Multi-scale Simulations of Massive Stratospheric Smoke Injected by the 2017 British Columbia PyroCbs

Abstract:

On August 12 2017 five Pyrocumulonimbus (pyroCb) over central British Columbia injected approximately 0.3 Tg of smoke aerosol into the stratosphere that circulated the globe for 15 months. A key unknown is the contribution of the pyroCb’s to the smoke’s ascent versus the lift provided by burning fuel. Further, the burning fuel was a complex mixture of trees (bark beetle killed forest) and slash piles (from logging practices) with the latter generating a more sustained and longer lasting burn. We first examine the source nonlinearity by a series of fire simulations comprised of slash only, conifer forest, and a mixture using our fire model (FIRETEC) that also predicts particulate smoke emissions. The simulations show that the mixed fuel simulation lofts the smoke higher due to stronger heating in the lower atmosphere than either the more intense but shorter burning forest simulation or the slower burning slash simulation. Next, results from these fine-scale simulations are integrated such as to provide sources of heat, aerosol, black carbon, and gaseous emissions for our larger-scale pyroCb simulations using a large-eddy cloud resolving model (HIGRAD). Maps of the actual August 12 fire sources and locations, observed meteorology and terrain are used, making our pyroCb simulations the most realistic to date. Because of the dry lower atmosphere, condensation of cloud droplets and activation of ice particles are the primary latent heat release mechanisms. Bulk parameterizations for condensation and freezing based on experimental data relevant to the observed atmospheric and aerosol conditions are used. We are able to explain the mass injected into the stratosphere via a combination of vertical motions from both the multiple fires consisting of a mixture of fuel and the pyroCbs. However, sensitivity studies are needed to quantify aerosol sinks and sources by cloud-aerosol processing, washout, and secondary organic aerosol formation in the upper troposphere that are highly uncertain.