The Chisholm firestorm: observed microstructure, precipitation and lightning activity of a pyro-cumulonimbus D. Rosenfeld1, M. Fromm2, J. Trentmann3, G. Luderer4,*, M. O. Andreae4, and R. Servranckx5 1Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel 2Naval Research laboratory, Washington, D.C. 20375, USA 3Institute for Atmospheric Physics, University of Mainz, Germany 4Max Planck Institute for Chemistry, Mainz, Germany 5Canadian Meteorological Centre, Meteorological Service of Canada, Montreal, Canada *now at: Umweltbundesamt, Dessau, Germany
Abstract. A fire storm that occurred on 28 May 2001 and devastated the town of
Chisholm, ~150 km north of Edmonton, Alberta, induced a violent
fire-invigorated cumulonimbus cloud. This pyro-cumulonimbus (pyro-Cb) had
overshooting tops of 2.5–3 km above the tropopause, and injected massive
amounts of smoke into the lower stratosphere. Fortunately, this event
occurred under good coverage of radar, rain gauge, lightning and satellite
measurements, which allowed in-depth documentation of the event, and gave us
an opportunity to study the cloud top morphology and microstructure,
precipitation and cloud electrification of the pyro-Cb.
The combination of heat and smoke created a cloud with extremely small
drops, which ascended rapidly in violent updrafts. There appeared to be
little freezing up to the homogeneous freezing isotherm level of −38°C.
A cloud with such small and short-lived highly supercooled drops is
incapable of producing precipitation except for few large graupel and hail,
which produced the observed radar echoes and charged the cloud with positive
lightning. The small cloud drops froze homogeneously to equally small ice
particles, for which there is no mechanism to aggregate into precipitation
particles, and which hence remain in the anvil. The lack of significant
precipitation implies that only a small fraction of the smoke is scavenged,
so that most of it is exhausted through the anvil to the upper troposphere
and lower stratosphere.
Comparisons with other cases suggest that a pyro-Cb does not have to be as
violent as the Chisholm case for precipitation to be strongly suppressed.
However, this level of convective vigor is necessary to create the
overshooting updraft that injects the smoke into the lower stratosphere.
Citation: Rosenfeld, D., Fromm, M., Trentmann, J., Luderer, G., Andreae, M. O., and Servranckx, R.: The Chisholm firestorm: observed microstructure, precipitation and lightning activity of a pyro-cumulonimbus, Atmos. Chem. Phys., 7, 645-659, doi:10.5194/acp-7-645-2007, 2007.