Using Novel Lake-Based Snowfall Measurements from Sites in the Alps, Himalayas, and Rockies to Evaluate Snowfall Estimates from Atmospheric Simulations Using the High-Resolution Regional Met Office Unified Model

Abstract The complex terrain in mountainous regions makes it extremely difficult to accurately measure or model snowfall, which is a key component of the terrestrial water budget. This study addresses these challenges by using high-altitude frozen lakes as pressure-sensing surfaces to produce accurate observations of the water content of snowfall at a range of sites in the European Alps, west–central Himalayas, and central Rockies, which are subsequently used to test and constrain snowfall output from a 1.5-km resolution version of the atmosphere-only Met Office Unified Model (MetUM). The model resolution is on a similar scale to the size of the lakes, as well as sufficiently fine to represent the critical interactions between atmospheric flows and the complex orography that influences snowfall and especially extremes. The snowfall output from the MetUM is additionally fine tuned by adjusting the fall speed of snow particles so that it is best able to capture the observed snowfall amounts, especially for extreme events. The results presented here show that the MetUM is generally able to accurately simulate both the timing and amounts of the snowfall observations over mountainous regions. Moreover, the model is particularly good at representing extreme snowfall events, with our study also using model hydrometeor output to examine the microphysical conditions related to these conditions. Finally, we suggest that the model output can effectively be used as pseudo-observations and for the generation of high-resolution, long-term gridded snowfall products. Significance Statement Snowfall in mountainous regions is a vital source of freshwater, sustaining rivers that support both large populations and diverse ecosystems. However, the complex topography of these areas poses significant challenges for accurately measuring snowfall. This study tackles these difficulties by using high-altitude frozen lakes as natural pressure sensors to monitor snowfall water content across sites in the European Alps, the west–central Himalayas, and the central Rockies. These observations are used to constrain and refine snowfall simulations from a high-resolution version of the Met Office Unified Model. The results show that the model can accurately simulate both the timing and amounts of the snowfall observations and can effectively be used for the generation of long-term snowfall products.