Whether the first snowflakes of winter fill you with glee or make you groan, winter snowfall is a crucial water source for drinking, agriculture and hydropower for more than 1 billion people worldwide.

To plan water management and disaster preparedness during the rest of the year, hydrologists and resource managers need to know how much water each winter's snowpack holds. Currently, ground or airborne observations of that measurement - called snow-water equivalent, or SWE (pronounced "swee") - are collected at only a very limited number of locations around the world. However, NASA hopes in the future to launch a global satellite mission to track this precious resource from space.

To design a mission that can measure all the snow characteristics that make up SWE, scientists need to determine what instrument combination to use, since no one instrument can do it alone. Enter NASA's SnowEx field campaign, which measures snow properties like depth, density, grain size and temperature using a variety of instruments, on the ground and in the air. A potential future NASA global snow mission will combine multiple remote sensing instruments, field observations and models - and SnowEx is discovering the best combination for the job.

Meeting the measurement challenges
Measuring snow might seem straightforward, but each environment brings unique challenges for remote sensing instruments. For example, snowfall in forests gets caught in branches or falls underneath the tree canopy, making it more difficult to measure remotely than snow that falls on an open landscape.

To dig into those differences, SnowEx measures snow from the ground and by air. The ground and air teams take similar measurements to compare their results, gauging how similar instruments perform under different conditions.

"Airborne observations allow us to collect high-resolution data over a large area, allowing simulation of remote sensing observations we might get from a satellite, at a range of resolutions and spatial extents," said Carrie Vuyovich, SnowEx 2021 project scientist, lead snow scientist for NASA's Terrestrial Hydrology Program and a physical scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Ground observations do not have the same spatial coverage, but allow us to validate the sensing technique in multiple, diverse locations, and the small footprint simplifies interpretation."

This year, the SnowEx team will deploy airborne lidar, radar and imaging systems to measure snow depth, changes in SWE, and the albedo of the snow surface, while collecting similar and complementary data over the same locations on the ground to compare and validate results. The albedo is the fraction of energy from the Sun reflected from a surface, a critical snow property for modeling melt.

There are three primary goals for the SnowEx 2021 campaign. The first goal is to repeat the L-band InSAR airborne measurement time series that was cut short by COVID-19 in spring 2020. (InSAR is a radar technique that estimates snow depth similarly to lidar, tracking changes in how long it takes for radar pulses to travel from the aircraft to the bottom of the snowpack.) This year, the Jet Propulsion Laboratory's Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) instrument will fly on a Gulf Stream 3 (G-3) aircraft weekly over each of six sites in Idaho, Utah, Colorado and Montana, from mid-January through late March.

In 2022, NASA and the Indian Space Research Organisation (ISRO) will launch NISAR, a space-based InSAR mission to study Earth's surface, including land, water, ice and more. SnowEx's InSAR explorations will inform future snow research with NISAR and other radar missions. The team will use lidar to validate the InSAR measurements on the ground.

Secondly, the team will use a spectrometer - an instrument that measures the intensity of visible and infrared radiation as a function of wavelength - to study albedo. Measuring albedo with spectrometers is a component of NASA's Surface Biology and Geology (SBG) study, which is developing research initiatives to better understand Earth's land and water ecosystems as part of the National Academies of Sciences, Engineering and Medicine's decadal survey. This is the first year SnowEx is directly targeting high-quality albedo observations, which will be focused in forested, steep terrain during the melt period. The team will fly NASA's Airborne Visible / InfraRed Imaging Spectrometer-Next Generation, or AVIRIS-NG, over two sites in Colorado during March and April to collect these observations.

A third goal for 2021 is to investigate snow properties in prairie landscapes. Snow is difficult to measure on prairies using the same approaches as over mountains because of their shallower depths.

"Prairie landscapes are identified as a gap in our remote sensing capabilities," said Vuyovich. "The substrate - the ground underneath - affects the signals and the ability to measure shallow snow. In addition, the spatial distribution of the snow in that environment is different from other environments, and can be difficult to measure and validate. Wind plays a significant role in redistributing snow across the landscape, which includes fields, crops, stubble, and ditches, leaving deep drifts and bare patches."

