Detection of Rain-on-Snow Events across Alaska

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Snowmelt and rain-on-snow (ROS) events enhance the liquid water content of a snowpack, which affects snow properties such as depth, density, grain size, and extent. These changes are associated with transfers of latent and sensible heat and create a positive feedback that accelerates snowmelt processes. Wet snow can affect high-latitude ecosystems at multiple spatial and temporal scales, impacting the hydrology, carbon cycle, wildlife, and human interactions. Our knowledge of the spatiotemporal patterns in ROS events and their response to climatic variability is limited by the sparse network of in-situ climate observations and landscape heterogeneity in Alaska.


With this analysis, NPS researchers inform this knowledge gap by combining observations from overlapping satellite optical-infrared and microwave sensors to improve understanding of the timing and distribution of ROS events across Alaska. The NPS team utilized vertically and horizontally polarized daily brightness temperature images at 6 kilometers from the 19 and 37 gigahertz bands of the Advanced Microwave Scanning Radiometers (AMSR-E/2) and integrated them with the 500-meter snow cover extent product derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) to determine the timing and distribution of wintertime (November-March) ROS events during water years (WY) 2003-2016.

Results revealed markedly higher ROS frequency in the southwest and central portions of Alaska relative to other regions. Timing of the events was most prominent in November and December, coinciding with warm temperature anomalies. ROS events in Alaska’s North Slope were rare during the winter months and typically occurred during periods of above-normal temperatures.  Furthermore, findings determined that as the Arctic amplification of global temperatures continues, wintertime ROS events will likely become more prominent in Alaska’s southwest and interior regions.  The ROS record created from this work is publicly available to contribute to Alaska’s monitoring of snow cover properties as it will influence regional transportation, infrastructure, and wildlife populations moving into the future.

The total number of ROS events for water year 2014, which observed the most widespread distribution of ROS events across Alaska, particularly in the interior regions. Grey shaded areas were masked to reduce water contamination on brightness temperature retrievals.

Author Name
Caleb G. Pan, Peter B. Kircher, John S. Kimball
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