Some Characteristics of Turbulent Transfer over Alpine Surfaces During the Snowmelt-Growing Season: Niwot Ridge, Front Range, Colorado, U.S.A.

Field studies of turbulent transfer over snowfields and tundra vegetation surfaces on Niwot Ridge indicate that eddy diffusivities range through almost three orders of magnitude during the snowmelt-growing season (53 × 10−4 m2 s−1 < K′m < 4404 × 10−4 m2 s−1). Average K′m values are roughly three times higher over tundra vegetation surfaces (K′m = 1724 × 10−4 m2 s−1) than over snow (K′m = 504 × 10−4 m2 s−1) because the winds are weaker over snowfields (which tend to have leeward aspects) and the turbulent boundary layer is very stable (due to an inverted lapse rate of temperature). A plot of shear velocity versus average wind speed at 0.8 m above the surface reveals a strong linear correlation and the possibility of different regression lines for tundra vegetation and snow. Between site differences in mean wind velocity depend upon the prevailing wind regime. On days of westerly flow, ridge crests and west-facing slopes experience winds (at 0.8 m) two to three times faster than the winds blowing across sheltered leeward hollows. However, when the prevailing wind regime is upslope, average wind speeds are relatively low above surfaces of all aspects and the between-site differences in mean velocity are not statistically significant (99% confidence level). Estimates of sensible heat advection based on horizontal differences in observed temperature profiles suggest that advection normally accounts for less than 5% of the total sensible heat flux over vegetated tundra surfaces but the proportion may approach 20% over snow.