The Driving Thermodynamic Force of Pyrocumulus Formation

Large wildland fires can modify the local environment by altering the sensible and latent heat fluxes, inducing local pressure gradients, and by releasing particulate matter. This influence can be as small as a subtle change in local winds or as large as the development of a pyrocumulonimbus. The latter is being modeled and analyzed within this study.

Uncertainty remains within the wildland fire research community about whether sensible and/or latent heat drives the development of pyrocumulus. The main objective of the present study is to identify whether the moisture source for pyrocumulus development is primarily composed of moisture released by the fire or from environmental entrainment. Through the use of WRF-fire, a fire behavior model coupled with the Weather Research and Forecasting model, both sensible and latent heat will be evaluated for the Region 23 Complex fire in northwestern Nebraska. Output from the model simulations will be used to create a moisture budget to isolate the respective contributions to pyrocumulus development. WRF will be run in Large Eddy Simulation(LES) mode with 60 meter grid spacing. This provides a suitable modeling environment for analyzing how topography, fire fuels, and weather all played a role in the formation of pyrocumulus. The main objective is to identify whether the moisture for pyrocumulus is from the fire or from environmental entrainment. A control run without fire will provide a base line for comparison during analysis. The expected results of the study would be to show that the main thermodynamic force driving pyrocumulus formation is latent heat released from the moisture produced by the fire.