Modeling of biomass smoke injection into the lower stratosphere by a large forest fire (Part II): sensitivity studies G. Luderer1, J. Trentmann2, T. Winterrath1, C. Textor3, M. Herzog4, H. F. Graf5, and M. O. Andreae1 1Max Planck Institute for Chemistry, Dept. Biogeochemistry, Mainz, Germany 2Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Mainz, Germany 3Service d'Aéronomie, CNRS, Paris, France 4NOAA GFDL, Princeton, New Jersey, USA 5Department of Geography, Centre of Atmospheric Science, University of Cambridge, Cambridge, UK
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
convection is very sensitive to the heat flux from the fire. The
emissions of water vapor play a less significant role for the injection height
but enhance the amount of smoke transported beyond the
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.
Citation: Luderer, G., Trentmann, J., Winterrath, T., Textor, C., Herzog, M., Graf, H. F., and Andreae, M. O.: Modeling of biomass smoke injection into the lower stratosphere by a large forest fire (Part II): sensitivity studies, Atmos. Chem. Phys., 6, 5261-5277, doi:10.5194/acp-6-5261-2006, 2006.