Noah Molotch, associate professor of Geography, and INSTAAR hydrologist with a joint appointment at NASA’s Jet Propulsion Laboratory, and CU-Boulder colleague Leanne Lestak have been using 20 years of satellite data of snow-covered area, along with the SNOTEL data, to generate close to real-time estimates of snow water equivalent for use by the U.S. Bureau of Reclamation. Their modeling effort can fill the gaps that ground monitoring doesn’t cover, particularly at certain elevations. In a series of interviews with the  and , Molotch said:

 “Snowpack in the Sierra is an important water resource for California, supplying around one-third of the state’s freshwater supply. Winter storms typically bring generous amounts of snow, which melt as temperatures rise in April. The meltwater runoff helps replenish rivers, reservoirs and groundwater.

Because of a historically dry winter, the statewide snowpack stood at 38 percent of its average at the end of the season on April 1. The little snowpack that accumulated in the southern Sierra had fully melted by May 24, leaving no additional freshwater supply for the hot months ahead.”

“The map above shows the snow water equivalent for the Sierra Nevada, or how much liquid water was contained in the snowpack on April 1 this year compared to its 20-year average. The data, modeled weekly by the’s Institute of Arctic and Alpine Research, is used by water forecasters, managers, irrigators, public utilities and many other parties.”

““What matters most is how much snow is on the ground on April 1st, because that’s really the indicator of the total amount of snow that accumulated for the whole winter,” said Noah P. Molotch, a hydrologist with the monitoring project and a professor at the University of Colorado at Boulder. This year’s depletion of the snowpack followed lackluster winters in 2020 and 2021, making this the third dry year in a row for California.”

“Molotch said 2022’s winter snow is likely to rank among the smallest five annual snowpacks since 2000. The lowest snowpack occurred in 2015, when accumulation was less than 10 percent of the average. California has not fully recovered from 2015, Molotch said.”

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Featuring previews of AGU talks by Geography graduate students

Friday, November, 20^th at 12:00PM MDT, 11:00AM PDT, 2:00PM EDT
Add the Zoom link* to your Calendar: Outlook, Google, iCal

Meeting ID: 936 7520 5146
*This particular colloquium is limited to University of Colorado Boulder audience through Zoom authentication.

Water Towers of the West: Where are they and how have they changed?
Katherine Hale

Snow-dominated mountainous regions rely on accumulated snow and snowmelt for water supply, and the storage of water in the snowpack delays and extends the downstream delivery of meltwater through the spring and summer months, when water demand is highest. Climate warming, however, has reduced snowfall across the western United States, reducing the annual snowpack and shifting snowmelt toward earlier in the year. These hydrologic changes alter the timing of surface water inputs (SWI) associated with rainfall and snowmelt and ultimately the catchment and regional hydrologic signatures. In this research, we derive an index of seasonal snowpack water storage, termed the Water Tower Index, which is based on differences in seasonality between precipitation and SWI. We define mountainous water towers as regions where there is significant temporal misalignment between precipitation and SWI, indicating a delay in meltwater production due to water storage in the snowpack. We used output from the Variable Infiltration Capacity (VIC) model from 1950-2014 to evaluate trends in WTI associated with regional warming. Evaluating the western U.S. by snow fraction, we determined that, on average, areas with an annual average snow fraction of >0.25 have experienced a significant decrease in annual WTI. This suggests that the delay in the timing between falling precipitation and SWI generation is becoming smaller, likely due to changing precipitation phase (snow to rain) and earlier snowmelt, both a product of climate warming. Future changes in WTI have broad implications for water availability and regional annual hydrologic behavior.

