FLOOD10041

• County: Boulder
• Basin: South Platte
• Title: Twentieth-Century Change in the Climate Record for the Front Range
• Date: 1/1/2000
• Prepared For: Boulder County
• Floodplain - Doc Type: Educational/Technical/Reference Information

Arc/ie, An/tlTC/ic, tlnd A/pint Rtstarcll. Val. 32. No.2. 2000. pp. lJ5-N6

"

Twentieth-Century Change in the Climate Record for the Front Range,
Colorado, U.S.A.

Nick Pepin

D<:panmc"tofGeog,~phy.
Bucl,,"gham BUIlding. Lion Terrace.
Un,ve..i'yof Porumou'h. Ponsnlou,h
pOl 3HE. U-K
n,cholas,pep,[e-mail]

Ahstract

The IOng-tenn climate records of lhe Moulllain Research Slation (MRS) al Ihe
Univenity of Colorado cover a range of elevations from the lower montane foresl
wne (-2000-2400 m), through Ihe upper mOlUane forest (-2400-2800 m) and
suhalpine forest (-2800-3300 m) to the alpine tunllJa (>3300 m) On NiwOI
Ridge, Colorado. Temperature rec6rds from all foW' MRS sites and the additional
high plains site of LongmolU (1509 m);u-e analyzed for Ihe period 1952-1997,
after extraction of much extra data from the origInal thermograph chans. The
records are adjusted for instrumental changes where necessary and all fourr ecords
are Judged to be homogenous. Conlrasting temporal Irends are uncovered al the
various elevations with warming at middle elevations and absolute cooling ahQve
the treeline in the alpine tundra. The resulting increased surface-based lapse rales
do nO( arise from changes in relative frequencies of airflow lypes as is shown by
a synoptic analysis based on objeclive airllow indices. Lapse rale increases are
most syslematic for synoptic classes wilh westerly components lind during fall,
wimer, and spring. Climate at high elevations of the Front Range appears to be
responding in an unusual way to global-warming influences.

Introduction

Due 10 Ihe recent inleresl in climalc vanabllilyandchange
(Houghlon ct aI., 1990, 1995) it is beconung increasingly im-
ponant to oblain reliable homogenous long-term insU\lmemal
records. This is especially the case al high elevations where it
is genernlly believed that any fulure climale change associated
with global warming is likely to be amplified (Benislon et a1.,
1994; Beniston and Rebetn. 1996; BenlSlon et aI.. 1997; Giorgi
elal., 1997). Moreove.rin topographically comple:o; region. it is
difficult to model .patial climatic pallems, and bence GCMs do
llOI perfonn so well in theu- attempt to .imulale local climate
variability and change (Barry, 1990. 1992; Giorgi et a1.. 1991).
Observational studies of change in high-elevation areas (Brown
et al., 1992; Diu; and Graliam. 1996; Diaz iUld Bradley. 1997;
Gamel( et a1" 1997) are thus becomlllg important bolh as a cli-
mate change detection I{)(}l and in order to verify GCM modeling
studies for mounlainolls te&ions (Giorgi et a1.. 1997).
One of the best altitudinal transeCIS o{climale dala in the
world is lhe Rocky Moumain From Range climate InUlseCI de.
veloped by the Mountain Re""archSlalion. UniversilyofColo-
rado at Boulder from Ihe early 1950s(Marr.I967).1bispaper
examines change in the 46-yr temp.:rature record (1952-1997)
Wilhspecillcreferencetoelevationalcompansonbetweensites.

The Front Rug, dimatDlogicaJ S.rvey

11le University of Colorado, lhrollgh the Mounlain Re-
search Station (MRS). set up a climate monitoring program in
theFrontRangeinI952(Marr,I967;Marretal..1968a.I968b).
lIS purpose at that time was to provide detailed climate infor-
mation for ecological work being undenaken in avarielyoflife
wnes. but especially on Niwot Ridge. an alpine lUndrllCCOSYS-
lem in the lee of the Con~nental Divide at around 3700 m above
se1l level. 11le mun climale stations (named AI. B1. CI, lUld

1

C 2000 RegenlS of the University of Colorado
1523-0430100$7.00

Dl)werechosenlorepresentlhefollrmajorecologicalronesof
the eastern slope of the Front Range;u dellned by Man (1967).
These ue, in order of ascending elevalion, the lower monlatle
forest,uppermonlane forest, subalpine forest, and alpllle tundra.
The treeline liesatarDllIId3350-3500m(Greenland.1989l. Lo-
cauollS of the sites. along with the additiooalhigh plains sile oc
Longmant (1509 m), are shown in FiJUR 1. Despite variable
vegetation IYpe!i,llll sites are ridCe-topJocatioos in order 10 erad-
icate loealtopographicaJ inftueneea as much u possible. espe-
cially local temperature iuvenionsthat are common in the dry.
often cloud-free climate of ~Iondo. The MRS sites cover a
range of over 1500 m in elevation from 2195 m al AI (Ponde.
rosa) 10 3749mal Dl (NiwOl). Furtho;.rdetails for each Stle are
given in Table I.
The a1tiludmal range of 2200 m between Longmom and Dl
in sllcha shortdislance (-30 lun) is impressive,promplingpasl
sludy of lhe Fram Range recurd. Many reports have been wnnen
with ecology in mind (Marr. 1967. 1968a. t 968b; Greenland $1
al.. 1985). A thorough c1imalological study was uQdenaken by
Barry (1972. 1973). That work was e:O;l"ndcd by Greenland
(1987,1989). Pasl analyses of climate change lIIlhe Rocky
Mounlain region include Browl\ el at. (1992) who ulilized paired
mountainlhish plains stalions in Cll(ora<.lo, Temperatures at D I
were paired Wilh Burlington (a high plams sile in eaSlem Col.
orado). At both sites muimum temperalures decreased during
the li,ne period analyzed (I95J-1991) but mimmum lempera-
tures increased at Burlinglon (and other high plains 5lations) but
decreased at 01. Thus increases in lhe lapse rale of minimum
temperalUre were uncovered. but lh.e trend in daylime lapse rates
(maximum temperatures) was weak and statistically in5ignifi-
cant. Results within Colorado were vanable, however. depending
On the station~ chosen. OIher station pairs yielded differenl
trends. Imponantly, lapse rale clt.anges Identified were contrasl-
ing when Burlinglon was compared with Cl rather than Dl.

N. PEPIN I 135