What is the surface mass balance of Antarctica?

This study estimates the surface mass balance (SMB) of Antarctica, by making an intercomparison of five different regional climate models (RCMs) all simulating the Antarctic climate from 1987-2015. This study was led by Dr. Ruth Mottram from DMI and was a collaboration between several European institutes in Antarctic research, amongst other DTU Space and DMI, the study was published in The Cryosphere

The SMB is the sum of accumulation and ablation on an ice sheet surface. Accumulation is precipitation, which in Antarctica is primarily snowfall, and ablation consists of sublimation, evaporation, and runoff.

Our research shows that, when RCMs are forced by the ERA-Interim reanalysis data set, the integrated Antarctic ice sheet ensemble mean annual SMB is 2329 ± 94 Gigatonnes (Gt) per year. However, individual model estimates vary from 1961±70 to 2519±118 Gt per year, this spread corresponds to approximately 2 mm of global sea level per year. The large differences are mostly explained by different SMB estimates in West Antarctica and over the Antarctic Peninsula. Integrated over the continent all the RCMs show a consistent interannual variability, which is strongly correlated with the forcing data ERA-interim. In the interior of East Antarctica, the annual mean SMB is below 25 mm of water equivalent per year, in West Antarctica, it is greater than 1500 mm water equivalent per year. To evaluate the individual model performances of simulating near-surface climate, we have used in situ measurements of near-surface temperature, firn temperature, surface pressure, wind speeds, and SMB measurements. No one model outperforms the others, the models have different strengths and weaknesses for different variables in different regions. However, in areas with complex topography, e.g in West Antarctica and the Antarctic Peninsula the resolution of the models is extremely important, the higher resolution is, the better the topography is resolved, leading to changes in orographic precipitation and katabatic winds.

Annually resolved SMB integrated over the entire ice sheet for the different RCMs, in the period 1979-2018. All RCMs are driven by ERA-Interim and except for MARv3.10 and RACMO2.3p2, SMB is calculated according to Equation 1. The ensemble is a mean calculated from all 6 RCMs in the period 1987-2015 where there is data from all the models. All trend lines are calculated for the period 1987-2015.
Sub-figure a show the SMB ensemble mean for the common period. Sub-figure b-g shows the difference between each model and the ensemble mean.

Full study:
Mottram, R., Hansen, N., Kittel, C., van Wessem, M., Agosta, C., Amory, C., Boberg, F., van de Berg, W. J., Fettweis, X., Gossart, A., van Lipzig, N. P. M., van Meijgaard, E., Orr, A., Phillips, T., Webster, S., Simonsen, S. B., and Souverijns, N.: What is the Surface Mass Balance of Antarctica? An Intercomparison of Regional Climate Model Estimates, The Cryosphere, https://doi.org/10.5194/tc-15-3751-2021

Unique DTU space study on Artic icemelt

The inland ice sheet that covers Greenland is shrinking and the melting is happening faster. This is confirmed by a new, detailed study carried out by researchers in the Department of Geodesy and Earth Observation, led by senior researcher Sebastian Bjerregaard Simonsen. The new research was recently published in the journal Geophysical Research Letters.

Research shows that from 1992 to 2020, more than 4,400 Gigatons of ice melted away. This melted ice has contributed to a 12 mm increase in the water level of the world’s oceans.

The research is based on altitude measurements over the 28 years with the ESA satellites ERS-1, ERS-2, ENVISAT, CryoSat-2 and Sentinel-3A. The unique thing about a new study is that the satellites cover the entire period with an uninterrupted time series and are based on the same measurement methods year by year. The Danish research team is the first to translate the long European time series of height changes into mass loss.

“On average, this corresponds to 158 Gigatons of ice melting each year over this period of nearly three decades. But there are big differences during the period and a clear trend towards the melting accelerating,” says Sebastian Bjerregaard Simonsen.

In the 1990s, the average melting rate was 57 Gt ice/year. In the 2000s it was 163 Gt ice/year. And in the 2010s, the average was 241 Gt of ice/year.

Read more here or find the full article at:

Simonsen, S. B., Barletta, V. R., Colgan, W., & Sørensen, L. S. (2021). Greenland ice sheet mass balance (1992‐2020) from calibrated radar altimetry. Geophysical Research Letters, 48, e2020GL091216. https://doi.org/10.1029/2020GL091216

(Photo. Icebergs and large pieces of ice near Jakobshavn in Greenland. Photo: DTU Space/S.B. Simonsen)

How much meltwater is retained within the snow on the Greenland ice sheet?

