HR: 08:30h
AN: A11C-03    [PDF]
TI: Pyro-Cumulonimbus: Strongly suppressed precipitation by smoke-induced extremely small cloud drops up to the homogeneous freezing level
AU: * Rosenfeld, D
EM: daniel@vms.huji.ac.il
AF: Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, 91904 Israel
AU: Servranckx, R
EM: Rene.Servranckx@ec.gc.ca
AF: Meteorological Service of Canada, 2121, North Service Road Trans-Canada Highway, Dorval, Que H9P 1J3 Canada
AU: Fromm, M
EM: mike.fromm@nrl.navy.mil
AF: Computational Physics, Inc, 8001 Braddock Rd. Suite 210, Springfield, VA 22151 United States
AU: Andreae, M O
EM: andreae@mpch-mainz.mpg.de
AF: Max Planck Institute for Chemistry, Biogeochemistry Department P.O. Box 3060, Mainz, D-55020 Germany
AB: Pyro-cumulonimbus (Pyro-CB) is defined here as a convective cloud that feeds directly on the heat and smoke of a major fire, with its top exceeding the -40C isotherm level. The microstructure and precipitation of Pyro-CBs that were formed over large forest fires were observed by in situ measurements at the Amazon, by radar in Canada and the USA, and by satellite multispectral retrievals in all of these locations and several others. The satellite measurements of cloud microstructure show that the cloud drop effective radius is extremely small, and at the mid and high latitudes it does not exceed the precipitation threshold of 14 micrometer even near the cloud top. This implies that the smoke provides sufficiently large activated cloud condensation nuclei that create large number of small droplets to the extent that they do not coalesce with each other or rime with ice particles up to the homogeneous freezing level, which is slightly below the -40C isotherm level. The lack of significant precipitation forming processes is manifested as very weak radar echo intensities reflected from the Pyro-CBs in spite of their vigor and depth, often penetrating the tropopause. This is especially conspicuous when compared with neighboring normal microphysically continental cumulonimbus clouds, which are composed of much larger particles and produce strong precipitation radar echoes. In situ measurements in tropical Pyro-CBs at the Amazon validated the remote sensing inferences, and showed that the large ash particles are incapable of producing fast-growing precipitation particles in the Pyro-CBs. Some of the implications of these observations are: 1. Pyro-CBs have little precipitation in spite of their vigor and extent, so that they do not help to mitigate the fire that created them. 2. Furthermore, "dry" lightning that do form with little precipitation in the Pyro-CB can ignite the forest downwind and so enhance the spreading of the forest fires by jumping forward 3. The lack of precipitation means also lack of washout of the smoke from the cloud, so that much of the smoke is detrained with the anvil at the upper troposphere and even the low stratosphere.
DE: 0320 Cloud physics and chemistry
DE: 1803 Anthropogenic effects
DE: 3324 Lightning
DE: 3354 Precipitation (1854)
DE: 3362 Stratosphere/troposphere interactions
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting