References

ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-7-645-2007

The Chisholm firestorm: observed microstructure, precipitation and lightning activity of a pyro-cumulonimbus

Rosenfeld

¹ Fromm

² Trentmann

³ Luderer

⁴ ⁶ Andreae

M. O.

⁴ Servranckx

⁵

Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel

Naval Research laboratory, Washington, D.C. 20375, USA

Institute for Atmospheric Physics, University of Mainz, Germany

Max Planck Institute for Chemistry, Mainz, Germany

Canadian Meteorological Centre, Meteorological Service of Canada, Montreal, Canada

now at: Umweltbundesamt, Dessau, Germany

09 02 2007

7 3 645 659

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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. <br><br> 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. <br><br> 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.

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