Sublimation of Snow Resources

Snow sublimation is an important but poorly understood component of the water cycle. It occurs when snowpack is lost to the atmosphere, turning directly into water vapor without melting first and bypassing the liquid water phase entirely. The National Science Foundation-funded Sublimation of Snow Project seeks to advance knowledge of snow hydrology by collecting data on snow and atmospheric conditions at and above the snow surface in Colorado’s East River Watershed. The SOS Project is the most intensive study of mountain snow sublimation ever conducted.

Drought
The US West is getting drier. After more than two decades of severe drought, there is an urgent need for a more nuanced and accurate characterization of water availability, especially in mountains, which serve as critical water towers. Snow sublimation is part of the story – yet the extent to which snow sublimation contributes to the increased dryness in the US West is not well understood.

Water supply
People who live in or downstream of mountains often look to winter snowpack to estimate their water supply for the coming spring and summer. However, the amount of water stored in the snow changes throughout the season, and some snow is lost to the atmosphere during sublimation. How much snow sublimates and the conditions that cause sublimation are still not fully understood, and the amount of water lost in any given season may be a large or small portion of the snowpack.

In 2021, water users in the Colorado River Basin experienced an unexpected mismatch between winter snowpack and the resulting spring and summer runoff. Snowpack across headwater basins reached around 80% of average, but snowmelt translated into Colorado River streamflow that was only 30% of average (not close to 80% of average which might be expected). Where did the water go? A combination of factors may be involved (as this blog illustrates), including dry soil conditions at the start of winter, lack of spring precipitation, increased evaporation, overestimates of snowpack, or an increase in sublimation. Understanding the dynamics of snow sublimation will help answer this question.

Snow ecology
Snow conditions across landscapes shape survival for both people and wildlife. Snow depth, snowflake shape, water content, snow density, and other factors influence everything from avalanche risk to den insulation to animal movement. Sublimation plays a role in changing snowpack conditions such as altering the snow density and snow depth that species rely on during the winter months, or people assess when making backcountry avalanche decisions.

Sublimation occurs when water molecules leave the surface of an ice crystal as a gas (water vapor), skipping the liquid phase entirely. Think about the “fog” that dry ice emits: as a result of carbon dioxide (CO2) sublimating from the dry ice, what you see is actually tiny water droplets forming (condensing) in the cold air. During sublimation, snow and ice do the same thing, but at a slower rate.

Evaporation is the way in which we typically think of water disappearing, going from liquid water to the gas form of water vapor.

Under the right conditions, when temperatures are still cold enough that the snow doesn’t melt on balance, solar radiation from the direct sunshine provides enough energy for the snow to change phase. Water molecules (in all forms – solid, liquid, gas), especially around freezing, are changing phase all the time, back and forth between phases. When it’s dry and windy (and not warm enough for melting), the balance shifts so that more snow sublimates.

The mechanism for this has to do with partial pressure and the triple point (see the phase diagram on our class website). More specifically, the partial pressure of water vapor in the air can be low enough, and the temperatures cold enough, that water can be below its triple point, where snow sublimates. Meanwhile, in more humid environments, the partial pressure of water is high enough that water is above the triple point, and snow mostly melts. Note: the triple point diagram is referring to the partial pressure of water pressure, which occurs at 0.01 C and at 0.6 kPa. At sea level, the typical partial pressure of water vapor is 0.5 to 3 kPa, so around that range, and sometimes lower at higher and drier locations.

Another thing to think about is that all of these phase changes are really STATISTICS! So we get some water molecules coming and going from the air all the time, and the net sublimation (or the lack thereof) is really the total probability of more going in one direction than another. If you have lower water vapor in the air, you statistically will lose more more water vapor molecules from the snowpack. If you have more energy going into the molecules (from high solar radiation), you have more movement and more likelihood of water molecules moving into the gas phase.