Towards an Operational Seasonal Sea Ice Forecast System for the Chukchi Sea
Jed Lenetsky

We utilize statistically-modeled ocean heat and volume transports through the Bering Strait along with additional predictors to create skillful predictions of sea ice retreat and advance dates in the Chukchi Sea, a key region for shipping and other growing economic activities in the Arctic. Inter-annual variability of June and September oceanic volume transports through the strait can be captured using along-strait surface winds and modeled Ekman transports in the Bering and East Siberian Seas. Combining the modeled volume transports with sea surface temperatures in the Bering Sea in turn enables skillful representations of June and September ocean heat transport (OHT) variability. These modeled OHTs, along with the linear trend in retreat and advance dates, explain 65% and 74% of retreat and advance date variance in the Chukchi Sea at one-month leads. Addition of the retreat date anomaly into the advance date model improves the explained variance of the model to 82%. Using only variables initialized July or earlier, we can explain 75% of the ice advance date variance. Forecast skill can be further increased by improving parameterizations of Bering Strait water temperatures, especially in spring and early summer. These findings are significant in offering a path towards operational forecasts of sea ice retreat and advance dates in the Chukchi Sea in the absence of direct observations of OHT from moorings in the Bering Strait.

Regional Trends in Snowpack Cold Content for the Inter-mountain Western United States
Jeffrey Schmidt

Trends for snowpack temperature, density, depth, SWE, and cold content were evaluated for the 1992-2020 winter seasons using the regional Kendall test (RKT) on novel snow-pit data collected from over 50 site locations across the Rocky Mountains, western United States. RKT is a non-parametric test with the proven power of monotonic trend detection in geoscience systems over linear regression by grouping data from certain geographic regions. According to both snow temperature and SWE, the RKT indicates that internal snowpack cold content has shifted toward warming the more massive northern region snowpack and maintaining a colder, less massive snowpack in the southern region. Variations in internal snowpack temperature explained most of the interannual variability in snowpack cold content magnitude in both regions. In the northern region, Thiel Sen’s slope for snowpack temperature was positive indicating the snowpack was warmer in recent years. Thiel Sens’s slope is more suitable for hydrological trend detection than ordinary least squares line fit because it is insensitive to outliers and missing values (which are expected in hydrologic systems). Thiel Sen's slope for SWE trend was generally flat in both regions, indicating little detectable change. High variance around the Thiel Sen’s slope fit for SWE reflects the low correlation that SWE has with cold content interannually. The southern snowpack trend is moderate cooling and the northern snowpack trend is pervasively warmer, which is controlled more by internal snow temperature than SWE. We conclude that northern region snowpack warming promotes less cold content in the future. Measuring and monitoring snow temperature changes is valuable for knowing the spatial-temporal arrival of spring by observing the snowpack in advance of isothermal conditions. Internal snow temperature and SWE data can be useful in multiple fields, including streamflow forecasting, climate change interpretation, and water resource management, among others.

Climate and topographic controls on the variability of snow water equivalent and snowmelt in a continental alpine watershed
Kehan Yang, Keith Musselman, Keith Jennings, Noah P Molotch

Seasonal snowpack is an essential component in the Earth’s surface hydrological cycle and energy balance. Recent climate warming has caused decreased peak snow accumulation, altered snowmelt rates, and earlier snow disappearance in many mountain ecosystems. Understanding and characterizing the spatial and temporal distribution of snow, often reported as snow water equivalent (SWE), is of crucial importance for assessing water availability to surrounding environments. In this study, we provide a comprehensive assessment of the long-term interannual variability of SWE distributions and snowmelt in the alpine Green Lakes Valley located in the Colorado Front Range. We leverage a physically-based energy and mass balance models, satellite observations of Fractional Snow Covered Area (FSCA), and long-term quality controlled daily meteorological data to estimate SWE distribution. Using a 23-year record of SWE distribution (i.e. 1997-2019), we evaluate the impacts of topographic and climate variables on interannual variability of SWE and snowmelt. Specifically, we use a linear regression model to compare metrics of elevation, aspect, slope, wind exposure, vegetation fractional coverage, and air temperature against metrics of SWE variability, including the temporal coefficient of variation in annual maximum SWE, the standard deviation of annual maximum SWE, the range in maximum SWE, maximum and average snowmelt rate, and snow disappearance date. The historical relationships between SWE distribution and topography have the potential to elucidate potential ecosystem response to future changes in snowpack and associated impacts on eco-hydrologic processes.

