Laboratory and Numerical Modeling of the Formation of Superfog from Wildland Fires

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, September 2014
Dr. Marko Princevac, Chairperson

Prescribed burns are a common tool used by wildland managers to reduce hazardous fuel accumulations, enhance
wildlife habitat, and stimulate plant regeneration.  In 2011, an estimated 8.18×10⁶ha were treated with prescribed fire in the United States, 2.62×10⁶ha were burned for forestry purposes in the southern U.S. Smoke management for prescribed burning has long been a concern because of the potential  impacts on air quality and visibility.  In rare cases a combination of smoke and fog has crossed over major roadways leading to visibility less than 3 meters, a condition known as superfog resulting in traffic accidents.  On the morning January 9, 2008, on the I-4 in Polk County Florida, a superfog event resulting from a nearby prescribed fire caused a 70 car pileup which resulted in 5 fatalities and 38 injuries.  In 2011 wildfires caused low visibility events resulting in numerous highway closures over a 3  month period at the Great Dismal Swamp National Wildlife  Refuge.  There  were isolated vehicular accidents caused by low visibility despite the best efforts of highway management.  In December 2011 marsh wildfire smoke caused superfog conditions leading to a major car pileup on the I-10 in New Orleans, LA. The accident caused 2 deaths and 61 injuries.  In January 2012 a superfog event formed from a nearby wildfire on the I-75 near Gainesville, FL.  The pileup included 7 semi-trucks and 12 cars.  This tragic incident claimed 10 lives and left 21 injured.  Research on the physics and conditions for superfog formation were investigated to assist land managers in the planning of safe prescribed burns in the future.

Superfog is currently hypothesized to form during the smoldering phase of a wildland fire in the night hours.  The smoldering phase releases primarily water vapor and particles that can act as cloud condensation nuclei (CCN).  Mixing between the cool ambient air, hot water vapor, and CCN will lead to condensation into droplets.  The presence  of  numerous  droplets  in  air  causes  extreme  light  scattering,  thereby  reducing  visibility.    Physical modeling  of  droplet  formation  and  measurements  of  droplet  size  distributions  has  been  investigated.    A combination of 2-dimensional advection diffusion modeling with heat and vapor flux from soils have been applied for  stable  boundary  layer  conditions.    This  model  has  been  developed  into  an  executable  program  for  land managers known as the Superfog Assessment Model (SAM).

Laboratory  experiments  in  a  wind  tunnel  using  pine  needle  fuels  beds  investigated  the  impacts  of  ambient temperature, humidity, fuel moisture content, and fuel bed configuration.  Heat and vapor fluxes from smoldering fuel beds were measured to be approximately 1.6 × 10^-3g m^-2s^-1 and 1 W m^-2.  Boundary layer experiments in the wind tunnel were used to validate the SAM model.

Through  physical  modeling  and  laboratory  experiments  parameters  likely  to  form  superfog  have  been  found. Superfog is likely to form when the wind speeds is less than 1.0 m s^‑1, temperature is less than 5^oC, relative humidity is greater than 80%, fuel moisture content is greater than 40%, particles with size 1 mm or smaller, and concentration of particles 10⁵cm^-3 or greater.