Ribbons of Atmospheric Moisture, and Growth through Extremes

In regions of complex topography like near California’s Sierra Nevada, rapid and significant intra-storm shifts in the amount and intensity of rain versus that of snow can be beneficial, but can also contribute to flooding, ice, or snow events that impact hydrology and communities. These extreme shifts, identified by vertical changes in the altitude where snow melts into rain, or the atmospheric snow level, have not previously been catalogued.

In this study, we design a detection algorithm for extreme snow level changes which are defined here as one-hour vertical changes of a magnitude equaling or exceeding 400 meters. We consider snow levels obtained from 10 vertically-pointing ground radars across California. The past six cool seasons, 1 October to 1 May each water year, are included. This unique network of radars allows for high temporal resolution, i.e., 10-minute, observations during precipitating storms. In addition to defining and detecting extreme changes in snow level, extremes are described in terms of seasonality and variations by water year and radar. We also consider associated vertically-integrated atmospheric moisture values.

Atmospheric rivers are defined as low-tropospheric corridors of enhanced moisture which form over near-tropical regions of the Pacific Ocean. These narrow plumes can travel to reach the West Coast contributing up to half of California’s annual water supply. In this study, we discover more than half of the detected extreme snow level changes occur during atmospheric rivers. Our research suggests high-magnitude positive snow level changes are more often associated with high-magnitude integrated water vapor transport values than with low-magnitude moisture. Additionally, we find the largest number of both extreme rises and descents during December through March, with fewer extremes overall at southern radars compared to northern sites.