Global snow water equivalent observations from space

Vast regions of the world depend directly on snowpacks for freshwater resources, food security, and ecosystem services, yet we know neither the annual nor seasonal baseline stocks of snow water equivalent (SWE) and their distribution worldwide. Such observations are crucial for understanding the role of snow in the Earth’s water, energy and carbon cycles, and are needed for low latitude-high elevation snowpacks of the tropics, for mid-latitude mountain and prairie snow, and for the vast snow in the polar regions. Even now, climate-driven changes in snowfall, snow accumulation, and seasonal snowpack dynamics are having a transformative impact globally. Mid-latitude climates are expanding northward, as a new water cycle begins to emerge as the transition zones between seasonal frost and permafrost moves north.

Specifically, seasonal trends in Northern Hemisphere hydro-climatology indicate increasing precipitation in the fall season as well as an increase in the frequency and intensity of extreme precipitation events. The fall trends are consistent with increasing atmospheric water vapor concentrations at higher latitudes over the last thirty years. CloudSat observations show that snowfall dominates precipitation poleward of 50^oN, and further, indicate that a significant fraction of low to moderate intensity events accounting for nearly 50% of precipitation are currently not detected by Earth Observation Satellites.

However, technological improvements are changing this situation. Whereas satellite-based snow cover area mapping has long been possible, quality snowpack water measurements at high spatial resolution have been elusive, yet urgently needed for a hydrologic world in transition. Global measurements at high spatial resolution are now possible due to novel SAR microwave capabilities, better and less expensive satellite platforms, and significant advances in snow modeling and data-assimilation.

To better understand the observational requirements of a snow mission, a global SWE climatology was developed using the ERA5-Land hourly reanalysis data and used to assess the space-time variability of global seasonal snow accumulation and ablation processes. The assessment indicated that snow event times varied in different regions and at different times of the year. Along coastal regions of North America, Europe and Asia, and at high elevations, median snow return periods were 3-5 days. In continental climates, the return periods ranged from 7-14 days. In the winter, return periods of 20 days and longer dominated on the lee side of the Western Cordillera and the Urals. A west-east spatial flip in the occurrence of snowfall from the fall to the winter season between Europe and Asia was detected, whereas in North America the seasonal flip took place between regions above vs. below 60^o. In addition, there are large spatial variations in the timing of snow accumulation and melt across different snow environments, with high intermittency in snow covered areas, particularly at low latitudes below 45^oN. These continental-scale shifts in snowfall and snow evolution reflect seasonal changes in storm tracks and atmospheric conditions that we must come to understand as we adapt to new cryo-hydrologic regimes.