Antarctic downslope winds affect ice sheet snowfall

Jacopo Grazioli (EPFL/MeteoSwiss)
Alexis Berne (EPFL – Switzerland)
Richard Forbes (ECMWF)
Jean-Baptiste Madeleine (LMD – France)
Hubert Gallée
Christophe Genthon
Gerhad Krinner (all Université Grenoble Alpes – France)

 

A collaboration between European scientists has shed new light on the effect of strong winds flowing from the high Antarctic plateau on the Antarctic ice sheet surface mass balance, by bringing together new observations of snowfall at an Antarctic research station with a year of analysis data from ECMWF’s Integrated Forecasting System (IFS).

Snowfall over Antarctica

The continent of Antarctica is often considered to be a desert as so little precipitation falls in the interior. However, precipitation around the coastal regions is much higher, with significant snowfalls from incursions of frontal systems associated with the Southern Ocean storm track. This snowfall increases the mass of the Antarctic ice sheets and is an integral part of the ice sheet surface mass balance. It is therefore important to quantify the amount of snowfall at the surface across the continent. In situ observations in Antarctica are sparse and satellite data cannot directly observe the precipitation at the surface. A dedicated campaign was therefore needed to help to understand the relevant precipitation processes.

New observations

A year-long observational field campaign was initiated in November 2015 to monitor precipitation at the Dumont d’Urville (DDU) research station in Adélie Land on the coast of East Antarctica. This provided an unprecedented time series of weather radar measurements of the vertical profile of precipitation. The data recorded the radar reflectivity of the precipitation through the year and how it changed in the vertical due to the microphysical processes of snow particle growth and sublimation (solid to vapour). This led to the discovery that the process of sublimation of snow falling through the lowest layers of the atmosphere has an important effect on the accumulation of Antarctic precipitation at the surface. This process had previously been neglected in the Antarctic ice sheet mass balance.

Snowfall sublimation over Antarctica.
Snowfall sublimation over Antarctica. The left-hand map shows the total annual amount of snowfall (water equivalent depth) reaching the ground, with significant snowfall around the coastal regions. The middle map shows the total annual amount of snowfall that sublimates before it reaches the ground, highlighting the effect of the dry low-level air from localised but persistent katabatic flows, particularly along the East Antarctica coast. The right-hand map shows the ratio of annual snowfall sublimation to the maximum snowfall in the vertical column, with locally up to 50% of the snowfall sublimating before it reaches the surface. The blue line indicates the 1,000 m altitude of the orography. Data are from 24-hour forecasts produced by ECMWF’s Integrated Forecasting System from November 2015 to November 2016. (Figure from Grazioli et al., doi:10.1073/pnas.1707633114).

Katabatic winds

The DDU research station is at a location that is frequently affected by strong downslope winds flowing from the high Antarctic plateau, called katabatic winds. These winds are channelled by the topography and are particularly persistent in specific regions around the coast. The katabatic winds are dry and the air warms adiabatically as it descends, leading to low relative humidity in a layer above the surface. Very close to the surface, air is moistened by sublimation of surface snow, but aloft the low relative humidity layer leads to significant sublimation of falling snow particles as observed by the radar at the DDU station.

Modelling the sublimation of snowfall

The profiles of falling snow at the station location were compared with results from three numerical models including the IFS. All three confirmed the important role of snow sublimation caused by katabatic winds. The IFS operational global analysis and 24-hour forecasts for the whole year were then used to quantify the impact of sublimation on falling snow over the entire Antarctic continent. The IFS results show that the total Antarctic continent cumulative precipitation near the ground was 17% lower than its maximum level higher in altitude, due to snowfall sublimation. The largest reductions were around the coast in the regions of persistent katabatic winds, particularly in East Antarctica, where the data suggest precipitation is as much as 35% lower than it would be without sublimation.

The new radar observations and modelling results from this scientific collaboration have, for the first time, identified and quantified the impact of snowfall sublimation in Antarctic katabatic winds. This will help to inform our understanding of the Antarctic ice sheet mass balance, which is essential for predicting how sea levels will evolve.

Further information can be found in an article published by Grazioli et al. in the Proceedings of the National Academy of Sciences (doi:10.1073/ pnas.1707633114).