Hauling field gear up Gothic Mountain…
Abstract Sublimation is an important yet uncertain component of the water balance in snow-dominated regions. Models that parameterize sublimation as a function of temperature show that sublimation has increased over recent decades, and this has been identified as a possible reason for declining Colorado River streamflow. However, sublimation increases with wind speed and water vapor gradients and cannot be well explained by temperature alone. Thus, we hypothesize that most accumulation season sublimation occurs during distinct weather events. To test this hypothesis, we combined 2 years of meteorological observations spanning site-to-synoptic scales from three field campaigns in a Colorado mountain valley and identified the events that produced the most sublimation, classifying them by duration and intensity. We found that approximately 60% of winter sublimation occurred during a small number of discrete events (14% of the season). These events coincided with either short sunny, dry periods or long storm events that brought new snowfall followed by blowing snow. Using these classified events and synoptic-scale reanalysis, we trained a random forest classifier to estimate long sublimation event occurrences and test whether they increased over the past four decades. The model performed well (average precision = 0.73; balanced accuracy = 79%), and results were most strongly associated with winter mean 500-hPa wind speed and total observed precipitation. Event occurrence varied considerably year to year with no significant long-term trend. In all, our findings show that accurately representing seasonal sublimation requires capturing discrete events driven by identifiable synoptic and local meteorological conditions. Significance Statement Sublimation, defined as the direct phase transition from ice to water vapor, plays an important, yet uncertain, role in determining how much snow is available to melt at the end of each winter. We wanted to reduce this uncertainty by identifying the types of weather events that cause the most sublimation to occur. We found that a majority of winter sublimation (60%) occurs over just a small fraction of the winter (14%), and large-scale weather dynamics can be used to indicate when these sublimation events occur. Our results provide new insight into how large-scale processes impact sublimation in complex terrain and point toward a better understanding of the major processes that cause sublimation to vary over time.