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• Message from the Department Chair, pg 2
• ​Mara Goldman: Reaction to Coronavirus, pg 3
• Page Hartwell: An Undergraduate's Perspective on COVID-19, pg 4
• Satellite-based snowpack information to inform water resource management during the COVID-19 pandemic, pgs 5-7
• Professors Seeking COVID-19 Funding, pg 8
• Human Geography Dimensions of COVID-19 in China, pg 8
• New Faculty: Introducing A. Marie Ranjbar, pg 9
• Narrating Nature: book by Mara Goldman, pg 10
• Emily Yeh: Sabbatical Report: Pastoralists of the Upper Yangtze, pgs 11-12
• Alumnus Update: Brooke E. Marston, pg 13
• Department News, pg 14
• Donor Support, pgs 15-16

All previous newsletters are on our Newsletters page.

For a more enjoyable reading experience, open the newsletter file and adjust your browser window to the same size as the newsletter page. The Table of Contents and other links are active within the document. 

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From base at 9,500 feet, scientists examine climate to top of tundra

Motorists cruising the famed Peak to Peak Highway north of Nederland as they savor the scenery or head out for a visit to Brainard Lake might well sail right past a modest signpost for one of the highest elevation long-term ecological research sites in the world.

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The Undergrad Snow Internship Program has been hosting undergraduates at the CU Mountain Research Station for over 20 years and includes alumni like Jen Morse and Noah Molotch, who help run the program today. The long-term data collected through this program has helped drive published papers due to the continuous collection of snowpack data. The collection points in the Subalpine (C1) and the Alpine Tundra (Niwot Saddle) provide a snow profile across an elevation gradient that is supported by metrological data and the Niwot SNOTEL Station. 

The students work hard to hike uphill in varying weather conditions: high winds, blowing snow, and the occasional sunny day. The varying harsh weather conditions on Niwot Ridge give the students experience in backcountry decision-making and understanding the threshold of collecting good data when conditions are not ideal. Their decision making is supported by training in Wilderness First Aid and understanding of the research site’s protected areas and emergency procedures if events were to turn bad. 

The measurements made within the snowpack include density measurements of snow water equivalent (SWE), snow depth, grain type and hardness.  The hard work they put into mentally and physically overcoming the conditions is rewarded by the ski down back to the Mountain Research Station (although it may not be filled with powder turns). 

The outreach of the Snow Internship Program is an important piece to furthering education about the ever-changing Colorado snowpack. From outdoor recreationalists, to local stakeholders such as the Colorado Water Congress, and grade school students, the undergraduates demonstrate the process of digging a snowpit and describing what each measurement is used for. The students are using their field experience to help others understand the importance of snow measurements and how fun they can actually be. And with a view as great as the continental divide along Niwot Ridge the beautiful sights can make any day worth it.

Niwot Saddle with snow pit site located in the upper right.

Photo: Jen Morse, Mountain Research Station PRA instruction new Snow Interns on protocol for the Snow Federal Sample to measure Depth and Snow Water Equivalent.

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32 degrees Fahrenheit is commonly considered to be the air temperature threshold for rain versus snow, thus informing meteorological forecasting and climate simulations. The new findings, however, show that coastal areas have a cooler threshold for rain, meaning that even temperatures below freezing might not produce snow. Inland and mountainous areas, meanwhile, are likelier to see flurries even when temperatures are several degrees above freezing.

“In Denver, Colorado, it might be 40 degrees and snowing. But in Charleston, South Carolina, it could be 28 degrees and raining,” said Noah Molotch, Director of the Center for Water Earth Science & Technology (CWEST) at ���Ҵ�ý�ƽ������ and a co-author of the study. “This study shows these fine-grain differences on a hemisphere-level scale for the first time.”

