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<br />
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<br />
<br />Cloud photograph. ~ere routInely obtained
<br />v~.th a Supet' 8-mm t!.-~-l.apse CHmura at I-minute
<br />Int~t'Vals throughout, -:~l~ duyl.lgltt hours. 'I'he time-
<br />:i.lpS~ C8m~rat locatc:!~ at~11~s City, was aimed ov~r
<br />t!.,~ rain gugc network. '!hctlc data were used to
<br />ic!cmclfy the types o~ c.!..ouc.ls pU:-J::Ilng over the rain
<br />g,3.~l:! network and to con: t('m sturm origins 1n the
<br />cas~ of locally deve:o?~ng sy~t~ms.
<br />
<br />National ~~ather Servic~ synoptic data.
<br />including surfac.e anl u!>pcr aIr charts. were also
<br />',sed in classifying s~orm types by determining the
<br />~1~e:3ence of fronts or t1~per level troughs.
<br />
<br />3.
<br />
<br />CLASSIFICA~IO~ OF STORM PERIODS
<br />
<br />Data from t~e rain gage network were used
<br />to determine all stOnt p"riods Juring th" fieid
<br />~eason. A storm ~a$ ~efincd us any time period
<br />c.!urlng which rain was oT)scrved by more than 2 of
<br />the:: 109 gages, and w1:fc!\ vus separated in time from
<br />other storm p"riocs ~., !!lore th..n 3 hours.
<br />
<br />Once stono ;>eriods had been, delineared
<br />they were classif1e~ ~slng the satellite. photo-
<br />gt'aphic, and Nationa.~ ';..'~ach~r Servic~ synopt.ic data
<br /><.l.vallable. ~tuch of 1;:tis analy~.ts was ac.comlJlished
<br />by Maddox and Reynol~~ ~1.976) and by Hartzell and
<br />e~rnes (1976). It v~s found that all ~torm9 pro-
<br />cueing rain over the ~e~\Jork could be classified as
<br />ot':~ of th~ following ~.!.v(: types ~
<br />1) mountain-generateJ Cbj
<br />2) coid front;
<br />3) upslope;
<br />4) upper-level ~rough; and
<br />5) airmuss-genera~ed Cb.
<br />
<br />Mountain-generated Cb's were those storms
<br />w!'tic:h formed over the !!'.ountain::i of southwes tern
<br />Yontana or northern '.:yo!!l1.ng and later passed over
<br />the rain gage ne t,,",or~c.
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<br />Table 1 lis~s ~he 40 storms obtierved by
<br />the rain gage net~ork ~~ring 1976. It includes:
<br />(",) time of occurrence. (2) number of gages ~ith
<br />1:~in, (3) volume of ra~.n produced, assumIng each
<br />
<br />gage represents 15.5 km2. (4) rainfall averaged
<br />for the entire n"t~ork, (5) lIV"ru~~ tl.infall for
<br />only those gages r~c~iving ruin. (6) t'~1~ 9tan~ard
<br />dt:vla!:lon in rulnfnll llIOOlJnt for gi]ges receiving
<br />rain, llnd (7) storm typt: u~.!.ng th~ numbering
<br />::lystem above. I
<br />
<br />Next, the storms of table 1 ~ere examined
<br />to find what hours of the day m,~ ~~lt 'lei !)reft:rt:ntial
<br />for storm occurrcnc~s. Th~ fre<!uency :of each storm
<br />type ~as det"rm,lned at 3-hour intet'/a19. ng. 2
<br />prt:::lents the results of this analyst,:; ;wlth t!1e
<br />follo~ing significant f~atures: I
<br />1) Mountain-generat"d Cb' s had a' large
<br />diurnal variation i.lnd JOu~t oftt:l1 uroC'u!ced rain in
<br />th~ network during the evening iln~ nlg'!1ttlt!lt: hours
<br />with a maximum just after durk. ~
<br />2) Airmass-generated Cuts tenc~d to peak
<br />after mid-afternoon. ulthough ~Htnp.!.e size 1s limited.
<br />3) Colu fTonts and uP?eT-l~~el t~oughs
<br />possessed similar rainfall occurrence cycles with
<br />late morning minimums.
<br />4) In gcnerai, the lea.t active precipitation-
<br />producing period ~as 0900-1100 ~!DT. ' S:urface heat- ,
<br />in,g may ,be a significant factor in ,t!1e, rainfall
<br />production cycle for most of the storm; types during
<br />the May - July ~erlod.
<br />
<br />34.
<br />
<br />- (1l - Ma~~r^'~ ~[mmO C9
<br />- -- (1) . C~lO r~a~r ,
<br />,-,- 13l - e?S~m I
<br />..... (.0\) ~ !JP~rn ~!'n:~ I T!'/O!.!GH
<br />.-'- (51 - AI;::,~.'oSS fi[:llWEo C9
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<br />30'
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<br />2- '''__
<br />18-70 21-23 00-02 OH5 as'a! D!"l' : Il'l( 15-17MOT
<br />TIME Of DAY
<br />
<br />10-
<br />
<br />6.
<br />
<br />Figure 2. Storm timing for the five storm ,;ypes
<br />during 1976.
<br />
<br />4. RAINFALL AUGMENTATION MODELS
<br />It is of interest to cons.!..ce'r how the
<br />amount of ~ea9onal rainfa2..1 rn~.gnt b~ n'odlfiec.
<br />using hypothetical seedIng models ii?p.!.llcu to che
<br />var.toua storm types. 'J.'lie: variable-change models
<br />of Huff and Changnon (1972) \J~re chos." as appro-
<br />priate. for they probahly cover tht: t:n'cire range
<br />of conceivable $eeding-inc'.lced c!lang(;:S'~. Table 2
<br />outlines the percentage rainfa~l chang~s assigned
<br />to storms of differing' net'Wor~\. mean ra'infall by
<br />these models. Note that ':he selJera~ !!!odels are
<br />based only on total ~torm rain.fall. 1':11;: selection
<br />of the model most closely fitting t!1e ,microphysical
<br />situation for each cloud tYPt: is beyon,c. the 9cope
<br />of thi. paper.
<br />
<br />The model percentages were ap?lie~ to the
<br />natural storm data of Tab~~ 1. and ~~~ rainfall
<br />change induced by each mocel was ca~c~~ated for
<br />each storm type during May through J'JIy, 1976.
<br />
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