The Dynamics of Megafire Smoke Plumes in Climate Models: Why a Converged Solution Matters for Physical Interpretations
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2022-10-03
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract
As the climate system warms, megafires have become more frequent with devastating
effects. A byproduct of these events is the creation of smoke plumes that can rise into
the stratosphere and spread across the globe where they reside for many months. To gain
a deeper understanding of the plume dynamics, global climate simulations of a megafire
were performed at a wide range of grid spacings from 2.0° down to 7 km, including a 7
km nonhydrostatic experiment. The analysis focuses on how the resolved dynamics af fects the specification of the plume characteristics such as injection height and black car bon (BC) mass. Prior studies initialize the smoke plume at one or a few grid points and
this is shown here to produce severely dissipative dynamics. In order to validate such
simulations with observations, enhancements of the plume characteristics to offset the
dissipation is necessary. Using a numerically converged simulation, sensitivity tests show
that to approximate the observed stratospheric lifetime, a reduction in BC fraction by
50% is necessary for external mixtures. The vorticity dynamics of the plume is also an alyzed with a Lagrangian budget to understand the mechanisms responsible for the evo lution of a collocated anticyclonic vortex. The results can be distilled down into a sim ple conceptual model. As the plume rises, the air diverges at the top of the updraft where
the largest concentrations of smoke are found. This divergence induces a dilution of the
background cyclonic absolute vorticity producing an anticyclonic vortex. Vortex decay
occurs from opposite arguments.