Abstract
Abstract. The first observations of smoke-induced density currents originating from large wildfires are presented. Using a novel mobile Doppler lidar and additional in situ measurements, we document a deep (~ 2 km) smoke-filled density current that propagates more than 25 km at speeds up to 4.5 m s−1 near a large forest fire in northern California. Based on these observations we show that the dynamics governing the spread of the smoke layer result from differential solar heating between the smoke-filled and smoke-free portions of the atmospheric boundary layer. A calculation of the theoretical density current speed agrees well with the observed propagation speed. Additional lidar and photographic documentation of other smoke-filled density currents demonstrate that these previously unknown phenomena are relatively common near large wildfires and can cause severe and unexpected smoke inundation of populated areas.
Highlights
Smoke from forest fires adversely affects human health (Johnston et al, 2012), reduces visibility, and alters the earth’s radiative energy balance (Penner et al, 1992)
Using a novel truck-mounted Doppler lidar, radiosonde system, and automatic weather station (Clements and Oliphant, 2014), we show that these density currents form due to differential solar heating between smoke-filled and smoke-free portions of the atmospheric boundary layer and can unexpectedly spread smoke counter to the ambient wind and over large distances (∼ 30 km)
Satellite images show a widespread upslope southwesterly flow developing in the early afternoon of 20 September, which is likely a manifestation of the Washoe Zephyr, a thermally driven wind system that penetrates across the crest the Sierra Nevada and becomes a downslope afternoon wind (Zhong et al, 2008). While this broader wind reversal is important in the displacement of the smoke layer, it is quite clear from the photographic sequence that the local spread characteristics are strongly affected by its density current dynamics
Summary
Smoke from forest fires adversely affects human health (Johnston et al, 2012), reduces visibility, and alters the earth’s radiative energy balance (Penner et al, 1992). Mesoscale circulations of this sort have been observed due to heterogeneous snow cover (Johnson et al, 1984), shading by thunderstorm anvils (Markowski et al, 1997), and gradients in soil moisture and surface albedo (Rife et al, 2002) Their impact on pollution transport, and in this case smoke dispersion, is a topic of considerable interest because their flow characteristics are highly nonlinear and can cause dispersion against the mean wind (Simpson, 1997). Using a novel truck-mounted Doppler lidar, radiosonde system, and automatic weather station (Clements and Oliphant, 2014), we show that these density currents form due to differential solar heating between smoke-filled and smoke-free portions of the atmospheric boundary layer and can unexpectedly spread smoke counter to the ambient wind and over large distances (∼ 30 km) Based on these observations and additional photographic evidence, we propose that smoke-induced density currents are relatively common near wildfires and must be considered for improved smoke dispersion forecasts and managing smoke impacts on communities
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