A new look at the role of fire-released moisture on the dynamics of atmospheric pyro-convection
Gunnar Luderer A C , Jörg Trentmann B and Meinrat O. Andreae AA Max Planck Institute for Chemistry, Department of Biogeochemistry, PO Box 3060, D-55020 Mainz, Germany.
B Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Becherweg 21, D-55099 Mainz, Germany.
C Corresponding author. Present address: Potsdam Institute for Climate Impact Research, PO Box 60 12 03, D-14412 Potsdam, Germany. Email: luderer@pik-potsdam.de
International Journal of Wildland Fire 18(5) 554-562 https://doi.org/10.1071/WF07035
Submitted: 16 February 2008 Accepted: 29 September 2008 Published: 10 August 2009
Abstract
We investigate the contribution of the moisture released by wildland fires to the water budget and the convection dynamics of pyro-clouds forming atop fires. Using an approach based on stoichiometric principles and parcel theory of convection, we assess the relative contribution of sensible heat and latent heat to the convection energy. We find that moisture release is of much lesser importance for the fire convection than the release of sensible heat from the combustion. We conclude from theoretical considerations that it is highly unlikely that the decrease of the cloud base of pyro-cumulus compared with that of ambient free convection is due to the fire-released moisture alone, in contrast to what has been suggested previously. In addition to the analytical results, numerical simulations of a specific case study are presented. They show that the fire-released moisture accounts only for a small portion of the total water in the pyro-cumulus cloud. Also, the effect of the fire-released moisture on the convection dynamics and the height of injection is found to be small compared with the effect of the sensible heat release from the fire.
Acknowledgements
We thank B. M. Wotton and J. Goldammer for the discussion on fuel moisture values. G. Luderer was funded by an International Max Planck Research School Fellowship. We thank the Max Planck Society for supporting the present work. We thank two anonymous referees for their constructive comments that helped to improve the paper.
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Appendix
In order to assess the respective effects of temperature and humidity perturbations on the lifting condensation level, we can apply a linearization approach
to Eqn 5:
Note that TLCL itself is a function of the temperature at the reference level T and the lifting condensation level pLCL ; hence both its partial derivatives need to be considered as well. From Eqn 6, they can be calculated as
The differential temperature dependence of the saturation water vapor partial pressure is given by the Clausius–Clapeyron equation (e.g. Rogers and Yau 1989)
so we can convert
Substituting Eqns 5, A6, A4, and A3 into Eqn A2 yields
Rearranging results then in the form presented in Eqn 7:
