Cold Smoke: Density currents cause unexpected smoke transport from large wildfires

Doppler lidar observations of a smoke-filled density current emanating from a large wildfire in northern California are presented (e.g., Fig. 1). The density current caused a surface-based layer of optically thick smoke to unexpectedly spread against the mean wind, covering a distance of ~25 km. Density currents of this sort constitute a previously unobserved mechanism for smoke transport and are unlikely to be well resolved by operational forecast models. Our analyses show that the density current results from differential heating between smoke-free and smoke-filled portions of the atmospheric boundary layer. The leading edge of the thermally direct circulation forms a meso-front, which exhibits clear density current characteristics including and elevated head and Kelvin-Helmholtz waves. Using thermodynamic and kinematic observations we find good agreement between the frontal structure and density current theory. This result, combined with scaling arguments, conclusively demonstrates the observed circulation is due to thermal contrasts and that smoke particulate loading is not important in this case. Additional photographic observations of a smoke-filled density current from the King Fire in September 2014 are used to examine the broader implications our findings on smoke transport processes near large wildfires.