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

[摘要] A fire storm that occurred on 28 May 2001 and devastated the town ofChisholm, ~150 km north of Edmonton, Alberta, induced a violentfire-invigorated cumulonimbus cloud. This pyro-cumulonimbus (pyro-Cb) hadovershooting tops of 2.5–3 km above the tropopause, and injected massiveamounts of smoke into the lower stratosphere. Fortunately, this eventoccurred under good coverage of radar, rain gauge, lightning and satellitemeasurements, which allowed in-depth documentation of the event, and gave usan 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 smalldrops, which ascended rapidly in violent updrafts. There appeared to belittle freezing up to the homogeneous freezing isotherm level of −38°C.A cloud with such small and short-lived highly supercooled drops isincapable of producing precipitation except for few large graupel and hail,which produced the observed radar echoes and charged the cloud with positivelightning. The small cloud drops froze homogeneously to equally small iceparticles, for which there is no mechanism to aggregate into precipitationparticles, and which hence remain in the anvil. The lack of significantprecipitation implies that only a small fraction of the smoke is scavenged,so that most of it is exhausted through the anvil to the upper troposphereand lower stratosphere.

Comparisons with other cases suggest that a pyro-Cb does not have to be asviolent as the Chisholm case for precipitation to be strongly suppressed.However, this level of convective vigor is necessary to create theovershooting updraft that injects the smoke into the lower stratosphere.

[学科分类] 大气科学