Observations of Pyrocumulus and Pyrocumulonimbus in Northern California

Abstract:

Pyrocumulus (pyroCU) clouds form due to condensation within wildfire convective plumes. In extreme cases PyroCU transition to deeper Pyrocumulonimbus (PyroCB), which are capable of generating precipitation, downdrafts, and lightning. Both PyroCU and PyroCB have important implications for smoke injection depth, cloud microphysics, and fire behavior. However, despite their importance, these pyroconvective clouds remain incompletely understood and are seldom observed. For example, key questions remain as to what processes control the condensation level and maximum plume height.

To address these questions, we present two cases studies of pyroCU and pyroCB associated with large wildfires in northern California in 2014 and 2015. The condensation level in these plumes is directly measured using a scanning Doppler lidar, which discriminates between smoke and condensed water. Lidar data, and additional data from nearby WSR-88D radars, are also used to measure the maximum plume height, which in one case reaches ~12km MSL. These observed plume heights and condensation levels are subsequently examined in the context of atmospheric profiles obtained from radiosonde data near each fire. From these data we discuss the comparative importance of latent heat release from combustion, environmental moisture, and atmospheric stratification on determining condensation level and plume height.