Cloud Physics Modifies Precipitation Isotopes in Wintertime Storms Crossing Western Canadian Mountain Ranges: Case Studies from 2016-18 compared to CNIP/GNIP and High Mountain Data Sets

Isotopic composition of precipitation was analyzed to improve our understanding of the cloud physics controlling wintertime storms crossing western Canadian mountains. This was used to track hydrometeor drift during strong cross barrier flow, embedded and terrain induced convection, and precipitation enhancement mechanisms near the snow level. This study was motivated by large differences in the forecast versus the observed precipitation reported by the Pacific Storm Prediction Centre using operational numerical models. In this study the high resolution (1 km and 2.5 km) numerical model forecasts from Canadian Meteorological Centre (CMC) containing variations of the most advance advanced multi-moment hydrometeor cloud physics models (based on the Milbrandt-Yau scheme) were compared to the GEM Regional model (10 km) which does not allow for hydrometeor drift and has stronger terrain blocking recently added. Wintertime forecast problems were reported greatest for intense Pacific storms with strong cross barrier flow. Focus in this study was placed on conditions favorable for the existence of super-cooled liquid water, and for near freezing conditions where the snow level may drop lower than expected in heavy precipitation and then suddenly jump upwards when freezing rain was reported. These wintertime forecast challenge cases often occur when a strong warm moist airflow exists aloft between about 1-3 km asl, and interacts with mountainous terrain. To address these forecast situations isotopic sampling of precipitation was carried out over the short term (from hours to days), and compared to longer term precipitation data sets available (from water samples, lake sediment cores and ice cores) from western Canada. For this study, storm-mountain interactions were tested on the scale of the actual terrain across Coquihalla Highway (ranging from sea level to 1.2-1.5 km asl with nearby summits 2 to 3 km asl). Isotopic samples were measured for individual storms at selected locations, and along transects from the BC coast to the Alberta Rockies during 2016 to 2018. Monthly precipitation samples from the western Canadian stations in the Global Networks of Isotopes in Precipitation (CNIP/GCIP) were obtained by Health Canada in cooperation with Meteorological Services of Canada and isotopic compositions were measured by the International Atomic Energy Association in Vienna, Austria to compare with similar data obtained during the last half century. For longer term isotopic comparisons, published data from isotopic cores (spanning into the last ice age) and precipitation studies from Mount Logan in the St. Elias mountains of southwestern Yukon were used to help explain puzzling isotopic shifts and unexpectedly heavier isotopic compositions observed at the highest elevations (3-5 km asl) and downwind of the mountains at Jelly Bean Lake. These observations are consistent with warmer, wetter and stormier than normal wintertime conditions observed in the Yukon in recent years. A comparison with new GNIP monthly isotopes since 2016 is underway to assess the data quality and utility for climate and weather analysis applications. In this study, data shows how air mass water isotopic signatures were modified by the standard Rayleigh fractionation processes as hydrometeor fallout occurred when storm clouds equilibrated and migrated upwards and inland over the mountains. Rapid non equilibrium cloud processes were necessary to explain deviations during the strongest storms with strong convection. Different methods for formation of ice hydrometeors were examined using the droplet freezing versus the multiphase (water ice, vapor) Wegener-Bergeron-Findeisen process theory for during storm events with focus on heavy snowfall, freezing rain and changing snow level events. Most results may be explained by how mid-latitude extra-tropical cyclones are fed by a warm moist airflow aloft (called an “Atmospheric River”) and how these Atmospheric Rivers vigorously interact with terrain. New cloud physics theories ( i.e. Bolot et al, 2013) can explain why unusually heavy isotopic data and sudden shifts are observed with heavier snowfalls near the summit of Mount Logan and just downwind. Stunning new satellite based isotopic data sets are on the horizon and may prove to be useful if they can be validated using operational cloud physics models. In summary, results from this study demonstrate how new models of cloud physical processes can be verified using stable water isotopes, and how merging model and isotopic data can help us address climate change, adopt new satellite data and better forecast difficult winter weather in Canada.