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<br />f
<br />
<br />. :
<br />
<br />T..I. J VaI'U,t_ ~.. ....lIn. _J.l. u...J I.. ':"1', ...1....11'
<br />...."..,11',.. ,.,..'.11. .\H.., h.." ..".. 1.1.."....... 0"1),
<br />
<br />S....R ,.... "l.hl,
<br />
<br />'I1I1'1.III.",.;.n'6'&\.'1,.",_ ''''101....''''01.1
<br />
<br />(-I
<br />
<br />1......1...
<br />
<br />-)0 .r\ -11)1,1
<br />
<br />no 100 n
<br />
<br />1,1 ~ n.'
<br />
<br />- "
<br />
<br />"
<br />
<br />.ILt. n.,
<br />
<br />..
<br />
<br />u..r n.'
<br />
<br />I.
<br />
<br />Fig. 3 !,resents the rainfall change
<br />induced by eac~ variable modd 011 the five "torm
<br />types previou"ly discussed. The natural ra lnfall
<br />whh:!l o~c.urr.atJ. ~uring th~ fIeld ~~u::lon 1::1 compared
<br />\11th th~ modifi~d rainfa.l1 proJuced by cL\ch varl-
<br />a~.~c-c!l,mge mo~el. Based on che results of Cabr leI
<br />(196'7) and Woodley et a1. (l977l, model B may be a
<br />rcali::lcic and sO!ncw~ontJcrva.tivc estimate of
<br />clout! seeding effectiveness. Ie will be used 1n
<br />th~ ~lucceedlng ~!.SCUSS1011 Uti an example whenev~r
<br />appru{>['late.
<br />
<br />Some significant features of ' Fig. 3
<br />
<br />incl'Jde:
<br />, , ''1:) . As'wod2':c be'.expectcd, moc:!els E, A, B, and
<br />C !lroc.uced. increases in tht:! natural raInfall from
<br />eac.h storm ty!?.e ....hile models X, Y, and Z decn:ased
<br />the natural ra..:nfall.
<br />2) OVera:l. mountain-generated Cb's, cold
<br />front storms. ~!'lcl upper level troughs produced the
<br />greate9t changes in precip.~tation over the field
<br />::{L.ason. For i,;:y.nm?le. using model fl. a rainfall
<br />in<.: rl:C\se of Y. ~ would re::J uJ. t if all 1.0 fj to rms in
<br />the ~!ay - July !,eriod were s~edeu. However. if
<br />Just the 15 !!lo~~,;a1n-generated Cbt s 8 cold t't;'ont
<br />storms. ~nd 10 ~?per-level troughs were seeded 29 nun
<br />of additional rainfall would re"ult.
<br />3) Alrmass-generated Cb's produced just 5 mm
<br />of rainfall during the May - July fleld season.
<br />Even using t~e !nost optimistic modtd (E). only an
<br />nd~itional 5 ~ of summer rainfall could be obtained
<br />from this stom type. Using model il, only 3 mm
<br />\Joule! be receivec during the field season.
<br />
<br />,"-
<br />
<br />90-
<br />
<br />~
<br />
<br /> po-
<br /> 70.
<br />~ ,:;0.
<br />)~
<br />..J
<br />..J !.o-
<br />.,
<br />."
<br />'" .10_
<br />..
<br />~
<br />-. :'0_
<br />~ "0.
<br /> 10.
<br /> O.
<br />
<br />I-MOUNTAIN GENERATED CB
<br />;,~ " 2'COLO FRONT
<br />~"B ~ 3-UPSLOPE
<br />
<br />l~!! b~li~!~::::::'~'~~~;::::: "
<br />
<br />:':, \ " : :'<:~.1 I
<br />;;; ::'.: ~v ::!r;'!,1~ I
<br />f. "j" ". f' . ",~{.... I
<br />h~t':;':-'I;lijHl~B~YI ~^ ~A
<br />J;" ,,' . t:'~.. l. ;..,t!! i' 9 ..:.
