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

Abstract

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 sensible Q h and latent Q e heat. In the lower snowpack, daily snow temperature changes greater than 18C day 21 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 snowpack warm-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 10 W m 22 and rarely had a magnitude larger than 20 W m 22 . 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 (100-400 W m 22 ) 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.

Climate controls over ecosystem metabolism: insights from a fifteen-year inductive artificial neural network synthesis for a subalpine forest

“…Evapotranspiration (ET) was calculated from latent heat flux. For further discussion of the eddy flux measurements at Niwot Ridge, see Monson et al (2002), Turnipseed et al (2002Turnipseed et al ( , 2003, Yi et al (2008), and Burns et al (2014Burns et al ( , 2015Burns et al ( , 2016. The data and further details on processing are available from the Niwot Ridge AmeriFlux web site (http://urquell.…”

Observations and simulations of the seasonal evolution of snowpack cold content and its relation to snowmelt and the snowpack energy budget

“…These methods suggest that cold content develops through both meteorological and energy balance processes, but few direct comparisons to observed cold content exist. This is likely due to the inherent difficulty in measuring cold content, which requires either time-intensive snow pits or colocated snow depth, density, and temperature measurements (Burns et al, 2014;Helgason and Pomeroy, 2011;Marks et al, 1992;Molotch et al, 2016). The lack of validation data introduces significant uncertainty into the dominant process by which cold content develops.…”

“…The entirety of the LTER is situated above 3000 m with treeline occurring at approximately 3400 m (Williams et al, 1998). Dominant vegetation in the subalpine is lodgepole pine, aspen, Engelmann spruce, subalpine fir, and limber pine (Burns et al, 2014). The alpine is characterized by several tundra vegetation communities of grasses, forbs, and shrubs, whose distribution is linked to patterns of snow depth and soil moisture (Walker et al, 1993(Walker et al, , 1994.…”

“…From 2008 to 2012, annual precipitation in the alpine and subalpine averaged 1071 and 752 mm, respectively (Knowles et al, 2015) and the ratio between above-and below-treeline precipitation varies annually as a function of upper-air flow regimes (Kittel et al, 2015). The majority of annual precipitation is snow, with estimates of the proportion of snowfall ranging from 63 to 80 % of total precipitation (Caine, 1996;Knowles et al, 2015). Over our study period, December, January, and February mean air temperature was −10.3 • C in the alpine and −6.2 • C in the subalpine.…”

“…Over our study period, December, January, and February mean air temperature was −10.3 • C in the alpine and −6.2 • C in the subalpine. Dominant wind direction is westerly, but the subalpine site also experiences easterly flow during intermittent upslope events (Blanken et al, 2009;Burns et al, 2014). Elevated wind speeds in the alpine, averaging 10 to 13 m s −1 in winter, exert a primary Figure 1.…”