Extreme Rises in Remotely Sensed Atmospheric Snow Level Across California's Mountain Regions

Atmospheric snow level or the rain-snow transition elevation is the height above mean sea level where snow is melting to rain. The atmospheric freezing level is defined as the level in the atmosphere where the air temperature is zero degrees Celsius and lies above the snow level as it takes time for the frozen precipitation to melt while approaching warmer air temperatures nearer the ground. The snow level is a key factor in forecasting precipitation type in regions of complex topography where ground elevation can vary significantly from one place in each basin to another.

California is situated within the coastal southwestern United States, and is home to various complex topographic features within the North American Cordillera. Falling precipitation interacts with the orography which comprises of high impact drainage basins such as the American River, the Feather River, and the Russian River watersheds.

Past studies focused on Western U.S. mountain regions (White et al. 2010; Neiman et al. 2014) show that operational models underforecast observed freezing level values that are based on remotely sensed snow level observations from vertically pointing radars. In cases with a high freezing level, the magnitude of these negative forecast biases increases, becoming more hydrologically significant. White et al. 2010 finds negative forecast biases up to 900 m when freezing level is at or above 2.3 km. These elevated freezing levels are associated with periods of maximum precipitation which present challenges for flood control and safety, and emphasize the importance of accurate forecasts.

Other studies have detailed how the snow level varies spatially and temporally. For example, Minder et al. 2011 and Minder and Kingsmill 2013 examine the mesoscale properties and terrain geometry that result in a depression in snow level altitude near the Sierra Nevada’s windward slopes compared to the upwind observed snow level altitude. Marwitz 1983 describes the thermodynamics that contribute to a lowering of bright band height as air intersects the windward slopes of the Sierra Nevada. Lundquist et al. 2008 explains how the snow level altitude along the Sierra Nevada’s windward slopes can be determined by making adjustments to remotely sensed snow levels upstream of the slopes based on the location of the profiler relative to the elevated basin and the time of day.

Our study looks at the boreal autumn through spring when California experiences its maximum annual precipitation, predominantly a result of landfalling atmospheric rivers (ARs). ARs are long, narrow ribbons of high integrated water vapor transport concentrated in the lower troposphere. They typically form within the warm sector of an extratropical cyclone, carrying moist, warm air from the subtropics. Upon making landfall along the western coast of the United States, ARs may produce precipitating storms which can lead to heavy rainfall and snowfall events with widespread implications for surrounding watersheds.

The goal of this study is to examine factors contributing to extreme rises in atmospheric snow level which may present complexities and challenges for forecasting. In order to do so, this study identifies, defines, and investigates rapid, significant rises in snow level throughout California during recent cool seasons using surface observations, remotely sensed radar bright band observations, radiosonde data, and high resolution modeling to fill in spatial and temporal gaps when necessary. The correlation of extreme snow level rises with distinct dynamic features typical of the region, focusing on warm fronts and landfalling ARs, is examined.

Bright band height observed by vertically pointing radars serves as a proxy for snow level altitude as outlined by White et al. 2010, and provides the information needed for detecting changes in snow level at each radar site during each time period. Radiosonde and reanalysis data allow for investigation into the vertical atmospheric structure of winds, temperature, moisture, and stability at each time and location. Observations are considered over time and space (coastal to inland, further north to further south, and vertically) to form conclusions on the nature of dynamic factors contributing to these sudden rises. Snow level is crucial in determining the type of precipitation each area is exposed to. Because higher snow level altitudes are associated with both more significant forecast biases and heavier precipitation periods, sudden rises in snow level can have pronounced impacts on the local and widespread hydrologic responses of this region.