Snowoff Date Trends

A scientist works on weather station equipment high in the mountains of Alaska.

Seasonal snow is important to Alaska's ecosystems. Snow cover feeds rivers and streams, provides moisture to the soil, and insulates the ground from extreme cold and wind. Snow cover also provides important reflection of the sun's rays back into the atmosphere (albedo), whereas ground not covered in snow absorbs solar radiation and warms the earth.

As temperatures warm, Alaska is seeing more snow-free days every year, which further warms the earth in a positive feedback cycle. With a longer snow-free season, we have seen an earlier onset of snow melt and seasonal flooding in the spring, earlier and longer potential growing seasons, changes in the permafrost active layer conditions related to vegetation growth and carbon, and reductions in plant diversity.

This study used remote sensing to determine snowoff dates from 1988-2016 to identify trends in seasonal snowpack across the Alaska region. The work compares passive microwave (PMW) snowoff dates with snowoff dates derived from other established geospatial records, including the Moderate Resolution Imaging Spectroradiometer (MODIS), Interactive Multisensor Snow & Ice Mapping System (IMS), and Landsat snow persistence records, and extends the snowoff record for Alaska an additional 13 years, back to 1988. In contrast to optical remote sensing products, daily PMW retrievals are not impacted by polar darkness or cloud cover. PMW snowoff dates were robust to increasing forest cover, but showed relatively early snowoff bias over much of the Alaska Interior, potentially due to the closed forest structure. The PMW record also showed earlier snowoff dates in southwest Alaska, relative to the other snowoff records, perhaps due to the shallow seasonal snowpack in that region. In contrast, the PMW record indicated a delay in snowoff dates from colder regions of the state, including the North Slope, Brooks Range, and Alaska Range. These data are available as Geotiffs through the Oak Ridge National Laboratory Distributed Active Archive Center.

Learn more about weather and snow in Southwest Alaska.

A Long-Term Passive Microwave Snowoff Record for the Alaska Region 1988–2016

Abstract

Snowoff (SO) date—defined as the last day of observed seasonal snow cover—is an important governor of ecologic and hydrologic processes across Alaska and Arctic-Boreal landscapes; however, our understanding and capacity for the monitoring of spatial and temporal variability in the SO date is still lacking. In this study, we present a 6.25 km spatially gridded passive microwave (PMW) SO data record, complimenting current Alaskan SO records from Moderate Resolution Imaging Spectrometer (MODIS) and Landsat, but extending the SO record an additional 13 years. The PMW SO record was validated against in situ snow depth observations and showed favorable accuracy (0.66–0.92 mean correlations; 2–10 day mean absolute errors) for the major climate regions of Alaska. The PMW SO results were also within 10 days of finer spatial scale SO observational records, including Interactive Multisensor Snow and Ice Mapping System (IMS), MODIS, and Landsat, for a majority (75%) of Alaska. However, the PMW record showed a general SO delay at higher elevations and across the Alaska North Slope, and earlier SO in the Alaska interior and southwest regions relative to the other SO records. Overall, we assign an uncertainty +/−11 days to the PMW SO. The PMW SO record benefits from the near-daily temporal fidelity of underlying brightness temperature (Tb) observations and reveals a mean regional trend in earlier SO timing (−0.39 days yr⁻¹), while significant (p < 0.1) SO trend areas encompassed 11% of the Alaska domain and ranged from −0.11 days yr⁻¹ to −1.31 days yr⁻¹ over the 29-year satellite record. The observed SO dates also showed anomalous early SO dates during markedly warm years. Our results clarify the pattern and rate of SO changes across Alaska, which are interactive with global warming and contributing to widespread permafrost degradation, changes in regional hydrology, ecosystems, and associated services. Our results also provide a robust means for SO monitoring from satellite PMW observations with similar precision as more traditional and finer scale observations.

Pan, C. G., P. B. Kirchner, J. S. Kimball, and J. Du. 2020. A long-term passive microwave snowoff record for the Alaska region 1988-2016. Remote Sensing 12(1):153.