In situ microphysics observations of intense pyroconvection from a large wildfire

Abstract

This study characterizes the size and shape distributions of 10 µm to 6 mm diameter particles observed during six penetrations of wildfire-induced pyroconvection near Boise, Idaho, USA, by a research aircraft over the period 29–30 August 2016. In situ measurements by the aircraft include winds, atmospheric state, and bulk water content and particle concentration, size, and shape. These observations are complemented by data from airborne and ground-based radars. One of the penetrations is through a subsaturated smoke–ash plume with negligible cloud liquid water content that is characterized by an updraft of almost 36 m s−1. The size distribution of number concentration is very similar to that documented previously for a smoke plume from a prescribed fire, and particle shapes exhibit qualitative and quantitative attributes comparable to ash particles created in a burn chamber. Particles sampled during this penetration are most likely pyrometeors composed of ash. Pyrocumulus clouds are probed in the other penetrations where values of relative humidity and cloud liquid water content are larger, but updrafts are weaker. Compared to the smoke-plume penetration, size distributions are mostly characterized by larger concentration, and particle shapes exhibit a higher degree of circular symmetry. Particle composition in these pyrocumulus penetrations is most likely a combination of hydrometeors (ice particles) and pyrometeors (ash).