|
<br />
<br />e<juallength (8298 d each) and the lapse nile CQJllrll.Slbetween
<br />1he earlier and later periods examined. Within and between-Iype
<br />comnbutions 10 lapse rom; chana:e (for each synoptic class at 100
<br />mb) are summarized for !be daytime lapse rale on the upper
<br />~Iope ICI Vi DI) in Table 8. 1be final column ,ives the ralio of
<br />wi!hin'Iype cl1ange dIvided by belween-type chana:e, and almost
<br />wi!bollt exceplion, values :u-e well above I fOl" synoptic c1aues
<br />witl1 weslerly .;:omponems (the mou frequent classes). This re-
<br />inforces lhe dominance of within-type chNlje. For the more ea.st-
<br />erly .ynopllc types. ralios bel<.lw I mean relatively link within-
<br />type change. but abs<.llllte.;:lumgesin lapsc:r.lIeilR also much
<br />smaller (li"l IWO colullUls) and llR moslly llOI statistically sig.
<br />nificant.
<br />T<.I delermine whether weslerly and oorthMlsterly aiJ-ftow
<br />c1aueshave.;:hangedllow chanlcLerislics during the n:cocd (a
<br />pos.\ible cause <.If \he slIong wilhuHypc lapse nte changes iden-
<br />tified). time senes of S. W, F. and Z fOl" these synoptic clas.:;es
<br />were examined. An im;rease in the /1ow Slrellp associated willi
<br />weslerly c1a.sses for example (i1lQ'Ca.R in F) could enhance
<br />doWllSlope wanrung and flihn effects (Barry, pers. comm..
<br />1998), thus strengthenina: lap:;e rates. However, investigation of
<br />index changes for all synoptic classes (Table 9) IlIlCOvered very
<br />(ewslgniticamrelationsl).ips(a11 signifi"amo;:baqges are listed).
<br />In panicular. index cluma:es for the: nonhwc,stedy cl3S$ were of
<br />opposite sIgn to those UpeCled to ac<:ount for aay stronger lapse
<br />rales (i.e.. F decreased). Therefore sublle wilhin-type "hanges in
<br />air:flowchlll1l<:terdu nOt explain thewithin-typclapsc: rate chang.
<br />esobserved.
<br />
<br />Discussion
<br />
<br />The relative imponan.;:eof within-sYllOpti.;:typc cbanges in
<br />lapse rate. and the inabilily of between-airflow type <:llanies to
<br />ac:countfortemperaturechangesobserveds~IheCJlIlyI9SOs
<br />on lhe eaStern slope of the Front Ranie, implies processes ex-
<br />ternalto the Rocky Mounlain regioolO be infIUelltial. TIle strong
<br />sw1ace cooling trend (and stronger lapse nll(5) II biaber eleva-
<br />tions is marked when Ilie prevailing weslerly dow is SIrOn., but
<br />is less marlr.ed under relatively iDfrcquem upslope COlIditions.
<br />Thi. high.elevation .;:ooling I1as also been associated with in-
<br />creased wetneu al \he highest elevations. Amlual ptecipitalion
<br />hasin<:reasedatDI nverlhelast46yrllldseveral)'ear$inlhe
<br />1990s are amooa: the wellC:SI in tbc m:ord (W1Ilia.ms el aI.,
<br />1996). Increasedpredpitation a. high elevations would usually
<br />mean increased snowfall, and a deeper hiih-elcvation s.nowpack
<br />cOllldstrenglhenlapseralesthrnughlllincreaseduplandl1owland
<br />snow gradient.
