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). The multi-scale atmospheric dynamics affecting smoke dispersion cause these smoke impacts to occur both close to, and far from, large wildfires. For example, when smoke becomes trapped in mountain valleys it can recirculate for many days (Robock, 1988, 1991), whereas when injected aloft it can cause downwind impacts thousands of kilometers away (Pahlow et al., 2005; Fromm et al., 2010). Despite advances in multi-scale smoke transport modeling, forecasts for smoke dispersion continue to suffer from uncertainties in fire emissions, smoke injection depth, and the physics of fire–atmosphere interactions (Larkin et al., 2009; Goodrick et al., 2012). For example, numerical models struggle to predict smoke concentrations during complex synoptic-scale and topographic flow interactions, especially in narrow mountain valleys (Strand et al., 2012). Moreover, many operational forecast models neglect smoke radiative effects, leading to potential errors in smoke dispersion forecasts due to unresolved feedbacks between smoke and atmospheric circulations. These shortcomings are compounded by the lack of observational studies examining the physical processes and feedbacks that contribute to smoke dispersion, especially in complex terrain.

A key component of fire–atmosphere interaction is smoke modification of the earth's radiative energy balance due to the scattering and absorption of solar and terrestrial radiation (Penner et al., 1992; Hobbs et al., 1997). For smoke of sufficient optical depth, the reduction in downwelling shortwave radiation causes a substantive decrease in daytime temperature, suppressed convective boundary layer growth, or even persistent near-surface temperature inversions (Robock et al., 1988; Segal et al., 1989; Garrett et al., 1990). These effects are collectively known as smoke shading. Furthermore, since smoke minimally affects nocturnal infrared cooling, a strong positive feedback between nocturnal inversion duration and suppressed smoke dispersion can contribute to multi-week persistent inversions in mountain valleys (Robock, 1988, 1991). These inversions can subject communities to prolonged exposure to hazardous levels of small particulate matter (e.g., PM2.5), which has been linked to a host of deleterious health affects (e.g., Delfino et al., 2009; Wegesser et al., 2009; Holstius et al., 2012).

Smoke shading is also a hypothesized, but heretofore unobserved, mechanism for thermally driven mesoscale circulations, which can strongly affect smoke dispersion (Segal and Arritt, 1992). These circulations are dynamically similar to sea breezes and develop from spatial gradients in surface sensible heat flux (Segal and Arritt, 1992). The resulting difference in near-surface air temperature induces thermally direct flow, the leading edge of which is often delineated by a mesoscale front. The propagation of these “mesofronts” is well described by density current theory (Simpson and Britter, 1980). 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).

In this paper we present first observations and analyses of smoke-induced density currents, which are previously undocumented processes affecting smoke transport near large wildfires. 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.

The primary focus of this study is a smoke-induced density current originating from the Bald and Eiler fires, which burned in close proximity in northern California from 30 July–12 August 2014 (Fig. 1a). Both fires were started by lightning and grew quickly, producing towering convective plumes reaching depths in excess of 9 km. At night, when fire behavior moderated, smoke pooled in nocturnal temperature inversions forming within the broad, flat-bottomed Hat Creek valley (Fig. 1a). For example, on the morning of 3 August, radiosonde and lidar data show thick smoke confined within a 400 m, 7 K surface-based temperature inversion (Fig. 1b). Meanwhile, satellite images indicate that the adjacent Feather Lake plateau remained smoke free throughout the morning (Fig. 1c), allowing for significant differential solar heating between the two locales.

By midday this smoke-induced thermal contrast initiated a propagating density current that rapidly spread the cold, smoke-filled layer to the southeast across the Feather Lake plateau. The approximate sequential positions of the leading edge of the smoke-filled mesofront are traced from visible satellite images and in situ observations (dashed lines, Fig. 1a). Between local noon and 15:00 PDT (Pacific daylight time), the smoke spread more than 25 km, inundating a large area with optically thick smoke (Fig. 1d). Notably, the direction of smoke propagation was counter to the prevailing southerly flow aloft, which was as strong as 12.5 m s-1.

