Research Tidbit: Where will snow survive in a warming world?

Higher snowfall intensity is associated with reduced impacts of warming upon winter snow ablation

Climate change is transforming winters throughout the western U.S. Warmer temperatures and winter rainfall reduce the magnitude of snow accumulation and alter the timing of snowmelt. Snowcapped mountaintops, characteristic of the western U.S, melt earlier in the spring, sending water rushing downriver. Not only is snowmelt occurring earlier, but it is bringing less water with it – threatening snow-dependent ecosystems and communities. 

Despite increasing winter temperatures, more intense precipitation – another consequence of climate change – may be able to sustain some snowpack. Even if mean annual snowfall decreases, an increase in the intensity of snowfall events could prevent snow ablation, or the loss of snow due to melting, sublimation or evaporation. Some physically based models have suggested that more extreme snowfall events may reduce snow ablation. A recent paper by Marshall et al. (2020) is the first study to use empirical data to test this relationship between snow intensity and ablation across the mountainous western U.S. In this study, Marshall et al. (2020) analyze spatial patterns in snowfall intensity and ablation using both observational data from snow networks across the Mountain West and outputs from climate change simulations.

Using historical data from the NRCS Snow Telemetry (SNOTEL) Network and the California Department of Water Resources snow pillow network, Marshall et al. (2020) first calculated snowfall intensity and winter ablation for stations from the Sierra Nevada and Cascade Mountains to the Rockies. They calculated snowfall intensity by dividing total precipitation that fell as snow by the number of days in which snowfall occurred. Ablation was calculated as the percentage of total snowfall that melted before the day of peak snowpack each winter. Next, the authors used publicly available outputs from the Variable Infiltration Capacity model forced with global climate models to estimate winter temperatures, snowfall intensity, and ablation at the end of the 21st century. For each station in the snow networks, they compared snowfall intensity and winter ablation over a historical climate (1970-1999) and a late 21st century climate (2070-2099).   

Historically, ablation has been lowest in the continental western US and higher in the Cascade Mountains and high-elevation regions of the American Southwest. Snowfall intensity, on the other hand, has been highest along the Sierra Nevada and Cascade ranges on the west coast and has decreased farther inland, with fewer large snowfall events in the eastern Rocky Mountains. Both the in-situ data and historical model outputs show that increasing snowfall intensity generally reduces winter ablation, especially where average winter temperatures are above freezing (Figure 1). Even as warm winter temperatures cause melting, if enough snow falls during extreme precipitation events some snowpack will likely last throughout the winter. Come spring, this surviving snowpack will melt, enabling the delivery of at least some water to snow-dependent areas of the western U.S.

Figure 1. Snow ablation, the percentage of total snowfall that melts before peak snow accumulation each winter, modeled as a function of average annual winter temperature (°C) and snowfall intensity (mm/day). Especially when temperatures are above 0 °C, higher snowfall intensity can lead to less snow ablation. Marshall et al. (2020) Figure 2.

Unsurprisingly, by the end of the 21st century, increasing winter temperatures will result in most regions experiencing an increase in ablation (Figures 2a and 2b) and a shift in the regions of most intense snowfall (Figure 2c). The coastal mountain ranges, where snow intensity has historically been the highest, will experience fewer extreme snowfall events, while the Rocky Mountains will likely have increasing snowfall intensity. Despite the projected overall increases in winter snowmelt, there are regions where the projected melting may be reduced due to increasing snowfall intensity (Figure 3). For example, in the Rocky Mountains, more large snowfall events may reduce ablation by up to 6.3%. In contrast, in the Sierra Nevada and Cascade mountains, a decrease in snowfall intensity will likely exacerbate ablation, increasing winter snowmelt by up to 6%.  

Figure 2. Changes in average (a) ablation (%), (b) winter temperature (°C), and (c) snowfall intensity (mm/day), calculated between 1970-1999 and 2070-2099. Ablation increases at most stations and average winter temperature increases across the western US. Snowfall intensity shows a strong geographic pattern, with decreasing intensity along the West Coast. Marshall et al. (2020) Figure 3.
Figure 3. The impact of snowfall intensity on ablation, 2070-2099. Warm colors show areas where ablation will increase due to changes in snowfall intensity identified in Figure 2c and cool colors show areas where ablation will decrease due to changes in snowfall intensity identified in Figure 2c. Adapted from Marshall et al. (2020) Figure 4.

These findings raise the question, how can snowfall intensity limit melting, especially when winter temperatures are above freezing? There are several possible explanations. First, in a deeper snowpack, more energy is required to initiate melting (Kumar et al., 2012). Second, larger snowfall events add more negative energy to the existing snowpack than smaller snowfalls, making the snowpack even colder and more resistant to melting (Kumar et al., 2012). Additionally, Marshall et al. (2020) suggest that more intense snowfall events may reduce canopy interception. That is, in larger snowfall events trees reach their maximum snow holding capacity more quickly and unload snow to the ground more frequently.

By incorporating multiple types of data, Marshall et al. (2020) provide empirical support for physically based models of snowfall intensity and ablation and offer new estimates of snowpack dynamics in a warming world. The authors suggest that climate change models and projections should incorporate changes in the size of snowfall events, given the impact that snowfall intensity can have on winter snowmelt and the spatial variations in this relationship. The geographic divide between regions of increasing winter snowmelt and decreasing winter snowmelt underscores the need to understand regional patterns in snowfall intensity and ablation, especially in regions where economic and environmental health depend on spring snowmelt.

Dr. Adrienne Marshall is a postdoctoral fellow at the University of Idaho where she uses hydrologic models to study permafrost and soil moisture in boreal forests. She earned her PhD in Water Resources from the University of Idaho in 2019 where she was an NSF IGERT fellow. You can follow her on Twitter @amarshall813.

Reference: Marshall, A.M., Link, T.E., Robinson, A.P., & Abatzoglou, J.T. (2020). Higher snowfall intensity is associated with reduced impacts of warming upon winter snow ablation. Geophysical Research Letters, 47, e2019GL086409. https://doi.org/10.1029/2091GL086409.

Additional In-Text Citation: Kumar, M., Wang, R., & Link, T.E. (2012). Effects of more extreme precipitation regimes on maximum seasonal snow water equivalent: Extreme snowfall regime affects SWEmax. Geophysical Research Letters, 39, L20504. https://doi.org/10.1029/2012GL052972.

By Julianne Davis

University of North Carolina at Chapel Hill

Twitter @hydro_jules

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