Examination of the Effects of Aerosols on a PyroCb and Their Dependence on Fire Intensity and Aerosol Perturbation Using a Cloud-System Resolving Model

Abstract:

This study investigates how an observed event of pyrocumulonimbus (pyroCb) influences water vapor concentrations and cirrus cloud properties near the tropopause. These concentrations and properties near the tropopause have important implications for global energy budget and climate change. Specifically, this study focuses on how fire-produced aerosols affect this influence via a modeling framework with high resolutions and sophisticated microphysical representations. Results from a case study show that when observed fire intensity is high, there is an insignificant impact of fire-produced aerosols on the convective development of the pyroCb and associated changes in water vapor and the amount of cirrus cloud near the tropopause. However, as fire intensity weakens, the effects of aerosols on microphysical variables and processes, which are related to the intensity of convection, such as droplet size and autoconversion increase. Hence, results indicate that aerosol-induced invigoration of convection is significant for pyroCb with weak-intensity fires and associated weak surface heat fluxes. Thus, there is a greater aerosol effect on the transportation of water vapor to the upper troposphere and the production of cirrus cloud with weak-intensity fires, whereas these effects are muted with strong-intensity fires.

Note that traditional understanding generally focuses on the effects of fire-produced heat and water vapor and their associated surface fluxes on the pyroCb and does not consider the effects of fire-produced aerosols on the pyroCb. This understanding adequately explains the mechanics for pyroCbs in association with strong fires. This study suggests that when pyroCbs form over weak-intensity fires, those effects of fire-produced aerosols require consideration.