Sublimation, the process by which a solid changes phase to gas, is a largely unquantified component of glacier mass loss worldwide. A study on Nepal’s Yala glacier, recently published in Frontiers In Earth Science, quantified the glacier’s loss of ice to the atmosphere during the 2016-2017 winter. Researchers found approximately 21 percent of Yala’s annual snowfall was returned to the atmosphere via sublimation, a rate higher than most glaciers on Earth’s tallest mountain ranges.
Like classroom demonstrations with dry ice, sublimation can occur from a static surface. Snow sublimation is the loss of water from the snowpack directly to the atmosphere. Though Yala is one of the world’s most studied glaciers, a complete understanding of water balance and glacier mass has been limited. In addition, complex terrain and dynamic conditions often inhibit models from accurately estimating sublimation.
The process to measure the rate of sublimation is complicated: sublimation varies based on the time of year, hour of the day, cloud cover, complex terrain features, altitude, and specific atmospheric conditions like humidity and wind speed. Even in a static environment, these components are difficult to measure. Add dynamic environmental factors like drifting and blowing snow, ice that melts and refreezes (skewing energy balance calculations), and remote fixed instruments that rise and fall with the glacier itself, and you get a vague idea of the quantification problem faced by scientists.
Researchers utilize two primary methods to measure sublimation: the gravimetric method, which continuously monitors the weight at a specific part of the snowpack, and the eddy covariance method, a process of direct observation to measure and calculate atmospheric factors. The gravimetric method can incorrectly interpret wind-induced erosion of the snowpack as sublimation. The researchers, which were comprised of a team from Utrecht University and the International Centre for Integrated Mountain Development, were able to measure turbulent fluxes at Yala’s surface using the latter technique. Turbulent fluxes act on frozen water molecules the same way wind might affect leaves scattered on a surface; some are lifted and become airborne, while others remain grounded, depending on the wind strength, direction, and location. Through extensive and careful post-processing of the water vapor, air temperature, and vertical wind, the research team was able to accurately estimate sublimation.
Out of the myriad components affecting sublimation, the team condensed Yala’s sublimation rate into two primary determinants, wind speed and humidity, which vary depending on the time of year and day. Daily sublimation rates were separated into humid days and non-humid days. Less sublimation occurs on humid days, due to colder surface temperatures and a weaker vapor pressure gradient. When humidity is low, winds increase, resulting in a well-mixed atmospheric layer above the surface and a vapor pressure gradient ideal for sublimation. Sublimation varies greatly from location to location on the glacier.
The project required two trips: one to install equipment and a second to retrieve the data. Emmy Stigter, a doctoral student at the University of Utrecht in the Netherlands and principal author of the study, led the research team. “The fieldwork involves quite some hiking and a lot of logistical challenges,” she told GlacierHub. Yala is a four-day hike from the start of the Langtang Valley, which is a day’s drive from Kathmandu. The instruments required so much energy to power that the team had to lug a car battery up the glacier to ensure it would have sufficient energy to run during the research. Though the equipment was in place all winter, a data card was corrupted, limiting some of the team’s observations to just over a month in autumn.
During the 32-day study period, which occurred from October to November 2016, Yala lost 32 millimeters of water equivalent. This represents a significant share of the glacier’s net loss during the period (70mm). Yala’s one millimeter per day rate of sublimation is a pace higher than the Swiss Alps, Colorado Rocky Mountains, and Spain’s Sierra Madre. Due to the low atmospheric pressure, sublimation is most prolific at high altitudes, like that of the Himalaya. Only Kilimanjaro and the Andean peaks exhibit comparable rates of sublimation, according to the authors.
The researchers found that sublimation rates are highest in November and December and peak around one o’clock in the afternoon. Sublimation rates also differed depending on wind at the locations on the 1.5-square kilometer glacier; the faster the wind, the faster the rate of sublimation. Stigter’s team observed that rates were 1.7 times higher on ridges and .8 times lower at the bottom of the glacier.
Blowing snow, which was not accounted for in this study, may be a consequential factor leading to underestimation of mass loss to sublimation. Suspended particles sublimate on an order several times greater than the surface sublimation, as there is more ventilation and supply of dry air. One study showed that up to 30 percent of annual snowfall was removed in the Canadian prairie and Alaska due to blowing snow sublimation, while Antarctica lost up to 85 percent of its precipitation. Stigter is currently involved in a new study quantifying sublimation during wind-induced snow transport events.