Despite widespread warming in mountain regions, little research to date has explored the physical mechanisms driving the variable response of snowpacks to changes in climate, instead focusing primarily on empirical relationships, such as seasonal air temperature or elevation. In this work, we evaluate how differences in snowfall fraction, cold content, and the snowpack energy balance produce simulated changes to snow accumulation and melt at an alpine and subalpine snowpack in the Niwot Ridge Long Term Ecological Research site. For our analysis, we created a 23 years baseline simulation using the SNOWPACK model forced by historical hourly meteorological data from water year 1991 through 2013. We then perturbed hourly air temperature in 0.5°C increments from +0.5 to +4.0°C above baseline and increased incoming longwave radiation accordingly. For every 1°C of warming, peak snow water equivalent declined by 43.9 mm in the alpine and 54.3 mm in the subalpine, melt onset shifted 6.2 days earlier in the alpine and 8.8 days in the subalpine, the snow season shortened by 10.7 days in the alpine and 16.4 days in the subalpine, and melt rate increased by 0.2 mm d⁻¹ in the alpine while decreasing by 0.4 mm d⁻¹ in the subalpine. We found the alpine snowpack was less sensitive to warming for three primary reasons: (1) Snowfall fraction decreased less rapidly per 1°C of warming than in the subalpine; (2) Cold content still consistently developed throughout the snow season, preventing mid-winter melt events; (3) Changes to snowmelt rate were not significant because increases to the turbulent fluxes balanced decreases in the radiative fluxes with earlier snowmelt onset. Additionally, at 3°C of warming and greater, the subalpine site experienced a fundamental shift where significant melt could occur throughout the entirety of the winter as cold content was no longer large enough to buffer against positive energy fluxes. In some years, the subalpine snowpack became transient with several cycles of accumulation and melt per winter. This tipping point suggests sites with lower cold content—like the subalpine studied here—are likely to be more sensitive to producing increased winter melt as warming continues over the coming decades.

尽管山区气候普遍变暖，但迄今为止，很少有研究探索推动积雪对气候变化做出不同反应的物理机制，而是主要关注经验关系，例如季节性气温或海拔高度。在这项工作中，我们评估了Niwot Ridge长期生态研究站点中降雪分数，冷含量和积雪能量平衡的差异如何产生模拟的积雪变化，并在高山和亚高山积雪融化。对于我们的分析，我们使用SNOWPACK模型创建了23年的基线模拟，该模型受1991年至2013年水域的历史小时气象数据的强迫。然后，我们以每小时0.5°C的增量扰动每小时的气温，从高于基线的+0.5到+ 4.0°C，相应地增加了入射长波辐射。在高山地区^-1，而下降0.4 mm d ^-1在亚高山。我们发现高山积雪对变暖的敏感性较低，主要有以下三个原因：（1）每升高1°C，降雪分数下降的速度不及亚高山降雪速度；（2）在整个雪季中，冷含量仍然持续增长，从而防止了冬季中期融化事件；（3）融雪速率的变化不显着，因为随着融雪的开始，湍流通量的增加平衡了辐射通量的减少。此外，在3°C或更高的升温下，亚高山地区发生了根本性转变，整个冬季可能会发生明显的融化，因为冷含量不再足以缓冲正能量通量。几年来，亚高山雪堆变得短暂，每个冬季都有几次积累和融化的循环。