The HighFire Risk Project
|Citation||McRae, R.H.D. & sharples, J.J. (2011). Modelling the Thermal Belt in an Australian Bushfire Context. Proceedings, MODSIM 2011 19th World IMACS/MODSIM Congress, Perth.|
Modelling the Thermal Belt in an Australian Bushfire Context.
|Author(s)||Richard H. D. McRae & Jason J. Sharples.|
|Abstract||On many occasions fire crews tasked to suppress fires at night in rugged landscapes have been surprised by fire intensity on mid slopes. While it has long been stated that this is due to a thermal belt, no studies have been done into how this occurs or the magnitude of its impact on fire behaviour.|
Radiative heat loss at night leads to cooling of surface air and subsequent density-driven drainage and pooling. This can produce a nocturnal inversion below the mid-slopes of a valley, and has been the subject of numerous studies of its effects on agriculture. The resultant pattern in weather can also be significant for fire managers. Mid slopes can be significantly warmer and drier than valley floors or ridgetops, and are termedthe thermal belt. The formation of a thermal belt requires a continental air mass and light or no winds.
A terrain model has been developed to show the formation pattern and intensity of the thermal belt. It uses adiabatic lapse rates, slope position and drainage patterns to model distinct dynamic elements of the system. When combined these provide a useful indication of the extent of the belt.
Initially the model looks at a sunset temperature reading and uses the dry adiabatic lapse rate to estimate temperature across a local catchment. An ambient cooling rate of about 1°C per hour applies to the entire system. Then the effects of radiative heat loss from the ground are considered. The portion of this that is transferred to air in contact with the ground makes that air denser. This air then flows along fall lines until it reaches a drainage line. Air flowing along the drainage becomes density stratified due to unequal sideslope drainage times. The time taken to reach the exit-point of the catchment can be readily estimated in the field.
An additional consideration is pooling of cold air in the low points of the landscape. Some straightforward geometric techniques can be used to estimate the pooling depth at any point with time, as drainage flows from an increasing fraction of the area arrive.
While the current model is empirical, there is a need for further work towards a physical model. The model successfully indicates the formation of a thermal belt with time after sunset. As the thermal belt develops, and using assumptions about variations in dew point temperature, it possible to use the model to estimate changes in fire behavior over time, across the catchment. More work is underway to link this model into bushfire management requirements.
|Keywords||Intensity of Thermal Belt, Radiative Cooling, Rugged Terrain, Bushfire Management.|