Turbulence and Energy Exchange during Wildland Fires in the New Jersey Pinelands
Abstract
Understanding relationships between turbulence and energy exchange during wildland fires is essential for accurate predictions of fire behavior, firebrand production and transport, and smoke dispersion. However, field measurements within and above forest canopies during fires have been limited. We measured turbulence and sensible heat flux using vertical arrays of sonic anemometers and thermocouples on towers during planned wildland fires in the New Jersey Pinelands, ranging from low-intensity backing fires to high-intensity head fires with crown torching. Results from management-scale burns (7 to 185 ha) were compared to small-scale, 100 m2 experimental burns with similar surface fuel loadings and consumption rates. Above-canopy air temperature and vertical wind velocities were enhanced up to 13.8 and 4.3 times in buoyant plumes over fires compared to values measured above unburned control towers, with maximum 10 Hz differences between burn unit and control towers of 195 °C and 6.4 m s-1, respectively. Across all management-scale burns, peak convective heating and vertical wind velocities were highly correlated (r2 = 0.90 to 0.78 at 10 Hz to 1 min integration times), while peak values between heating and horizontal wind velocities were significantly correlated only at 10 Hz. Within individual burns, relationships between convective heating and vertical wind velocities in buoyant plumes were stronger for high-intensity head fires than low-intensity backing fires. Although convective heat flux decreased with height above flame fronts, slopes of the relationship between convective heating and vertical wind velocity increased with height for both management-scale and small-scale experimental burns. Integrated 10-minute sensible heat fluxes were much greater for head fires than for backing fires, averaging 14.8 kW m-2 vs. 2.2 kW m-2, respectively. When integrated over the duration of flame front passage and smoldering consumption, sensible heat fluxes were typically within the range of convective heat flux estimated from fuel consumption measurements. Our research provides key information to parameterize physics-based fire behavior models, and contributes to a better understanding of firebrand transport and plume dispersion during wildland fires.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A23J2943C
- Keywords:
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- 3379 Turbulence;
- ATMOSPHERIC PROCESSES;
- 3390 Wildland fire model;
- ATMOSPHERIC PROCESSES;
- 4301 Atmospheric;
- NATURAL HAZARDS;
- 4313 Extreme events;
- NATURAL HAZARDS