Wildfire Smoke Shading Observations: Impacts on Boundary Layer Mixing and Thermally Driven Smoke Transport

Abstract

Smoke shading from wildfire smoke cools the surface and atmosphere below the smoke by absorbing and scattering sunlight. This has the potential to create meteorological feedback and large air quality impacts, but observations of smoke shading and induced local flows are rare. Mobile truck-based Doppler lidar observations during the California Fire Dynamics Experiment (CalFiDE) in August and September 2022 provide a novel data set of smoke shading events. The truck-based lidar conducted measurements of horizontal and vertical winds while underway, along with qualitative smoke backscatter, allowing a study of boundary layer (BL) dynamics in and out of the smoke shaded areas. An in situ temperature sensor on the truck recorded air temperature. Smoke shaded versus clear-sky temperature differences of up to 8°C were observed over 12 km along a valley floor in complex terrain. Using vertical velocity variance as a measure of turbulent mixing, smoke shading reduced BL mixing by up to two orders of magnitude, preventing the development of an afternoon convective BL in some cases. Temperature gradients also resulted in thermally driven flows wherein the cold smoky air mass undercut warmer ambient air. One density current case was observed with a strong opposing flow resulting in a convergent updraft and shear-induced vortices atop the smoke-filled density current. In another case, the flow was impeded by complex terrain resulting in a slower-moving smoke front and a temperature gradient that lagged behind the front.