Extended Abstract (312K)4M.4
The mesoscale analysis and prediction of fire weather
Joseph J. Charney, USDA Forest Service, East Lansing, MI
Recent advances in the theoretical understanding of fire-atmosphere interactions and fire weather prediction have opened up broad new areas of active research into the mesoscale processes involved with fire events. By employing established mesoscale models and mesoscale analysis techniques, the atmospheric structures that can promote or inhibit extreme fire behavior are being explored in greater detail. This presentation will describe the new fire weather prediction tools, techniques, and theoretical developments that have been developed in recent research efforts. These fire weather prediction elements will then be associated with atmospheric predictability by comparing theoretical and simulated atmospheric structures that can influence fire behavior with both meteorological observations and observed fire behavior.
Mesoscale models have played a key role in the recent development of mesoscale fire weather predictions. By analyzing daily, real-time mesoscale atmospheric simulations and building upon atmospheric parameters developed to address severe weather, such as convective available potential energy (CAPE), certain types of erratic and extreme fire behavior can be associated with atmospheric stability and moisture characteristics. Additionally, the planetary boundary layer (PBL) processes routinely predicted by mesoscale models are intrinsic to fire plume behavior, smoke dispersion, and fire-atmosphere feedback processes that can have a profound impact on fire behavior. By developing modified CAPE calculations and exploring the development of PBL structures that can lead to the sudden appearance of dry, windy conditions at the ground, mesoscale models contribute to firefighting activities and potentially enhance firefighter safety.
Several important theoretical developments have accompanied the advances in recent modeling studies. First, the impact of the water vapor released in the combustion process on the ensuing plume growth and the magnitude atmospheric feedback on the fire has been addressed. Theoretical calculations and field observations suggest that the water vapor released by combustion can substantially augment the environmental moisture, leading to different plume rise, condensation, and instability mechanisms above a fire than would be expected from the environment alone. Additional studies have investigated the unbalanced circulations associated with jet streak dynamics and interactions, and the influence of mesoscale atmospheric boundaries such as fronts and sea breezes on observed fire behavior.
The new emphasis on mesoscale fire weather and predictability has contributed to the development of coupled atmosphere-combustion model and the analysis of their results. These models explicitly resolve the combustion process to predict the energy exchanges between the fire and the atmosphere, and allow both systems to dynamically and non-linearly adjust to each other. Understanding the mesoscale processes that contribute to the simulated fire-atmosphere interactions in this new generation of coupled models plays a vital role in the analysis and validation of these tools.
Mesoscale predictability will be explored in the context of how the processes and models described above are compared against actual fire behavior observations on the ground. Fire behavior prediction and analysis has been carried out by firefighting organizations for many years, but most of the fire behavior research focus has been on fuel impacts on fire behavior. While the atmosphere is acknowledged as having the potential to have a major impact on fire behavior, the nature of that impact is often a poorly understood. Comparisons between observed fire behavior and mesoscale predictions (models and theoretical predictions) of environments that can promote certain type of fire behavior can yield new insights into the predictability of the atmosphere. Specifically, the fact that fire behavior observations, particularly for large fires, are collected routinely and are often NOT thoroughly analyzed in the context of atmospheric processes means that there is a large, virtually untapped dataset of observations in existence that can be used to validate or understand mesoscale processes. The mesoscale processes associated with some extreme fire events also appear to have synoptic precursors that can be detected up to five days or more in advance of the fire event, which is an additional consideration for mesoscale predictability that is important for fire weather forecasting and research.
Session 4M, Mesoscale Applications Using Numerical Models
Thursday, 27 October 2005, 10:30 AM-12:15 PM, Alvarado GH
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