Antarctica: The wind sublimates snowflakes

"Until now, the extent of this important process, which is largely undetectable by satellite, was not fully appreciated," explains Alexis Berne, corresponding author of the study and head of EPFL's Environmental Remote Sensing Laboratory (LTE). Berne worked with a team of Swiss, French and British researchers in 2015 and 2016, using a new combination of instruments to take measurements at the Dumont d'Urville French research station on the coast of East Antarctica. The team used three instruments: a Doppler dual-polarization weather radar, a weighing precipitation gauge and a radar profiler. The polarization radar collected information on the type and intensity of precipitation, while the precipitation gauge weighed the accumulated snowfall every minute and helped calibrate the two radars' estimates. These two instruments were used to collect data from November 2015 to January 2016. The third instrument -- the radar profiler -- has been continuously collecting vertical profiles of the intensity of precipitation up to three kilometers in altitude since November 2015 at Dumont d'Urville.

At first, the researchers were surprised by the results they obtained. The sharp decline in precipitation recorded near the ground was not consistent with their usual observations. "We therefore worked off the hypothesis that the reduction in precipitation in the lower atmospheric levels was caused by snow crystals sublimating as a result of the katabatic winds," explains Christophe Genthon, CNRS Senior Scientist at the Institute of Geosciences of the Environment, based in Grenoble. These frequent, strong winds come from the continent's high plateaus. The Antarctic ice sheet is quite flat, so the winds can gain in strength and reach as far as the coast. This creates a thin bottom layer of air (up to 300 m) that is saturated with uplifted snow crystals. Above this, there is a second layer of air that is much dryer. Snowflakes formed in the cloud layer aloft, sublimate when they pass through this second layer, turning straight into water vapor. Over time, this reduces the contribution from precipitation to the ice sheet's mass balance. "This layer is in a blind zone for satellites because of the echoes from the surface, which explains why this phenomenon had not been detected by satellites," says Berne.

The researchers then found evidence of katabatic winds capable of triggering sublimation in most of the data collected by radiosonde at permanent research stations across East Antarctica. Using a series of numerical atmospheric models and comparing the results with the measurements taken in Adélie Land, they were able to quantify the impact over the entire continent. And they discovered that the process of sublimation has a huge influence on the accumulation of precipitation.

Data on the ice sheet's mass balance is essential for predicting how sea levels will rise or fall. Researchers generally expect global warming to result in higher levels of precipitation in Antarctica. But the impact of the katabatic winds on precipitation could challenge these forecasts and make them far more complicated. The team therefore plans to continue analyzing the continent. "We'd like to keep collecting data on coastal areas and look more closely at areas where the terrain is more complex. We also plan to use different types of atmospheric models for comparison purposes. Broadly speaking, we hope our work will help increase our understanding of how climate change will affect precipitation in Antarctica," explains Alexis Berne.