Superposition of Smoke‐Induced Density Currents and Diurnal Wind Systems During the California Mosquito Fire

Abstract Wildfire smoke can drive intense near‐surface density currents that rapidly inundate populated areas with hazardous air, producing extreme and prolonged smoke exposure with serious health consequences. Yet, the dynamics of these flows remain poorly understood. We document two such events during the September 2022 Mosquito Fire, when smoke‐driven currents caused abrupt and severe air‐quality degradation along the east side of the Sierra Nevada. Observations from a dense PurpleAir sensor network, meteorological instruments, and Doppler LiDAR revealed sharp PM 2 . 5 increases of 200–1,000 μg m −3 , temperature drops of 3–4°C, and wind shifts characteristic of density‐current propagation. The smoke currents evolved in concert with the regional Washoe Zephyr, illustrating how local radiative forcing and regional thermally driven flows can interact to organize and intensify smoke transport. The observed cooling—arising from strong radiative heat loss within optically thick smoke—enhanced horizontal density contrasts that drove and sustained the advancing flow. The currents exhibited a canonical structure: a sharp leading edge, a turbulent head with vigorous mixing, and deep smoke‐filled air intruding beneath the prefrontal layer. Propagation speeds of roughly 4 m s −1 matched theoretical scaling based on temperature deficits, current depth, and opposing winds. These findings show that radiative cooling within dense smoke layers can amplify and sustain smoke‐induced density currents, prolonging hazardous air‐quality impacts across complex terrains.