1. Craig Clements San José State University Shaorn Zhong Michigan State University Xindi Bian and Warren Heilman Northern Research Station, USDA Scott Goodrick Southern Research Station,USDA Turbulence Kinetic Energy and Fire-Induced WindsObserved during FireFlux Turbulence Kinetic Energy and Fire-Induced Winds Observed during FireFlux Seventh Symposium on Fire and Forest Meteorology 23-25 October 2007 Bar Harbor, Maine

2. Overview of Talk Photo by M. Patel 1. Observations Fire-Induced Circulations Fire-Induced Circulations Turbulence Kinetic Energy Turbulence Kinetic Energy Other turbulent statistics Other turbulent statistics 2. Summary and conclusions

3. Fire-Induced Surface Winds (Main Tower, 2-m level 1-sec) Time (CST) Thermocouple Vertical Velocity Wind Speed and Direction

4. Fire-Induced Surface Winds (Short Tower 2-m level 1-sec) Wind Speed and Direction Vertical Velocity Thermocouple Damage Convergence Zone

5. Comparison of Surface Winds Outside of Burn Plot

6. Model Comparison Cunningham and Linn 2007 FireFlux 2006

7. Upper Plume Structure (28 m Level) Wind Speed and Direction Vertical Velocity Temperature Water Vapor period of downdrafts

8. Thermal Structure of Fire Plume 250 200 150 100 50 Temperature (C) Downdraft ahead of Fire front

9. Turbulent Eddy Flux (Eddy-Covariance) Transport of a quantity by eddies or swirls. The covariance of a velocity component and any quantity. Net upward heat flux  0 Eddy Mixes some air down And some air up z  ´= _ w´= _  ´= + w´= + (adapted from R. Stull)

10. Main Tower Turbulent Sensible Heat Fluxes Instantaneous heat fluxes = ~0.8-1.0 MW m -2

11. is a measure of the intensity of turbulence simply the summed velocity variances Turbulence Kinetic Energy (TKE) During Fire

12. Photo by Laura Hightower

13. Wind Velocity Variances During Fire

14. IIIIII IV VVIVII I. Time rate change of TKE, or local storage of TKE II. Advection of TKE by the mean flow III. Buoyancy production or destruction IV. Mechanical or shear production V. TKE transport or dispersion VI. Pressure correlation or redistribution VII. Viscous dissipation Turbulence Kinetic Energy Budget

15. Buoyancy Flux During Fire

16. Turbulent Momentum Fluxes During Fire

17. A Conceptual Model for Fire-Atmosphere Interaction Weak convergence zone isotropic anisotropic Downdrafts entrain background air Wind shear causes tilted plume and turbulence generation Shear induced turbulence influences horizontal vortex

18. Fire-induced surface winds were 2-3 times stronger than ambient winds. A convergence region formed downwind of the fire front, but was shorter in duration than expected. Inflow velocities were much weaker than expected. Observed instantaneous upward vertical velocities were on the order of 10 m s -1 and downward vertical velocities = 5 m s -1 Directly measured sensible heat fluxes were ~28.5 kW m -2 occurred at higher levels in the plume rather than near the surface. However, estimated instantaneous heat fluxes at the surface were on the order of 0.8 - 1.0 MW m -2. Summary and Conclusions

19. The observed TKE during the grass fires increased due to the variance in the ambient wind component (fire direction) rather than the contribution from all three velocity components. The turbulence within the upper fire plume, is isotropic and equally driven by both buoyancy and wind shear. While near surface turbulence is anisotropic and driven by variance in the horizontal momentum rather than buoyancy. This suggests that although buoyancy is important, mechanically generated wind shear is responsible for the observed turbulence in grass fires. Summary and Conclusions