@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} }