We have just been involved in constraining the model range for firn model for the Greenland Ice sheet within the RETMIP project. The study was lead be GEUS and a summary of the findings is found below.

DTU Space conducting firn observations on the top of the Greenland Ice Sheet.

The thick snow that blankets the Greenland ice sheet provides a key service to the Earth’s system: the thick snow layer acts like a sponge when the surface of the ice sheet melts in the summer and prevents every year gigatons of meltwater to be poured into the ocean and contribute to sea-level rise. In a warming climate, we need computer models that can describe how this snow layer can retain meltwater generated at the surface of the ice sheet; and that is no easy task. Meltwater infiltrates and potentially refreezes in the snow depending on various parameters: the snow density and temperature, for example, or on how thick ice layers it contains. Multiple computer models are currently being used to simulate the retention of meltwater on the Greenland ice sheet. Yet, these models had never been evaluated on the same weather data input, until this new study.

A team of 23 researchers representing 18 research institutes and lead by Baptiste Vandecrux has evaluated nine snow models at four sites. These sites were chosen to represent the various climate present at the surface of the ice sheet: from cold and low-snowfall areas to warm and high-snowfall areas. What they found was striking: the snow models agree relatively well in the absence of melt, but the more melt was generated at the surface, the more models disagreed on where the water should infiltrate, whether it should refreeze and be retained or whether it should initiate a downslope flow towards the margin of the ice sheet and the ocean. Luckily, such a comparison exercise allows the team of researchers to identify key issues that snow models should address in the future. This greater insight on how snow models retain or not meltwater will hopefully allow the improvement of these models and allow a more accurate estimation of the current and future contribution of the Greenland ice sheet to sea-level rise.

The study was published in The Cryosphere and is available at : https://doi.org/10.5194/tc-14-3785-2020

Swath-like processing of CryoSat-2 data over sea ice

A new study by DTU Space and NASA JPL investigates the potential of performing “swath-like” processing of CryoSat-2 interferometric (SARIn) data over Arctic sea ice.

The proposed retracking method significantly increases the number of sea surface height retrievals, measured from leads, in the ice-covered Arctic Ocean reducing the uncertainty of sea ice thickness estimates from radar altimetry.

Left: In the lower part of the track, both sea ice and sea level elevations are extracted from single waveforms.
Right: sea ice freeboard and corresponding random uncertainty in March 2014 from the swath-like algorithm (a, b) and a processor not using the SARIn phase information (c, d). (e) is the difference (a)–(c), and (f) represents the percentage of variation of (b) with respect to (d). Red dashed lines represent the boundaries of the CryoSat-2 SARIn acquisition mask. Source: https://doi.org/10.1109/TGRS.2020.3022522

This technique might potentially be applied to data from the Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) mission, due to launch in 2027. While CryoSat-2 measures the sea ice in SARIn mode only along the Arctic coastline, the instrument onboard CRISTAL is planned to operate in SARIn mode over the entire ice-covered region, and this method could provide high-density sampling of the sea level and the sea ice thickness in both the Arctic and Southern oceans.

Full study:
Di Bella, Alessandro, Ronald Kwok, Thomas W. K. Armitage, Henriette Skourup, and Rene Forsberg. “Multi-Peak Retracking of CryoSat-2 SARIn Waveforms Over Arctic Sea Ice.” IEEE Transactions on Geoscience and Remote Sensing, 2020, 1–17. https://doi.org/10.1109/TGRS.2020.3022522.

The new CryoSat-2 baseline D is now ready

The Baseline-D data products.

In connection to the development of the ESA Cryosat-2 baseline-D, we at EO4CRYO have been involved in the validation effort of the new product, based on our airborne surveys at Austfonna, Svalbard.

(left) The airborne elevation measurement from laser scanning. (Right) The differences in geolocation of the radar echo in the new and previous baselines.

How to cite and access the paper.
Meloni, M., Bouffard, J., Parrinello, T., Dawson, G., Garnier, F., Helm, V., Di Bella, A., Hendricks, S., Ricker, R., Webb, E., Wright, B., Nielsen, K., Lee, S., Passaro, M., Scagliola, M., Simonsen, S. B., Sandberg Sørensen, L., Brockley, D., Baker, S., Fleury, S., Bamber, J., Maestri, L., Skourup, H., Forsberg, R., and Mizzi, L.: CryoSat Ice Baseline-D validation and evolutions, The Cryosphere, 14, 1889–1907, https://doi.org/10.5194/tc-14-1889-2020, 2020.