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Appendix 3 provides a discussion on the simulation process, from ignition, through to build-up, spotting and fire spread.

1.3 What is PHOENIX

PHOENIX is a bushfire characterisation model that integrates fuel, terrain, weather conditions and suppression to simulate a fire's development and progression in the landscape. It is used by land and fire managers to support fire management and land-use planning and to support decision making during bushfires.

PHOENIX is a mechanistic continuous, dynamic, empirically-based model that simulates fire characteristics such as fire spread, flame height, intensity, size and ember density and stores the results in a database (using spatially gridded data), and can also simulate some the effects of suppression efforts and the impact of fire on various values and assets.

At a minimum, PHOENIX requires fuel data as an input. However, there are several inputs, including terrain, weather, suppression, fire history and assets that are required for realistic simulations. These inputs must be prepared correctly in a spatial format.

PHOENIX produces a range of outputs including fire spread, intensity, flame height, ember density, burn frequency and asset impact. The outputs of a simulation can be viewed in GIS, Google Earth, as images and in spatially gridded data provided in ASCII, XML, CSV and Shapefile file formats.

1.4 How PHOENIX works

PHOENIX is a standalone executable program designed for operation under Microsoft Windows, but can also be easily converted to Mono to run on UNIX and LINUX operating systems. It has a graphical user interface (GUI), but can also be controlled via command line to allow integration with other software systems. It is standalone software that does not need to be installed and therefore can be run directly from a device such as a USB storage device. It is also designed to support running across more than one computer (where one computer is the master) or processor, for more complex simulations requiring more processing power.

In order to gain an overview of how the simulation process in PHOENIX works, an overview is offered below based on Figure 1. A more detailed explanation is offered in Appendix 3: The Simulation Process.

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To trigger a simulation the user specifies an ignition time and location. This is used to access the underlying input layers such as fuel, terrain and weather, in order to start the fire behaviour calculations.

Each new fire (including spot fires) has an initial build-up phase. The rate of build-up is based on the conceptual model described by Cheney (1981) modified so that the proportion of the available fine fuel and the wind speed affecting the build-up rate of spread is related to the proportion of the build-up.

Each point on the perimeter is dealt with individually using Huygen's spread principle (Andersen et al. 1982). Flame height is used to determine which fuel strata are incorporated in the fire behaviour. For non-grassland fuel types, flame height and maximum spotting distance for each point are based on a modified McArthur model (McArthur 1967), fire intensity and spread rates are based on a unique dynamic fire spread algorithm developed for PHOENIX and a modified CSIRO grassland model (Cheney et al. 1998) for fuel types that have no elevated fuel component.

The convection model in PHOENIX assumes that convection columns will form over the hottest areas of a fire. It uses this information, in conjunction with (terrain modified) wind speed and direction to determine potential ember impact patterns.

The convective strength and amount of bark fuel of each heat centre are used to determine the quantity of embers launched and the expected travel time for the embers once aloft. PHOENIX calculates the probability of embers igniting a spot fire. Once the probability of ignition is high enough, a spot fire is initiated. Any spot fires are run as independent fires and start by going through a build-up phase.

Perimeter expansion is modelled in discrete time steps. At the end of each time step, the fire perimeter is checked for any 'tangles' or coalescence with another fire such as a spot fire. As a perimeter grows additional vertices are added to the perimeter if required to keep the distance between each perimeter point less than the size of the Fire Grid defined for the simulation.

The computational sequence is now repeated for the new time step with fuels, weather and terrain conditions reassessed to find current conditions.