Modeling of biomass smoke injection into the lower stratosphere by a large forest fire Part II: Sensitivity studiesReport as inadecuate

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1 MPIC - Max Planck Institute for Chemistry 2 Institute for Atmospheric Physics 3 SA - Service d-aéronomie 4 NOAA GFDL 5 Department of Geography

Abstract : The Chisholm forest fire that burned in Alberta, Canada, in May 2001 resulted in injection of substantial amounts of smoke into the lower stratosphere. We used the cloud-resolving plume model ATHAM Active Tracer High resolution Atmospheric Model to investigate the importance of different contributing factors to the severe intensification of the convection induced by the Chisholm fire and the subsequent injection of biomass smoke into the lower stratosphere. The simulations show strong sensitivity of the pyro-convection to background meteorology. This explains the observed coincidence of the convective blow-up of the fire plume and the passage of a synoptic cold front. Furthermore, we performed model sensitivity studies to the rate of release of sensible heat and water vapor from the fire. The release of sensible heat by the fire plays a dominant role for the dynamic development of the pyro-cumulonimbus cloud pyroCb and the height to which smoke is transported. While the convection is very sensitive to the heat flux from the fire, the emissions of water vapor play a less significant role. The aerosol burden in the plume has a strong impact on the microphysical structure of the resulting convective cloud. The dynamic evolution of the pyroCb, however, is only weakly sensitive to the abundance of cloud condensation nuclei CCN from the fire. In contrast to previous findings by other studies of convective clouds, we found that fire CCN have a negative effect on the convection dynamics because they give rise to a delay in the freezing of cloud droplets. Even in a simulation without fire CCN, there is no precipitation formation within the updraft region of the pyroCb. Enhancement of convection by aerosols as reported from studies of other cases of convection is therefore not found in our study.

Author: G. Luderer - J. Trentmann - T. Winterrath - C. Textor - M. Herzog - H. F. Graf - M. O. Andreae -



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