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Jennings, Keith S. ¹ ; Molotch, Noah P. ²

¹ University of ÃÛÌÇÖ±²¥ Boulder/INSTAAR
² University of ÃÛÌÇÖ±²¥ Boulder/INSTAAR/CWEST

Knowledge of the physical mechanisms governing the evolution of the snowpack internal energy state and snowmelt production is limited by scarce in situ observations of snowpack properties apart from snow water equivalent (SWE). This project leverages a 20-year snow pit record from the Niwot Ridge Long Term Ecological Research site in ÃÛÌÇÖ±²¥â€™s Front Range mountains to quantify the meteorological and energy balance controls on cold content development and snowmelt generation. Pit observations of SWE, snow temperature, and cold content in the alpine and subalpine were used to validate and improve physically based snowpack simulations, which were forced with a quality controlled, serially complete meteorological dataset. Observations and simulations showed cold content increases primarily through additions of new snow rather than through a negative surface energy balance. In the subalpine, fluxes exert a larger influence on cold content as a greater relative portion of the snowpack interacts with surface energy exchange. Simulations indicated the average diurnal temperature range is highest in the snowpack’s upper layers and converges towards 0.3°C to 0.5°C as depth below the surface exceeded 50 cm. Increased maximum cold content tends to delay snowmelt onset, but the relationship is non-significant. Rather, the timing of peak cold content and total spring precipitation better predict snowmelt onset, explaining 68% of the variance in peak SWE timing. Cold content at 6AM is an indicator of daily snowmelt potential with snowmelt rates increasing as the starting energy deficit approaches 0 MJ m^-2. Additionally, snow melts more rapidly in the alpine where the turbulent fluxes play a larger role in snowmelt generation. At both sites, drought years with protracted melt seasons exhibit the lowest snowmelt rates and greatest proportion of peak SWE lost to sublimation.