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<br />Wor1d Meteorological Oryanizalion <br /> <br />The following are excerpts from the WMO <br />Statement on Weather ModifICation (WMO <br />1992) with reference to summer precipitatJon <br />augmentation: <br /> <br />CooWs douds appear to be c::arQdales for I"I"(l(Jficatioo <br />a<:axtling"bolhlt1estatic""'It1e_~ <br />hypoIt1esis. Fold expelin'on~ """ n-doud rncrophy.;ic<j <br />measurements experimental seeding IIiaIs 11 several <br />rngioos have shoYm that isdaled ().Jfl'dJs douds MVch <br />do not pro;... """ naturally can be _led 10 prodoce <br />"'" by ~ cloud seeding """""". It1e "",faD <br />an1C.U1lsfrcrnttleseisolateddoudsareverysmall. <br />Repats of Imted sua::ess have been obtained from <br />'_10 prove Olalstalisticaly sign<1icanlrainlal <br />an1C.U1ts can be prtdJced on a seascnaI basis from Ihese <br />"""'i and Ia-ger sy.;lems. Al1enwIs 10 sign<1icantty <br />enhance ranfall frcm curoolifoon clouds have <br />roncentrated their elfCl'tS on systems wtlidl poduce <br />rainfall naturally. <br /> <br />The cloud seeding conceptual <br />models <br /> <br />Natural prncipitalion efficiency <br /> <br />The large and mesoscale dynamics <br />determining the characteristics of the cloud <br />systems down 10 the small scale microphysics <br />determining the nudeation and growth <br />characteristics of water droplets and ice <br />partides all form part of the chain of events of <br />precipitation development. In figure 6. <br />Bruinljes (1999) attempts to hypothesize on the <br />physical path thai leads to the Initiation and <br />development of precipitation. This involves <br />various microphysical processes thai proceed <br />simultaneously but at different rates, with one <br />path becoming more dominant because of Its <br />greater efficiency under given atmosphenc <br />conditions <br /> <br />Cloud condensate over land is near1y a"-vays <br />initially available in the form of small cloud <br />liquid droplets. As the cloud develops. the <br />small droplets are typically less than 1 0 <br />microns in diameter at the cloud base. with <br />concentrations in hundreds per cubic <br /> <br />centimeter. Figure 7 shows cloud droplet <br />diameters (red dots) as small as 5 microns at <br />doud base to 20 microns at cloud top_ Cloud <br />droplet concentrations range from 1400/cc at <br />doud base to 250/cc at cloud top. <br /> <br />At doud base (2400m MSLI, the competition <br />fOf the water vapor excess among the droplets <br />is strong. and further growth by condensation <br />is restricted The fall velocity of these small <br />droplets is less than 1cmls. so they move With <br />the air o.ments in the cloud. Growth by <br />condensation to precipitation-sized particles <br />(30 microns) is not observed in Figure 7. and <br />does not occur. No precipitation is formed. and <br />the preciprtahon efficiency is 0%. <br /> <br />I <br />I <br />I <br />~ <br />l <br />~ <br />I <br />~ <br />4 <br />l <br />4 <br />4 <br />4 <br />4 <br />. <br />4 <br />. <br />4 <br />. <br />4 <br />. <br />4 <br />4 <br />. <br />4 <br />4 <br />4 <br />4 <br />4 <br />4 <br /> <br />3 <br /> <br /> . . -~.=--~ --. <br /> l .. "4TU <br /> - <br /> !. . ...., I <br /> <W<" ~ ~ )~ ~lJl <br /> "4TU. , <br /> I <_.._~- :,/11 <br /> } J <br />.. .~'" <br /> . $'<<)" :-- <br /> , ~il <~..ql'1. c- I <br /> I , <br /> . <br /> ..!I"kA'":J"Il4n.... <br /> <br />J <br /> <br />I f.uuA.'1" <br />F19ure 6 Pattlways by wtuch water vapor is transforrred <br />into vallCtlS types ofhydrcmeleors (Bruinl/eS. 1999) <br /> <br />UlP'~__~7 <br />~ IB I~ M ~ <br />"" <br /> <br />"" <br /> <br />"- <br /> <br />- <br /> <br />..._- <br /> <br />"- <br /> <br />,~ <br /> <br />I.~ <br />I <br />I,~. <br />. <br /> <br />--..0_' <br /> <br />._- <br /> <br />..- <br /> <br />, <br /> <br />-- <br /> <br />- ..................- <br /> <br />.~ <br /> <br />j <br /> <br />---- - <br /> <br />... <br /> <br />_L _ _"- . . <br /> <br />,,~""-_._... <br />p.......~_.j...1. .~o,-- <br />]t)OO_~_4... I <br />~ lOOC ,~ 1000 ~ <br />CDP_C.....<<rn') <br /> <br />F'9ure 7: CIoucl droplet dlameter (red dots) afld doud <br />cYopIet oonc:entrabon (blue squares) as a functlor1 of <br />altItude Data obtarned frcm !he SOM. reseach aircraft fer <br />a severe storm west of Arnanlkl on 7t" August 2004 <br /> <br />15 <br />