|
<br />.;.
<br />
<br />.:;;
<br />
<br />!~A!t OllSERVF.D CHARACn:!!IS'HCS OF SEMI-ISOLATED CONVECTIV~: CLOUDS
<br />IN EASTE!t'l MONTANA
<br />
<br />John MIddleton and Adin Super
<br />
<br />High P:ail\s Cuoperativt: Program
<br />Bureau of Reclamation
<br />~.l.t~::I Clty. 1-10n t ana
<br />
<br />1.
<br />
<br />IN'!!l.ODUC'EON
<br />
<br />'!he Hlg'l P~.ains Cooperative ~rogt'.:1m
<br />(HIPLEX) is current'.y involved in the desIgn of a
<br />randontized seeding ~x~)CrJ.ment for summer convt;;!ctive
<br />clouds in eastern \!l.IU'-_clll<l. The l1IPLEX D~::Ilgl\ Plan
<br />(Ac~(.~rmun t!t a.t.. 1'11t)) hus tent~ltlvely spccJ.flcd
<br />n convc:ctive com1J~cx Cl.~ on~ of the candlJaccs for
<br />the experiments.!. UIl L!: (or at least a 941mpllH~ unJt)
<br />..t:o,~.. the ..s~~lg..!..e .c-1J~.1.!.1 . t....Y~~.x:iment(1)hase 2) ~ The
<br />convective c:!.oud CVll';t.~e.X has been t~ntative~.y
<br />defined HS ~~iDg a c~ouc group visually scparable
<br />from other groups by c:ou<..!.-frce d19tances of two
<br />group diameters ct' l"or.ci typically 10-30 km. An
<br />lrn?ort;ant con:3.!.~€ t'D.!::!.O!1 is. obviously, the character-
<br />istics of the cor.vc~,:lve comp18x~9 (e.g.. th~it'
<br />sizes, !re,=!'..!ency of occurrence:, lifetimes, anJ rain
<br />pro~uction) a1"'.c t.!~c i11Ji.liC:y to ohjectively Identify
<br />thC:!!l. The siz~ ('~ t~l~ cump1~xe::; ure also Importunt
<br />for d~t~rm.!.l\ing t.~lt: rain gdgt: ~paclng r~'luir~UH!llt
<br />for eva~uction 01: C!IC c.xpt.!rimenc.
<br />
<br />'This pllpe-r: ceals l"Ilth lnleial efforts in
<br /><!eflning a convcetLve comple:x with radar data. Two
<br />days w~re se.!..ectt~l! ,~o i.!..lustratc differ.ences b~twe€.'n
<br />individual 20 d~l:: rill~'! ( echoc's and radar comp.lexc::i
<br />form~d ';)y u~ing :le'b.:rn;~ ~cho diameter separiltlon
<br />distances. One l)f ~~,c days, May 25, rcpr~9dnts a
<br />duy on ....hich con'/';'I.;J;.i.tJn was ast;\oc.f.ated with u cold
<br />fron~al pa~sagc. '~'~le second day, July 19. rcpt:'e~-
<br />ents a dny on ....hl'.C:l L~'~ echoes formed primarity '.Jy
<br />a!.r~.a:>9 C(l!1vcc~ilJn. "....,~h of th~ duys pro(~ut;c..t s.lgn-
<br />!..fican'': r.'ll.'~=a~."..:i .....i.':.'!.11 1.50 ~<m of 1'filt:~ CJr.y, w~.th
<br />grc~:~r ':.!I,'~! 2.5 CP' ',)1,,' Lng recorced,
<br />
<br />A stat !.s'.::'cCI.l SUI!\mary of the 20 d'h~ echo
<br />characteris~!.c~ Eo':" ~~\~ ~ntlre 1976 data bu::>w will
<br />b~ aV<ll~H~:tl for ~H'~~l!lltatioH at the conferenCe,
<br />
<br />2.
<br />
<br />P~~OCED~.."~!':
<br />
<br />~tgitizct.! radar data that were collected
<br />at !-tLlt!:s ~.!.ty, Hon:H1\i1. from April 15 through
<br />.!uly 30. !...l)7G. w.:n. tl~cd for this study. Tlu.~ 5./. em
<br />ra(tar hac it one-d~r-.rce beam \I~l:!.ch was swept tl\ roug~1
<br />360 c!egr~e.'3 of aZ!.J"'..!~~\ in eac!\ ot twelve on~-uegn:~
<br />st~p9 fro~ ~.0 t!11~OU~'1 12.0 d~grees elevation ullglc.