Snow sublimation can be measured in a few different ways:

• Mass Balance/Lysimeter: For sublimation to occur, the snowpack cannot be actively melting. But if temperatures are at or below freezing and no new snow is falling or blowing away, you can track sublimation by measuring how the mass of a known volume of snow changes over time by using a device called a lysimeter. This is essentially a box that is filled with snow, and is measured at the beginning and end of the day. If the mass decreases throughout the day, then snow must have sublimated from the box (assuming no snow blew into/out of the box).

• Eddy covariance: Eddy covariance is the measure used to observe the exchanges of gas, energy, and momentum between the snow surface and the atmosphere. In the atmosphere, water vapor generally moves upward, away from the Earth’s surface. You can measure water vapor concentration and wind speeds in the air to calculate the flux (movement/transport) of water molecules. You can then use the flux measurements to approximate the transport rate at the surface, where sublimation occurs. One way to envision sublimation is to think of the frozen surface of snow as a very large “supply” of water vapor relative to the drier atmosphere above it (when it is not actively snowing). Measuring the amount of water vapor in the air at different heights above the surface of the snow can give you a general idea of how much water vapor the snowpack is releasing into the atmosphere. To do this, you need to understand the turbulence of the air, which powers much of the transfer of energy and water vapor toward or away from the snow surface. Turbulence, the process that causes the rapid and seemingly random motions you feel on an airplane during a choppy flight, also affects sublimation. Turbulence is very efficient at transferring energy and water vapor toward or away from the snow surface. Turbulence is calculated by measuring horizontal and vertical wind speeds. By measuring the turbulence of the air and the amount of water vapor contained within it at very high frequency (20 times per second), you can determine how much water vapor is moving vertically away from (or toward) the snowpack. Sublimation is calculated as the net amount of water vapor moving away from the snow surface. However, if more water vapor is moving toward the snowpack, the opposite of sublimation (deposition) occurs, which can add mass to the snowpack (similar to how frost forms on a cool morning).

Sublimation is a tricky subject to study because (1) it’s a process invisible to the naked eye, (2) it’s difficult to take accurate and extended measurements in harsh, snowy, and mountainous environments, and (3) many models used to predict sublimation make many assumptions that do not represent mountainous terrain properly.

Recent years have shown greater variability and uncertainty in our prediction of water availability within the Colorado River. The SOS Project seeks to lessen prediction uncertainty with a better understanding of when, where, and how much sublimation occurs throughout the year.

The Sublimation of Snow Project uses a much larger suite of instruments than has ever been used before to track snow sublimation on an unprecedented scale in order to gain insight into what drives sublimation and to improve how we represent sublimation in models.

The SOS Project is also measuring snowmelt processes: identifying when snow starts to melt in the winter season and how fast this melt occurs. Better identifying this process will help understand the “snow disappearance” date — the date when ground snow cover completely disappears. It is important to understand how this date is changing each year because when the snow disappears, the energy balances at the earth’s surface change significantly, and the behavior of land surface and climate models also change.

The Rocky Mountain Biological Laboratory (RMBL) provides logistical and housing support to scientists and offers a unique and pristine environment for studying atmospheric and snow processes in complex terrain. RMBL provides the electric power and fast internet needed to make data collection reliable. The SOS Project was incredibly fortunate to be hosted at RMBL. According to our graduate students, RMBL is the “beez-kneez!”

How does Lidar work?
Lidar devices act like a rangefinder (like those used in golf to measure the distance to a hole). A Lidar device shoots a laser, which bounces off an object and returns back to the Lidar device, which then calculates how far the laser traveled before returning. A Lidar does this millions of times over the course of one scan (~60 seconds). Using these many data points of distance, the Lidar can recreate a surface. For example, if the Lidar is pointed at the ground, the returned lasers will allow you to create a detailed model of the surface.

How does the SOS Project use Lidar?
The SOS Project uses Lidar data in two main ways. The first is to characterize the snow surface and how it changes over time. Lidar scans were completed every five minutes throughout the winter season, providing a model of the snow surface every five minutes. Using these modeled surfaces, we can estimate how snow depth changes over the entire field site over the course of the winter. This is a tried-and-true method for determining snow depth.

