Abstract
Abstract. In this paper we present the first direct observational evidence that the condensation level in pyrocumulus and pyrocumulonimbus clouds can be significantly higher than the ambient lifted condensation level. In addition, we show that the environmental thermodynamic profile, day-to-day variations in humidity, and ambient wind shear all exert significant influence over the onset and development of pyroconvective clouds. These findings are established using a scanning Doppler lidar and mobile radiosonde system during two large wildfires in northern California, the Bald Fire and the Rocky Fire. The lidar is used to distinguish liquid water from smoke backscatter during the plume rise, and thus provides a direct detection of plume condensations levels. Plume tops are subsequently determined from both the lidar and nearby radar observations. The radiosonde data, obtained adjacent to the fires, contextualize the lidar and radar observations, and enable estimates of the plume ascent, convective available potential energy, and equilibrium level. A noteworthy finding is that in these cases, the convective condensation level, not the lifted condensation level, provides the best estimate of the pyrocumulus initiation height.
Highlights
Pyrocumulus clouds form when wildfire convective plumes rise to their condensation level and subsequently develop cumuliform cloud tops (American Meteorological Society, 2015a)
The observations presented in this paper demonstrate that plume condensation levels can exceed the height of the ambient lifted condensation level (LCL), sometimes substantially
During the Bald Fire the plume condensation level was more than 1 km www.atmos-chem-phys.net/16/4005/2016/
Summary
Pyrocumulus clouds (pyroCus) form when wildfire convective plumes rise to their condensation level and subsequently develop cumuliform cloud tops (American Meteorological Society, 2015a). In contrast to Potter (2005), Luderer et al (2006, 2009) use high-resolution simulations and theoretical sensitivity calculations to conclude that “the combined effect of released moisture and heat from the fire almost always results in a higher cloud base compared to ambient conditions.” They find that moisture released in combustion constitutes less than 10 % of the pyroCu/Cb water budget with the remainder of the plume water resulting from entrained environmental air. The pyroCu cloud bases and plume rise dynamics were measured using a mobile atmospheric profiling system (Clements and Oliphant, 2014) that included a scanning Doppler lidar and an upper-air radiosonde system which provided thermodynamic profiles immediately upstream of the fire perimeters From these data, our results clearly show that observed plume condensation levels are substantially higher than the ambient LCL. Additional aspects of the plume rise, including limiting factors on convective growth and the role of environmental moisture are examined
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