The research, which compiled nearly 18 million precipitation observations spanning over 100 countries and four continents across the Northern Hemisphere, was  in the journal Nature Communications.

The ability to differentiate rain from snow has important ramifications for Earth’s hydrologic cycle and water management, especially in drought-stricken areas of the American west. Winter snowfall is estimated to provide water storage for one billion people worldwide while climate warming could increase the amount of future rain-on-snow events, raising the risk of flooding.

“Snow and rain differ greatly in the ways they affect climate,” said Ben Livneh, an assistant professor in ���Ҵ�ý�ƽ������ Department of Civil, Environmental and Architectural Engineering and a co-author of the study. “Snow acts as a water reservoir and reflects incoming sunlight, whereas if the same amount of precipitation falls as rain, that can dramatically change water resource management decisions.”

To date, land surface models have typically predicted rain and snow based on a single, consistent air temperature threshold: snow below it and rain above it. But the ���Ҵ�ý�ƽ������ researchers found that the threshold is not static and that relative humidity and surface pressure play an important role as well.

“The rain-snow air temperature threshold is primarily a function of relative humidity and methods incorporating humidity and elevation are more likely to predict rain and snow correctly,” said Keith Jennings, a graduate researcher in ���Ҵ�ý�ƽ������  and the lead author of the study. “If you just use 32 degrees Fahrenheit across the board, your estimates will be wrong in lots of places.”

The continental U.S. had the most rain-snow variability of any country included in the study. Some of the coolest northern hemisphere thresholds were observed in the southeastern United States while the Rockies and intermountain West had some of the warmest thresholds.

The new study could inform the future of climate and land surface modeling as researchers look for ways to predict snowfall versus rainfall more accurately, especially in areas crucial for freshwater, agriculture and biodiversity. Future research will look to improve the map and simulations by incorporating even more meteorological data points from around the world.

“The great thing about this research is that anyone can observe these variables right in their own backyard,” said Molotch. “The topic lends itself well to future citizen science.”

NASA and the National Science Foundation provided funding for the research. INSTAAR graduate researcher Taylor Winchell also co-authored the study.

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Are you interested in the various processes related to snow in mid-latitude and polar areas? You will learn the physics and chemistry that underlie processes such as snow metamorphism, and apply this knowledge to real situations, including calculation of basin storage of water, runoff rates, acid snow, and avalanche dynamics. 

The course will cover snow formation in the atmosphere, snow accumulation and distribution, snow metamorphism, avalanche dynamics, snowmelt and runoff, remote sensing of snow properties, and case studies in the Rockies and Sierra Nevada. 

Prerequisites are a physical geography course or equivalent, and a parametric statistics course.

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Earlier snowmelt periods associated with a warming climate may hinder subalpine forest regulation of atmospheric carbon dioxide (CO2), according to the results of a new University of Colorado Boulder study.

The findings, which were recently published in the journal , predict that this shift in the timing of the snowmelt could result in a 45 percent reduction of snowmelt period forest carbon uptake by mid-century.

See entire article and hear interviews at 

Related  Earlier snowmelt decreases streamflow, reduces forests' ability to regulate atmospheric carbon dioxide

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A new University of Colorado Boulder-led study that ties forest "greenness" in the western United States to fluctuating year-to-year snowpack indicates mid-elevation mountain ecosystems are most sensitive to rising temperatures and changes in precipitation and snowmelt.

Led by CU-Boulder researcher Ernesto Trujillo and Assistant Professor Noah Molotch, the study team used the data -- including satellite images and ground measurements -- to identify the threshold where mid-level forests sustained primarily by moisture change to higher-elevation forests sustained primarily by sunlight and temperature. Being able to identify this "tipping point" is important because it is in the mid-level forests -- at altitudes from roughly 6,500 to 8,000 feet -- where many people live and play in the West and which are associated with increasing wildfires, beetle outbreaks and increased tree mortality, said Molotch.

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