<br />~~le::le "vo!.'J~e SCitns" \.J~re obtained. once each 5 m.!.n-
<br />utes whl!ncv~r rrcc :~l.r:Llclon CC~IOcS were ob~ervef.!
<br />within ~SO km lJf ':'tt: roeur. T!It:se radar datu were
<br />,roces9ud by COIT~utcr co pro~uc2 a composit~ B-~can
<br />(range V9. .l.~Zi.UH.l'.:.1) ~lrcsentat!.un '....hich consJ.sted of
<br />all t:choett g,r~atcr t;~!an 20 dnz from all 12 c.!.evlltion
<br />ansles cO!n?re::lset! 0'11:0 a sing:e ~.!.ane. The 5-rninute
<br />cOJ!1po:;iite :'.-scar ~ ....'et"c then hunc. analyzed to
<br />identify range ,nd az.~muth bouncar.!.es of indIvidual
<br />echoes defint.:c ty t~lc 20 dBz con,;our, using minimum
<br />
<br />separations bt:tween t:choes of 1 km in ronge and
<br />I c.egret: 1n azimuth. Each radar echo was given it
<br />unlqut! identifier th.:: fir9t time it was observed.
<br />ThJ.~, i,dentifier wu~ retained until the ec!lo either
<br />d.l:;isipated, propagut~d outside the observing area
<br />or merged into a 1areer ~cho. The individual
<br />botl1uluri~s were UHt;:t~ for further computer procesH-
<br />Jng Co provJ1de ec'lO time histories of total volume,
<br />ceutroid location, maxtmum tOp, maximum dBz, aver-
<br />a.gt: d~z. maximum raInfall -rat.,;, llv~rage rainfall
<br />ral:e) totLil rainfall un<.! other paramct~rs. SpceJ.:tic
<br />dcr:ails concerning tlle radar characteristics and
<br />the data processing are given by Schroeder ~.
<br />(1976).
<br />
<br />Tile computer generated output for each
<br />ind:tvidual 20 dEz radar echo contains a summary of
<br />"-v,,r.y tilt angle at 1Jhich the echo existed. T',e
<br />tiLl: summary records contaIn. the cell identifica-
<br />ti'JI\ number, time, tl1c Lingle, total area gl.Cuter
<br />ch.:m 20 dBz, centroid location, maximum dBz value
<br />cud location, total echu volume at the ci.lt angle.
<br />are... of echo in 5 dBz steps ranging from 20 d~z
<br />to .:: 65 dUz, the average rainfal~ rate, maximum
<br />rainEall rate and rain flux in !,m per 5 minutes. .
<br />Ru.tn flux it:! calculated for both the Mar~haJ.l-
<br />Palmer 7,-R relatioTlt>hip and the Z-R relationship
<br />derived by Smith !:.!:-a.:.1". (1975) using 10 cm radar
<br />in tho: northern lUgh Plains. There is also a
<br />volume SUlIIlUary record each 5 minutes which contains
<br />the SlJmmat!tm of all tilt records 1n addition to
<br />the max1tuul1l top height and locaticn..
<br />
<br />The indlv;ldual 20 dBz radar echo data are
<br />used to produce t!h~ echo lifetime, nw.x.lmum volume,
<br />~s ci1U<.\teJ total ralntal1 and movement. 'i'hey art~
<br />also useJ to com~ille the individual ~choe~ lnt.o
<br />nH..lar complexes by the following method, The
<br />St.n~milry data for each 5 minutes are computer pro-
<br />ce~sed to dt.':t~rmin(: the maximum urea at ,my tJ.lt:
<br />81's.le and the volume cencroid locatIon for each
<br />cE:ll every 5 minuti:s. The maximum area is used
<br />to calculate the dl&meter of a circ.!.e of equivalent
<br />HI'e:u, hereafter culled the equivalent edlO d.iameter.
<br />Th~: volume centroid locations of two cells are used
<br />to det~rmLne if the two cells are separat~d by a
<br />specified multiple of equivalent echo diumeter of
<br />tlLe: larger of the two echoes. If tll.;:: separation i9
<br />It~SS than the required number of ec!uivalent dia-
<br />mt!t:~r9, the t!:ChOt.':9 ilre considerec part of a com[)lex.
<br />EVt.:ry echo is checked with all otht:r echoes to cctl:.r-
<br />m..nc the ~t:paration and is included in the complex
<br />iJ: 1 t Is within the specifi.ed dJs tance. A chain-
<br />ing effect is al::Jo included. This assures that any
<br />ed\o wh:Lch is closer than the specified distance
<br />(a mu1t'iple of the equivalent dIameter of the
<br />large(:it of the two echoes) to an echo that is part
<br />0.: a com[11ex is also included within the complex.
<br />
|