During its southeastward excursion across the Feather Lake plateau the leading edge of the smoke-induced density current was intercepted 6 times with the truck-mounted Doppler lidar and instrumentation. The vertically pointed 1.5 µm lidar samples at ∼ 1 Hz using a range gate resolution of 18 m and a total range of 9.6 km. The lidar backscatter is sensitive to aerosol in the size range of a few microns, e.g. PM2.5, and thus provides high values of backscatter for forest fire smoke. The lidar also samples the line-of-sight velocity at a resolution of ∼ 1 cm s-1. A unique aspect of the truck-mounted lidar is the ability to sample in motion, thereby providing spatially and temporally resolved profiles of the atmospheric boundary layer, or in this case the structure of a smoke-filled density current as it moved across the landscape. Further details of the observation platform, including the radiosonde system, are available in Clements and Oliphant (2014).

The density current intercept locations and corresponding lidar backscatter are shown in Figs. 1a, 2, and 3, respectively. Common to each intercept are the canonical features of an atmospheric density current including a sharp frontal zone, elevated head, substantive temperature contrast, shallower following flow, and interfacial wave mixing (Britter and Simpson, 1978; Simpson and Britter, 1979, 1980; Simpson, 1997). For example, the near-surface air temperature (colored squares, Fig. 2) substantively decreases across the leading edge of the density current, which is defined by a sharp increase in lidar backscatter where the smoke layer undercuts the smoke-free boundary layer. The backscatter in the clear air is nearly zero due to pristine conditions outside of the fire-modified environment.

The intercept sequence also reveals changes in the density current structure between the onset, maturation, and eventual decay of the circulation. At the onset, the leading edge of the circulation is marked by an isolated smoke-filled updraft followed by shallow near-surface flow (Fig. 2a, b). The density current circulation subsequently deepens as the smoke layer advances to the east-southeast. For example, at intercept C the head region is observed to broaden and deepen (Fig. 2c) and the Doppler velocity data show increased vertical mixing (not shown). The increased mixing contributes to a more homogenous density current at intercept D (Fig. 2d), where the isolated plume at the leading edge is no longer apparent and the following flow is deeper and better mixed.

Intercepts E (Fig. 2e) and F (Fig. 3a) reflect the slowing and eventual decay of the density current circulation. For example, at intercept E the leading edge of the density current develops an elevated nose wherein the foremost smoke is found aloft. Laboratory experiments indicate that the height of the nose depends on the comparative magnitude of the density current speed and surface frictional effects (Britter and Simpson, 1978; Simpson and Britter, 1979). In this case the density current advance is slowing, perhaps due to flow–topography interaction or a decrease in the negative buoyancy. The backscatter at intercept E also indicates an embedded front within the following flow, suggesting that cleft and lobe instabilities cause portions of the front to laterally fold over on itself (Simpson, 1997; Mayor, 2011).

Finally, intercept F (Fig. 3a) corresponds with the foremost advance of the smoke layer, consistent with the satellite observations and the frontal isochrones in Fig. 1a. At that time the leading edge of the smoke layer was diffuse and ragged as it drifted back and forth across the truck-mounted instrumentation, which was now parked. Following the stagnation of smoke layer the truck was driven back to the Hat Creek valley. The lidar backscatter along the return transect reveals the full extent of the smoke layer blanketing the Plateau (Fig. 3b). Substantial wave activity and a continued multi-layered structure are noted and the truck temperature sensor indicates suppressed near-surface temperatures throughout the smoke layer, with the coldest temperatures corresponding to the regions of the highest aerosol backscatter, indicative of smoke shading. The western edge of the smoke layer, and a corresponding increase in near-surface temperature, was subsequently observed within the Hat Creek valley.

In this section we present additional observations of smoke-induced density currents near large wildfires. These data indicate that smoke-induced density currents are a surprisingly common feature of large multi-day forest fires. We also discuss the interaction of these density currents with other thermally driven flows in complex terrain.

