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Last modified
7/28/2009 2:34:51 PM
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3/5/2008 2:27:17 PM
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Weather Modification
Title
Observational and Numerical Studies of Cloud and Precipitation Development with a View to Rainfall Enhancement
Date
4/1/1992
Weather Modification - Doc Type
Report
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<br />..~ <br /> <br />. <br /> <br />. <br /> <br />[e <br /> <br />i <br />I. <br /> <br />! - <br />, <br />I <br /> <br />I. <br /> <br />! <br />L_.~< <br /> <br />.. <br /> <br />1981}. In clouds ~th warm bases coalescence usually occurs together with an active ice <br />multiplication process in the -5 to -80C region (Hallet and Mossop, 1974). The dominant <br />ice crystal habit will differ in both cases with hexagonal plates and dendrites being the <br />dominant graupel embryos in the former case and large frozen drops and columns and <br />c.apped columns in the latter. In the latter case precipitation development via the ice phase <br />is iDitiated earlier and more efficiently than in the former case (Johnson, 1987; Bruintjes et <br />al., 1987). The concentrations ofice crystals in the latter case are usually higher than in the <br />former case. Calculations of the ice enhancement ratios due to the Hallet-Mossop process <br />presently seems to underpredict the observed concentrations in clouds (Harris-Hobbs and <br />Cooper, 1987; Hobbs and Rangno, 1985; Rangno and Hobbs, 1991). To explain the higher <br />natural concentrations several other ice multiplication processes have been suggested in the <br />past (Hobbs and Farber, 1972; Pitter and Finnigan, 1989). Later chapters will attempt to <br />address some of these aspects. <br /> <br />Aggregates of ice particles are also found to act as precipitation embryos (Vall et al., <br />1982; Heymsfield, 1982). Hobbs (1974) showed that the probability of finding aggregates <br />in a cloud increases with increasing temperature and particle concentratio~. Heymsfield <br /> <br />. <br />(1986) in a modelling study concluded that aggregation appears to be an important growth <br />mechanism at temperatures where crystals grow rapidly along one crystal axis (-5 and - <br />150C regions) based on results obtained in the HIPLEX experiment. Measurements at <br />_6oe in HIPLEX clouds did indicate many aggregates. The physical mechanism of crystal <br />aggregation is not yet fully understood. It is therefore difficult to model aggregation and <br />many simplifying and potentially unrealistic assumptions have to be made when modelling <br />this process. Effects like enhanced aggregation due to electric multipoles in growing <br />ice postulated by Finnegan and Pitter (1988) will have to be considered. The electric <br />multipoles are formed due to soluble salt impurit~es in the crystals. Aggregation might be <br />a potentially important mechanism in cloud seeding experiments to enhance rRin;all. This <br />will be explored further in later chapters. <br /> <br />Since its discovery more than forty years ago both silver iodide and dry ice are still the <br />leading seeding materials used to seed clouds around the world. Both materials enhance <br /> <br />16 <br />
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