Simulating The Nonlinear Response Of Alpine And Subalpine Snowpacks To Climate Warming
Jennings, Keith S 1 ; Molotch, Noah P 2
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It is well known that climate warming invokes nonlinear responses in snow accumulation and melt within the seasonal snow zone. Previous research has shown that higher, colder sites are generally more resistant to warming-driven reductions in peak snow water equivalent (SWE) and snowmelt rate. While prior work has examined the proximate empirical causes (e.g., elevation and air temperature) behind this phenomenon, little research has been done on the physical mechanisms behind the differential response. Here we explore the energy balance processes controlling the nonlinear effect of climate warming on alpine and subalpine snowpacks in order to identify the underlying physical, not empirical, causes. To do this we performed a series of physics-based snow simulations using the SNOWPACK model at an alpine (3500 m) and subalpine (3000 m) site in the Niwot Ridge LTER in Colorado’s Rocky Mountains. Baseline runs utilized a 23-year quality controlled, serially complete forcing dataset and model output was validated on SWE, depth-weighted snowpack temperature, and cold content from a long-term snow pit record. We then utilized a pseudo global warming approach where we increased air temperature from 0.5°C to 4.0°C in 0.5°C increments with associated increases in downwelling longwave radiation. The subalpine site was more sensitive to air temperature increases, experiencing significant reductions in snowmelt rate (-20.3%) and peak SWE (-57.0%) over the 4.0°C range, while the alpine exhibited a non-significant snowmelt response and a significant reduction in peak SWE (-17.1%). We found two physical mechanisms behind the nonlinear response: 1) A significant decline in subalpine net energy exchange (Qnet) during the primary snowmelt period; and 2) A step change in the relationship between cold content (CC) and Qnet throughout the winter in the subalpine, whereby Qnet became consistently greater than CC, enabling persistent snowmelt. There was no evidence of the above two processes in the alpine simulations.