On the ground, teams dig snow pits - car-sized holes in the snow that reach down to the ground - and measure snow depth, water content, temperature, reflectance and grain size in the pit walls. Other team members on skis or snowshoes take handheld probe measurements of snow depth and albedo with field spectrometers. Using a snow micropenetrometer, measurements of the force on the probe tip provide detailed profiles of snow hardness and microstructure. The ground team also uses radar to rapidly measure how snow properties vary across the area of a typical satellite sensor pixel. The radar systems are mounted on snowmobiles or towed while skiing.

"This year we have some new instruments, like helicopter- and UAV-based lidar surveys, which allow us to adapt to weather and line up these calibration and validation surveys with the airborne radar. The low cost flight platforms allow more frequent surveys over a given area than from a fixed-wing aircraft, which is important for this time series experiment," said HP Marshall, an associate professor at Boise State University and SnowEx 2021's co-project scientist. Airborne lidar works by bouncing laser pulses off the surface and measuring the time it takes for the pulse to return. By tracking differences in timing across the landscape, lidar creates a 3D picture of the height and structure of the surface below. Scientists can calculate snow depth by comparing lidar measurements of the same area when there's snow, with surveys from when there is no snow.

In addition to collecting observational data, SnowEx's modeling research helps the team see how snow changes across different terrains and time.

"Modeling fills in the gaps in the remote sensing and ground observations," said Marshall. "In hard-to-measure areas like forests, models can use remote sensing observations in open areas to define precipitation patterns, allowing predictions of snow properties in the forest. Some of the remote sensing approaches that measure depth, such as lidar, also require models to estimate snow density, to allow conversion of depth to SWE. Between remote sensing acquisitions, models continue to simulate snow conditions. The models can be constantly updated when and where the remote sensing and ground observations of snow properties are available - all three approaches work together to provide the best estimates of snow conditions."

"People use models for different reasons," Vuyovich said. "Water managers could use models to help make decisions. NASA's Terrestrial Hydrology Program and SnowEx efforts will help design what we need from a satellite: what coverage, temporal frequency, accuracy and resolution are needed. Models can also help us fill in the gaps we may get between space-based observations."

Navigating a challenging landscape
In the sequential component of each campaign, SnowEx teams at sites across the western United States collect snow data weekly from December through May. Normally, this effort is punctuated by an intensive two- or three-week period of intensive data collection in one area, larger than the other site areas. In order to protect the teams during the ongoing COVID-19 pandemic, however, this year's campaign will only include the time series. At each site, only local teams within a 2-hour drive of their home base will collect ground observations over a limited area, to avoid the need for overnight stays or gathering in large groups.

"This is a pandemic world, and we're doing a lot virtually," Marshall said. "I'm excited that we're able to navigate this, that we have dedicated local field crews who can do this safely, and that we can still get on the snow. Our committed local field teams include students and researchers from many different government labs and universities, who deploy to their respective fields each week, on the same day as the overflights."

Most years, NASA and SnowEx partner with local schools and organizations to support citizen science efforts and educational opportunities, but this year, those activities will happen virtually, through blogs, videos, and remote data collection. SnowEx's primary outreach partner is the Winter Wildlands Alliance SnowSchool, a nationwide program with 70 sites that reaches 35,000 K-12 students. This year, they have developed virtual snow science activities to allow K-12 students to continue to learn about snow during the pandemic (https://winterwildlands.org/homeschool-snowschool/), as well as follow-on activities for schools that have been to Snow School in the past.

"We're excited this is happening," said Vuyovich. "With all of these challenges, we're excited that people are going to get out in the field and that we can continue to push forward. I may not see much snow here in D.C., so I'll be living vicariously through these photos and blogs."

To follow SnowEx 2021 in the field visit here

Related Links
Snow at NASA
It's A White Out at TerraDaily.com

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