The interaction of topography and ambient atmospheric conditions and their combined effect on surface climate are studied in the alpine fellfield on Niwot Ridge, Colorado Front Range. Insolation, net shortwave and all-wave radiation, surface and air temperatures, ground heat flux, wind speed, and evapotranspiration are analyzed in order to characterize moisture and energy exchanges. The spatial pattern of daily energy and moisture fluxes is influenced more by cloud cover than topographic position. The daily radiation load is usually highest on east-facing slopes because clear mornings are commonly followed by cumulus development during midday and early afternoon. In the dominant westerly wind regime, wind speeds on east-facing slopes are lower than elsewhere. Strong insolation produces high rates of evapotranspiration from the thin, sandy/gravelly fellfield soil when moisture is abundant. After a few consecutive precipitation-free days, large radiation loads produce high surface temperatures, especially on east-facing slopes where wind speed and thus the sensible heat flux are small. During the 1985 summer drought period, the impact of hot, dry weather on plant water stress was most pronounced on east-facing slopes. The well-defined distribution of Dryas octopetala on Niwot Ridge, near the southern extent of its circumpolar range, is an example of the control of topoclimate over plant distribution owing to water stress during periods of drought The absence of Dryas from south-facing slopes does not correspond well with present topoclimatic patterns. The present distribution of Dryas may be a consequence of topoclimatic patterns throughout the middle of the last millennium when the summer climate in the Colorado Front Range was hotter and drier than today. Summer droughts then were probably more prolonged, and a combination of clearer days, lower wind speeds, or an increased frequency of northerly wind would have created more intense water stress on south-facing than east-facing slopes.
All Science Journal Classification (ASJC) codes
- Environmental Science(all)
- Atmospheric Science
- Earth and Planetary Sciences (miscellaneous)
- Earth and Planetary Sciences(all)