Formation of Superfog from Wildland Fires - Theoretical and Physical Modeling

Smoke from prescribed burns can occasionally cause significant reduction in visibility on highways in the southern United States. Visibility reduction to less than three meters has been coined "superfog" and initial conditions for its formation have been developed previously. Accurate characterization and prediction of precursor conditions for superfog is needed to prevent dangerous low visibility situations when planning prescribed burns. It has been hypothesized that extremely hygroscopic cloud condensation nuclei from the smoldering phase of a fire can produce a large number of droplets smaller in size than in naturally occurring fog producing superfog conditions at relatively low liquid water content. A thermodynamics-based model for fog formation was developed. Laboratory generated superfog measured by a Phase Doppler Particle Analyzer determined that mean droplet radius was 1.5 mum and the size distribution could be modeled as a log normal distribution. Experiments in an environmentally-conditioned wind tunnel using longleaf pine needle fuel beds provided visibility, heat flux, temperature, humidity, and particle production data for model verification. Numerical modeling was used to approximate the growth of a superfog boundary layer with CH2O values of 2 g m-3 or greater in the Superfog Analysis Model (SAM) which successfully predicted previous superfog events.