Development of a New Superfog Screening Tool through Theoretical, Experimental and Numerical Investigation

Major car pile ups, such as ones in Florida on the Interstate Highways 4 and 75 in 2008 and 2012, have resulted from the formation of a Superfog.  Superfog is dense smoke cloud that reduces visibility to less than 3 meters caused by interaction of smoldering wildland fire emissions with ambient environment. Here we will present theoretical, numerical and laboratory modeling efforts that have been conducted to explain the origins of Superfog.  Thermodynamics of water vapor, droplet size distribution, liquid water content, extinctions coefficients and boundary layer development were explored.  Laboratory measurements of smoldering smoke and smoke boundary layer growth have been conducted for various environmental and fuel conditions.  In these experiments the favorable conditions for the Superfog formation were determined to be: fog droplet sizes less than 1 mm, minimum cloud condensation nuclei concentrations of 10⁵ per ccm, liquid water content greater than 2 g kg^-1, ambient temperature less than 40^oF, ambient humidity greater than 80%, fuel moisture contents greater than 40% by mass, and wind velocities less than 1 m s^-1 (2.2 mph).  Based on these results the Superfog Analysis Model (SAM) has been developed to aid land managers to quickly assess situations as favorable or unfavorable to the formation of Superfog.  SAM has been validated by laboratory experiments and has been successful in predicting previous Superfog events.  The theoretical formulations, experiments and the SAM development and features will be presented.