<br />:~:l~'l':7:J':!ifl~j::~11~:,:'~~ ~,!~c
<br />:1;.'; I - I 'J~l!'\:,i']\' ,...! II.,. INipl'I i"
<br />f':l:r"~.!~'lr.~Jrl"I~ <1'1; I 1"~lll'JI'D
<br />.Il! 'I,ll' r' . '.' 'ill'}).' I I 1-, i' ,r"ITtl;. -II' X E
<br />(.:tf1ll" ;'~~i ;l;rit,';." :l\~;~l: ,.: i t~ll~~Y N"f,Bc
<br />lP'~.l:i .:iji~t{ .."'~', .J~'l ;'::!dl:rd~!1i";:'fii"J(~
<br />~~~~~~Jj~~~.1U~~~~.~~L~~~~~~~~~9~~tj~~~~
<br />(I) (2) IJI (-II (~)
<br />
<br />NCM!3!::R
<br />of <;fORMS IS
<br />
<br />e
<br />
<br />10
<br />
<br />6
<br />
<br />Figure 3. Natural (N) and model~ed (E, A, B, C,
<br />X, Y, Z) sea::ionnl rainfall for the
<br />five storm types during May - July,
<br />1976.
<br />
<br />4) Upper-level troughs. though togel:her pro-
<br />ducing less natural rainfall .tho" the one. up~lope
<br />9torm, were more susceptible to moJel Inckea~es.
<br />That Is, all upper-level troughs uccounted for 17 mm
<br />of rainfall naturally, whereils t!\C single upslo~e
<br />storm produced 25 mm of rainfal.L VsJ.ng inodel B,
<br />uppcr-l~vel troughs would produce 8n udd.ltional
<br />13 mm of rainfall whil~ th~ ur~lop~ storm would
<br />increase the natural rainfall by only I. min.
<br />I
<br />
<br />To further understand the sigul~cance of
<br />these models, Fig. 4 was prepared. This flgure
<br />portrays the cumulative ralofnll ChUllgC plrol.~uced
<br />by each model for each sturm type. I
<br />
<br />-'------,
<br />!
<br />
<br />-100 . -8~
<br />
<br />MAY-lUlY m:ULATIVE RAlllrl,Ll CII,\NGI: (m m ~
<br />
<br />
<br />
<br />~'~~l)~'\'
<br />"\ \\
<br />
<br />
<br />~ ~ :.:- ,....
<br />
<br />
<br />3
<br />~
<br />I
<br />t
<br />I
<br />'-60 '-5'0 '-4'0 '-io '-io '-1'0' 0 . 1'0 . io 'io' 4'0' 5'0 ' 60
<br />MAY-JULY CUr,lUWIVE RW:fAlL CI\ANGl, (",I.)
<br />
<br />Figure 4.
<br />
<br />Modelled cumulative seasonal rainfall
<br />change for the' fiye storm tYP::::J.
<br />
<br />The cumulative change J.n both .geasonal
<br />rainfall amount and percent cotal from each storm
<br />type may be extracted from Fig. I~. For e'xample. I.
<br />u9.lng nl\Jdel il. it 1a seen that mountaJ.n-g~n(::rated'
<br />CblS accounted for il 13 mrn (B.SZ) 11lcrea~:f.: oVer
<br />th~ l1<.ltural precip1.taeJ.on. As wel.L, the l1e': effect
<br />of assuming model n valid for all storm~ 115 a cutn- ,
<br />ulative increase of 231. during t.:!u: E'l..::.LJ ~~ason.
<br />~!ollnta.ln-generated Cb'::I, cole! front~ and :upper-
<br />J.e'v~l troughs comblnecl yild.dcd 19.6% over. Lht:. total
<br />natural precipitation when model B wa~ ap'p.tJed..
<br />Thus, 85% of the potential benefit from n~odel B can
<br />be realized by concentrating on mountain-Igenerated
<br />Cb's, cold front storms and upper-level t:roughs.
<br />
<br />5.
<br />
<br />CONCLUSIONS
<br />
<br />This work was undertaken to accomplish
<br />two objectives. First, it was d~sircd t~ investi-
<br />gate natural storm characteristics, inclJding storm
<br />types, frequency of rain occurrence, durdcion,
<br />amount, areal coverage and time of rain occurrence
<br />near Miles City, Montana. Second. it was d~sired
<br />co see which types of storms could produce the
<br />greatest changes from the natural rai~fall using'
<br />hypothetical "eedlng models.
<br />
<br />Using the data bank discussed, the
<br />following factors were observed about the natural
<br />precipitation regime near Mile" City:
<br />
<br />-;
<br />
<br />f
<br />,
<br />I
<br />\
<br />i
<br />I ,i
<br />I
<br />
<br />.........
<br />
<br />-----_.'
<br />
<br />'.'
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