Craig Clements San Jos é State University Shaorn Zhong Michigan State University

1. Turbulence Kinetic Energy and Fire-Induced WindsObserved during FireFlux 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 Seventh Symposium on Fire and Forest Meteorology 23-25 October 2007 Bar Harbor, Maine

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

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

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

5. Comparison of Surface Winds Outside of Burn Plot

6. Model Comparison FireFlux 2006 Cunningham and Linn 2007

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

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

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

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

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

12. Photo by Laura Hightower

13. Wind Velocity Variances During Fire

14. Turbulence Kinetic Energy Budget IV I II III V VI VII 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

15. Buoyancy Flux During Fire

16. Turbulent Momentum Fluxes During Fire

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

18. Summary and Conclusions • 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.

19. Summary and Conclusions • 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.