92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Monday, 23 January 2012: 2:00 PM
Numerical and Physical Investigation of the Properties of Superfog
Room 339 (New Orleans Convention Center )
Christian Bartolome, Univ. of California, Riverside, CA; and M. Princevac, A. Venkatram, S. Mahalingam, G. Achtemeier, D. R. Weise, H. Vu, and G. Aguilar

Smoke and haze resulting from wild and prescribed fires are of great concern because of their impacts on air quality and visibility. Tragic events on the Interstate-4 in Florida and Interstate-10 in Mississippi in 2008 have resulted from extreme visibility reduction caused by smoke and fog combinations from nearby prescribed burns traveling over roadways. Such events of extreme visibility reduction, to less than 3 meters, are referred to as “superfog”. When planning prescribed burns, it is important for land managers to recognize conditions that will result in superfog. The physical modeling is conducted in a custom designed chamber with controlled mixing of hot, wet and smoky air from one side and dry and cold air from the other side of the chamber. Superfog forms when water vapor from combustion, live fuel, ambient and soil condenses on smoke particles as ambient air cools. Empirical relations developed for naturally occurring advection fogs relate visibility to the liquid water content (LWC). These relations suggest a relatively large LWC (~ 5g m-3) needed for superfog formation. Such high LWC is thermodynamically difficult to achieve and is possible in the atmosphere only under extreme conditions. It has been hypothesized that extremely hygroscopic cloud condensation nuclei (CCN) from the smoldering phase of a fire can produce a large number of droplets smaller in size than in naturally occurring fog. Consequently, it is feasible to achieve superfog conditions at relatively low LWC (~2 g m-3) superfog. Laboratory generated fogs resembling near superfog visibilities have been measured by a Phase Doppler Particle Analyzer (PDPA) system to determine particle number density and size distribution. Measurements indicate that mean droplet diameter of 3μm was larger than expected for superfog while producing similar low visibility conditions. A sensitivity study of droplet size distributions and number densities was carried out to understand the impact on visibility and LWC. It was found that lower LWC (~2 g m-3) is sufficient to form superfog when droplets of mean diameter of 2 μm or less, with log normal size distribution, are present. It was found that the required number of droplets has to be larger than 105 droplets per cubic centimeter. If droplet size distribution has large deviation in radii, then much larger amounts of LWC are needed for significant visibility reduction. The presence of CCN from smoke has a significant impact on the droplet size distribution. Superfog formation is modeled for various concentrations of solute CCN. It was found that for high concentrations of solute pollutants, water vapor will readily condense to a large number of droplets to form superfog. In this presentation modeled and measured effects of the pollutant CCN concentration, LWC and droplet size distribution on visibility will be discussed.

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