<br />C<.Impanson with changes in ElUOpe is inlriiuiDj:. In \he
<br />European Alps, wannmg has been amplified It ruab elevations
<br />(BelllslOll et aJ.. 1994), but unlike Colorado this lias also been
<br />as.ociated wllh ~ .;:hange in cin;ulation, namely an increase in
<br />the Nonh Atlanllc Oscillallon index (NAO) with more frequent
<br />anllcyclones over central Europe especially in winleI. TIle rela-
<br />live warmth of such synoptic cor>ditious increues wilh elevalion
<br />(GiorgI el al.. 1997). III C<.Ilorado no sySlernaricchanges incir-
<br />culatioo frequencies have beell identified. Chanaes experienced
<br />al high elevali<.lns in the maritime uplands of the U.K. lITC more
<br />sinular 10 the Colorado lapse rale changes. United Kinadom up-
<br />land litesilfe showing much less wlll'ming than adjr.cCIll lowland
<br />ilfus(Garnetletal..I991)andrco;:entworlr.(Pepin,lInpublisbed)
<br />suggests 1hestrengthening in lap:;e nlte 10 bc: mcntapparenlun-
<br />der westerly synopti" lilualions. Cbanies II bi&b elevations in
<br />the Front Range appear to be mOre similar 10 dtu",a in northern
<br />and western Europe and increased uposure 10 Ihe mobile mid-
<br />
<br />1441 All.cnc, ANTAIl.CTIC, AND ALPINE REsEJt.IICH
<br />
<br />latilude westedies may be a common factor. In suppon of this,
<br />the lower slopes of \he Front Range are more i$Ol:lled from upper
<br />tropospherk ilirftow and subjecI to more local cJimali" illJluences
<br />incJudin, mesoscale mversions (Barry. 1913). 'The local lapse
<br />rate weakening belween Al and BI is more likely to be locally
<br />.;:ontroJled.Colllinuinllresearchinterellinthehillh-elevalioncli-
<br />mate of the Fronl Range is made more important therefore by
<br />\he idcathal hiih-elevationareas may have global significance.
<br />Ground-based stations give lapse rateS whicl1 will differ
<br />from those inlhe free-aiJ-. A comparison of the IWO is clearly an
<br />imponalll exteDJion of tl1is work. In particular this will sllow
<br />whelher the hia:h-elevalion surf~ce cooling experienced i5 also a
<br />free-air phen<.lmenon.lf IllH this suggesls a reduction in the pl~-
<br />teau healini effeclofthehillh Rocky Mounlilins. wrnchwould
<br />require either increased sllOW cover, reduced insolalioll or in-
<br />creased cold ttir advection (higher willd speeds). This also I1lgh-
<br />lighuthe need for coherent study of other climalic elemenlS,
<br />especially wind speed, preclpitatinn (including snowfall). solar
<br />radiation inpul and cloud cover. Unforu.nately reliable dala are
<br />oot available foc very long periods. Precipilau<.In re<:nrding is
<br />probleflllltic at hillh elevations due 10 sheltering facl<.ln and lhe
<br />large &(DOunt of solid precipitation (Williams et a1.. 1998). Wind
<br />and solar radiati<.ln d.ala are only reeenlly available (since the
<br />1990s) and doudiness data is Ihe hardest to oblain. Snow cover
<br />data is also i'ill<:hy. 1be develOpmelll of lhe SNOTEL database
<br />(see the website; hnp;lIwww.wr"c.dri.edulsnotel) is interesting
<br />but at presenl rco;:ords are shon. A final limilation of the present
<br />an.a..Iysis is thercstriction 10 \he eaStern slope of the Front Range.
<br />Changes identified in this paper m~y nOt apply on Ilie weStern
<br />slope of Colorado and in other pans of the U.S.
<br />
<br />giSl at the Mounlilin Research Sl~tioll. who l1e1ped with data
<br />handling and &Ieally improved the manuscript. Michael Hart-
<br />man. the Long-Tenn Ecologi"aJ Research Dala Manager. helped
<br />with data manipul~uon and storage. I also thank: \he numer<.lus
<br />climalologists ,II the Mounlttin Rese:u-c11 Station over the years
<br />woo have slnlll8led agilinst advet"S<: comtitions to keep meleo-
<br />rological observalions gOillg. Sleven Muller kirtdly provided
<br />Figure l. Mark Losleben provided Figure 5. Other tigures were
<br />produced usinglhe stalistical package STATA (StalaCorp 1995).
<br />Pressure dala for c~lculalLon of synoptic indices was provided
<br />by Will Spangler al Lhe Nali<.lnal Center for Atmospheric Re-
<br />se:u-ch in Boulder. Clare Goodess ~t the Climallc Researcl1 Unit.
<br />University of EasI Angli~. advised on Ilie us<: of a1g0rilhmS in
<br />the synopti.;: claSSlficalion.1 also thank Roger Barry and Nolan
<br />Doesken forlheir helpful insights.
<br />
<br />References Cited
<br />
<br />Summary
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<br />
<br />Acknawledgmeats
<br />
<br />TIle research for this paper wa.s undenaken as a visiting
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<br />N. PEPlN/145
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