thurston and KJ Tory and Jeffrey Kepert} } @article {bnh-2386, title = {Improved predictions of severe weather to reduce community impact: Annual project report 2014-2015}, number = {150}, year = {2015}, month = {11/2015}, 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. So far, we have studied ember transport in smoke plumes, pyrocumulus clouds, the meteorology of the Blue Mountains bushfires of October 2013, and the east coast low event of April 2015, four studies which span a wide range of time and space scales and require a range of different methods.

We are building on our existing modelling of bushfire plumes by using it to study ember transport.\  Embers are added near the base of the plumes and their trajectories calculated from the model winds. \ At higher wind speeds the embers travel further downstream than at lower wind speeds, as expected.\  However, the lateral spread is much broader for lower wind speeds. \ Understanding the spread in landing positions will facilitate the development of computationally affordable and physically realistic means of calculating the expected spotfire distribution in fire spread models.

We have further extended our plume modelling to begin a study of pyrocumulus development. Intense fire plumes in suitably moist environments can lead to cloud development, with intense updrafts and if rain develops the possibility of strong downbursts.\  Such pyro-convection may lead to enhanced and unpredictable fire spread, increased ember transport and spotting, and further ignitions from pyrocumulonimbus lightning. \ Our simulations produce realistic clouds, including the formation of rain and strong downdrafts. \ We will now examine some more cases, with the eventual aim of providing a preliminary forecast tool for pyrocumulus formation.

Although the Blue Mountains fires of October 2013 persisted for several weeks, much of the spread occurred on a single day. While this was expected to be a day of high fire risk, the extreme fire spread was unpredicted and the causes unknown. \ Our high resolution simulations helped identify the downward extension of high winds aloft in the vicinity of the fire ground, due to mountain wave activity. \ In addition, the marked wind change on that day was associated with a dry slot, known to worsen fire behaviour due to extremely low humidity.

East coast lows are intense low-pressure systems that form close to the east coast of Australia, most commonly along the New South Wales coast. They can produce severe wind, wave and flood impacts as in the event of 20-23 April 2015, which we are studying. For the first time, we are conducting this study with an ensemble of 24 simulations, rather than just a single forecast.\  Each simulation begins from a slightly different initial state, giving 24 different, but plausible, forecasts that represent a range of possible outcomes. Collectively, these simulations accurately predict the position and intensity of the low, the strong winds and the rainfall. \ The differences between them will be analysed to determine how predictable aspects of the event were.

}, issn = {150}, author = {Jeffrey Kepert and KJ Tory and W. Thurston and Simon Ching and Robert Fawcett} } @article {bnh-2404, title = {Improved predictions of severe weather to reduce community risk}, year = {2015}, author = {Jeffrey Kepert and W. Thurston and Simon Ching and KJ Tory and Robert Fawcett} }