Snow Temperature Changes within a Seasonal Snowpack and Their Relationship to Turbulent Fluxes of Sensible and Latent Heat

摘要

Snowpack temperatures from a subalpine forest below Niwot Ridge, Colorado, are examined with respect to atmospheric conditions and the 30-min above-canopy and subcanopy eddy covariance fluxes of sensibleQ_h and latent Q_e heat. In the lower snowpack, daily snow temperature changes greater than 1°Cday~(-1) occurred about 1-2 times in late winter and early spring, which resulted in transitions to and from an isothermal snowpack. Though air temperature was a primary control on snowpack temperature, rapid snowpackwarm-up events were sometimes preceded by strong downslope winds that kept the nighttime air (and canopy) temperature above freezing, thus increasing sensible heat and longwave radiative transfer from the canopy to the snowpack. There was an indication that water vapor condensation on the snow surface intensified the snowpack warm-up. In late winter, subcanopy Q_h was typically between 210 and 10Wm~(-2) and rarely had a magnitude larger than 20Wm~(-2). The direction of subcanopy Q_h was closely related to the canopy temperature and only weakly dependent on the time of day. The daytime subcanopy Q_h monthly frequency distribution was near normal, whereas the nighttime distribution was more peaked near zero with a large positive skewness. In contrast, above-canopy Q_h was larger in magnitude (100-400Wm~(-2)) and primarily warmed the forest-surface at night and cooled it during the day. Around midday, decoupling of subcanopy and above-canopy air led to an apparent cooling of the snow surface by sensible heat. Sources of uncertainty in the subcanopy eddy covariance flux measurements are suggested. Implications of the observed snowpack temperature changes for future climates are discussed.