@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} }