Dynamics of wildfire and atmospheric boundary layer interaction and implications for extreme fire behaviour under quiescent weather conditions
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Much remains unknown how the atmospheric stability profile influences updraft acceleration or deceleration within fire plumes, which influences local horizontal convergence and divergence and fire intensity. Therefore, there is a need to address largely unanswered and unexplored questions such as how vertical variations in stability affect wind speed at the ground. The main aim of this thesis is to improve understanding of the fundamental role of the atmospheric boundary layer (ABL) stability in the characteristics of a wildfire's convective plume and associated feedback processes.
Chapter 1 provides background information on current knowledge of fire-atmosphere interactions; more specifically the influence of ABL stability on the convective plume dynamics of wildfires and basic fire-induced circulations. ABL characteristics and the basic concept of atmospheric stability are also reviewed.
Specific research questions addressed in Chapter 2 was: what atmospheric stability profiles and ABL structures does Advanced Regional Prediction Systems (ARPS) model simulate in complex terrain under weak synoptic forcing to investigate fire-atmosphere interaction? The main goal of Chapter 2 is to use an atmospheric numerical model to investigate ABL temperature and wind structures associated with weak synoptic forcing in mountainous terrain, in preparation for undertaking simulations of wildfires in complex terrain in the following chapter. The selected model configuration successfully produced the diurnal evolution of temperature and wind structures in the ABL over the valley.
A research question addressed in Chapter 3 was: what atmospheric processes are involved when the convective plume of wildfires interacts with ABL over complex terrain? The main goal of this chapter is to identify processes associated with fire-atmosphere interaction in complex terrain dominated by a thermally-driven diurnal mountain wind regime using a hot patch in the ARPS model to represent effects of the intense surface heating by wildfires. Fire- induced circulation was characterised by inflow layers, that is relatively cool mixed layers and overlying warmer return flow layers, penetrative convection over the surface convergence zone, subsidence and associated internal gravity waves.
A research question addressed in Chapter 4 was: how does interaction between the identified fire-induced circulation and the ABL stability affect surface wind variability that primarily controls fire spread and intensity? A primary goal of this chapter is to gain more insight into how the atmospheric stability profile influences the entire fire-induced circulation and surface heat flux using a series of idealized 2-D simulations. It was found that the role of K- H instability at the inflow-return flow interface was to cause a density difference in the inflow layer as a result of mixing. This variation in the inflow layer depth generated the surface u- velocity maxima at the surface directly underneath the wave trough where the K-H waves formed due to vertical transport of momentum and the surface wind reversal below the wave crest due to a hydraulic jump. Results suggest that the combination of a deeper initial mixed layer, stronger surface heating, and a weaker capping inversion is favourable for the formation of surface wind reversal near the fire and thus extreme fire behaviour potential.
A research question addressed in Chapter 5 was: how does the third dimensionality in the ARPS model affect the formation of surface wind reversal near the fire as simulated in the 2-D models in Chapter 4? Overall, thermodynamic and flow structures associated with the feedback processes between free convection and the ABL remained very similar and are thus not very sensitive to the model dimensionality. Thus likely to be reasonable from the 2-D simulation. However, the overall magnitude of fire-induced circulations was lower in the 3-D simulation as compared to the 2-D counterpart. Surface wind reversal associated with the density variation within the inflow layer, a hydraulic jump phenomenon, and rotor formation were key findings from the 2-D simulations in Chapter 4, and these features were generally reproduced in the 3-D simulation but with a much reduced reversed surface flow depth and weaker reversed flow velocity.
In Chapter 6, a summary of key findings is presented and the major contributions to generation of new knowledge in the subject area are discussed. The use of the coupled fire- atmosphere model, even though idealised and simplistic, is shown to be valuable for better understanding of the complex relationship between ABL stability and plume dynamics. It is concluded that a developed fire-induced circulation involving penetrative convection can result in surface wind reversal that could cause extreme fire behaviour by convective heating of unburnt fuel without the influence of background mean flow. Limitations of this work and some suggestions for future research are made at the end.