Winter Warm Spells and snowpack ablation in western North America

Winter warm spells (WWS) are extreme temperature anomalies that might impact the snowpack. WWS amplify snowmelt and sublimation in mountain regions with uncertain consequences to timing and volume of water resources. Most studies focus on the spring season when snowmelt rates and streamflow response are high. However, winter snowmelt events are important in places where the snowpack and air temperatures are closer to the freezing point during winter, and thus it will become important in other regions in a warmer climate.

This study aims to understand the effect of WWS on snowpack ablation patterns in the mountainous western North America and how they might change under a warmer climate. For this, we use two convection permitting regional climate model simulations to represent historical (2001-2013) and future atmospheric and the surface conditions. The future simulation is performed with a Pseudo Global Warming approach for a high emission scenario (RCP8.5). We verify WWS using gridded maximum daily temperature observation, and winter ablation using snow pillows. Then we characterize WWS and relate them to snowpack ablation.

Although days with ablation during WWS represent a small fraction (8.3%, 0.6 days on average), 55% of total ablation occurs during WWS over regions with significant snowpack (mean peak snow water equivalent over 150 mm). Consistently, a larger ablation rate (53%) is found during WWS than non-WWS events. Total ablation during WWS increases about 157% in a warmer climate; however, the extreme ablation (99^th percentile) rates show slight decrease (5%). Classifying the domain based on its humidity and temperature, we found that ablation rates during WWS in humid regions are larger in a warmer climate than those of the dry regions, which is explained by the differences in the energy balance and the snowpack cold content. WWS predominantly drive snowmelt (93.8%) rather than sublimation (6.2%), which has relevance to water resources such as flood risk, soil moisture, and streamflow response. Furthermore, the median snowmelt rate during WWS found to increase in response to warming by 179% compared to the median sublimation rate (125%). This study provides a comprehensive description of the impact of extreme temperature events and a warmer climate over our changing snowpack. We acknowledge financial support by Centro CRHIAM Project ANID/FONDAP/15130015, and the Anillo project ACT-210080.