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

Abstract

The long-term climate records of the Mountain Research Station (MRS) at the University of Colorado cover a range of elevations from the lower montane forest zone (~2000–2400 m), through the upper montane forest (~2400–2800 m) and subalpine forest (~2800–3300 m) to the alpine tundra (>3300 m) on Niwot Ridge, Colorado. Temperature records from all four MRS sites and the additional high plains site of Longmont (1509 m) are analyzed for the period 1952–1997, after extraction of much extra data from the original thermograph charts. The records are adjusted for instrumental changes where necessary and all four records are judged to be homogenous. Contrasting temporal trends are uncovered at the various elevations with warming at middle elevations and absolute cooling above the treeline in the alpine tundra. The resulting increased surface-based lapse rates do not arise from changes in relative frequencies of airflow types as is shown by a synoptic analysis based on objective airflow indices. Lapse rate increases are most systematic for synoptic classes with westerly components and during fall, winter, and spring. Climate at high elevations of the Front Range appears to be responding in an unusual way to global-warming influences.