Figure 5 presents additional mobile lidar backscatter and truck air temperature observations obtained adjacent to the Eiler fire on 2 August, the day before the density current examined in Sect. 3. These data show a north–south propagating smoke-filled layer accompanied by a 2 K thermal contrast between the smoke-free and smoke-filled portions of the boundary layer. Compared to the observations in Sect. 3, this smoke-layer possesses fewer distinct density current characteristics and propagates over a more limited geographic extent. Nevertheless these data provide additional evidence for smoke shading as a mechanism to produce substantive horizontal gradients in the near-surface temperature capable of driving mesoscale circulations that affect smoke transport.

Further, it is interesting to note that the density current timing on both 2 and 3 August is similar (e.g., near noon), and we speculate that these density currents may superimpose on thermally driven flow reversals from down valley to up valley winds. While we lack sufficient observations preceding the Bald and Eiler fires to assess how smoke alters the strength and timing of these terrain-driven flows, it stands to reason that the up valley afternoon flow will be enhanced due to the differential smoke shading superimposed on the typical along valley and along slope temperature gradients. In this sense, the smoke-filled density current may behave similarly to sea-breeze circulations embedded in upslope flows in coastal topography, a superposition which is known to produce a stronger circulation than either process acting independently (Mahrer and Peilke, 1977).

Additional evidence for the propensity for smoke-induced density currents and the interaction of density currents with diurnal mountain wind systems is presented in Fig. 6, which shows a sequence of photographs extracted from a time lapse of a smoke-filled density current on 20 September 2014. In this instance the smoke layer spread more than 30 km from its source (the King fire; http://inciweb.nwcg.gov/incident/4108/), crossed the crest of California's Sierra Nevada, and propagated across Lake Tahoe. Inspection of the photographs and time-lapse animation (available at: http://youtu.be/SLVKUEoMGwI) indicate the presence of canonical density current features at the leading edge of the smoke layer. The resulting smoke inundation of the Tahoe Basin forced the cancelation of a major sporting event due to hazardous levels of PM2.5 (http://www.ironman.com/triathlon/news/articles/2014/09/tahoe-cancellation.aspx#axzz3RIVH89F0).

Compared to the Bald and Eiler fire cases, the King fire density current is more clearly embedded within a larger scale diurnal wind reversal. Satellite images show a widespread upslope southwesterly flow developing in the early afternoon of 20 September (not shown), 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. For example the leading edge of the layer advances to the southeast in opposition to the background synoptic-scale flow and exhibits a series of lateral cleft and lobe instabilities.

At issue for future studies is the degree to which diurnal mountain wind systems are altered by smoke shading. Previous investigators have established that smoke can delay, or even prevent, the break up of nocturnal temperature inversions, but no comprehensive studies of smoke-modified thermally driven circulations in complex terrain have been conducted. The smoke-induced density currents examined in this study are likely just one manifestation of these smoke-shading effects, and a more comprehensive assessment of smoke-modified boundary layers is needed to better understand local transport of smoke from wildfires.

In this paper we have presented a set of novel observations of smoke-induced density currents and established that the driving mechanism in their propagation is the thermal difference due to reduced solar insolation beneath the smoke layer. Our results indicate that these self-propagating mesoscale fronts can cause severe and unexpected smoke impacts, especially since their flow can oppose the ambient wind. Additional observations indicate that smoke-induced density currents are relatively common near large multi-day wildfires. Our findings also imply that smoke-modified boundary layers might contribute to changes in fire behavior that impact both firefighter and community safety. For example, smoke-induced density currents can contribute to rapid wind shifts, drastic reductions in visibility, and delayed inversion breakup. Since smoke modeling and forecast tools typically neglect smoke radiative forcing they can not account for these phenomena, and due to the inability of satellites to resolve fine-scale boundary-layer processes there is a need for observational studies within the fire-modified environment to improve our understanding of the broader range of smoke-modified boundary layers and other fire–atmosphere interactions and feedbacks.