The SOS Project is also using Lidar in a more experimental way. When it snows or when snowflakes are picked up from the snow surface and blown around by the wind, the Lidar lasers bounce off of snowflakes suspended in the air. The more snowflakes in the air, the more the lasers will bounce off of suspended snowflakes. Thus, the Lidar can indicate the intensity of snowfall or the intensity of blowing snow. This method is experimental and is still being developed.

Wind moving over the landscape, blowing snowflakes around, creates another source of sublimation. Because they are surrounded by dry air, blowing snowflakes are especially likely to sublimate. Blowing snow also increases the rate of sublimation, because during blowing snow events, both airborne snowflakes and the snow surface can sublimate.

The SOS, SAIL, and SPLASH campaigns all concurrently collected data in Colorado’s East River valley during the 2022-2023 winter season and provided a great opportunity for collaboration and understanding processes across scales

The SAIL and SPLASH campaigns operated for longer periods of time. While the SOS Project focused on measuring sublimation, blowing snow, and near-surface meteorology, the SAIL project was more focused on large atmospheric processes. SAIL instruments measured valley-scale wind fields, synoptic (high atmosphere) winds, and synoptic weather conditions (temperature, humidity, pressure) up to 30 km above the Earth’s surface.

The SPLASH campaign’s measurements were more similar to the SOS measurements, but the SPLASH campaign deployed fewer instruments at multiple locations throughout the East River valley. While the SOS campaign provides a lot of repeat measurements of sublimation at one site, the SPLASH campaign provides a single measurement of sublimation at three sites, all about two kilometers apart up and down the valley.

The three campaigns are designed to complement each other. The SOS campaign provides repeat high-resolution, near-surface measurements at one site. The SPLASH campaign provides near-surface measurements at multiple sites distributed throughout the valley. The SAIL campaign provides near-surface, valley-scale, and larger atmospheric scale measurements throughout the East River valley and in the atmosphere above.

While the impacts of climate change on snow sublimation are still being studied, there are a few different hypotheses about what those impacts may be.

• Blowing snow from weather events: Climate change may change the trajectory of storm systems, which could either enhance or limit the occurrence of blowing snow events. An increase (or decrease) in blowing snow events would, in turn, enhance (or diminish) sublimation.
• Warmer weather may cause less blowing snow: Warmer conditions can increase the density of falling snow. This denser snow is harder to incorporate into the atmosphere when weather gets windy. An increase in snow density from warmer temperatures could therefore decrease sublimation since wind blowing across the snow surface can no longer pick up snow and transport it in the air.
• Less snow may mean less sublimation: A decrease in the amount of snow that falls could translate into decreased sublimation, though sublimation may then affect a larger percentage of total snow that does fall.
• Warmer temperatures may create more sublimation: Higher temperatures can accelerate sublimation by increasing the amount of energy available to power the transition from solid to gas.

There is currently large uncertainty in model-estimated sublimation amounts. The data collected by the SOS Project will help improve our basic understanding of physical processes like surface and blowing snow sublimation. Improving our basic understanding of these processes will help us improve the models that represent those processes and could eventually lead to improvements in the models used to predict Colorado River flows.

Also of note, the East River Valley where the SOS campaign took place is a headwater basin of the Colorado River. Ninety percent of the Upper Colorado River Basin’s flows come from high-altitude basins like the East River Valley, so improving our understanding of water pathways in this location helps us learn more about these water-critical headwater catchments.

Sublimation is rarely measured over an entire winter season — and when it is, those measurements typically take place at one height in one location. The SOS Project is unique in that we took measurements at many different heights within a specific area, which can offer new insights into the variability of sublimation when measured at different locations. The SOS Project will also inform best practices for future sublimation studies around the world through innovative measurement techniques and more refined instrumentation needs, and SOS Project data will provide insight into the conditions that lead to sublimation in other mountainous and semi-arid regions. With over a billion people across the globe reliant on snow for water resources, understanding every facet of the snowpack is vital.