NASA’s ICESat-2 altimetry mission provides observations to quantify changes in freeboard, thickness, and sea surface height of the ice-covered Arctic and Southern Oceans. This mission's objective is to add to the sea ice records from NASA’s ICESat, Operation IceBridge, as well as those from ESA’s altimeters. The ATLAS instrument on the ICESat-2 observatory (launched in September 2018) uses six high-resolution beams and micropulse altimetry to profile the sea ice and ocean surface. In this talk, we briefly describe the phenomenology of photon distributions in the sea ice returns, our approach for finding the heights of the ice and water surfaces, and the achievable height precision. Early results and assessment of data quality of the ICESat-2 acquisitions over the Arctic and Southern Oceans will be shown.

The correct initialisation of model sea ice thickness is important for improving forecasts of sea ice extent, concentration and thickness. Under the SEDNA (Safe maritime operations under extreme conditions: the Arctic case) project, work is underway to assimilate CryoSat-2 SIRAL (Synthetic Aperture Interferometric Radar Altimeter) observations of Arctic sea ice freeboard, processed by CPOM, into the Met Office's Forecast Ocean Assimilation Model (FOAM). The operational version of FOAM currently assimilates observations of sea ice concentration from OSI SAF SSMIS (Special Sensor Microwave Imager and Sounder), sea level anomaly from satellite altimetry, sea surface temperature from various satellite and in situ platforms, and temperature and salinity at depth from moored buoys and profilers including Argo and marine mammals. FOAM uses the NEMO ocean model coupled to the CICE sea ice model, and a 3D-Var FGAT (First Guess at Appropriate Time) data assimilation scheme, NEMOVAR. Output from this system is also used to initialise the Met Office's seasonal forecasting system, GloSea.

Previous studies have made use of monthly averaged ice thickness observations, but here we use daily observations of sea ice freeboard along CryoSat-2 orbit tracks. The freeboard observations (height of the sea ice above the ocean surface) are converted to ice thickness within the FOAM system, using modelled parameters. The observations themselves can be noisy, but the assimilation system makes use of observation uncertainty information to determine the impact of the observations on the analysis. Observations also undergo quality control, including the rejection of those which deviate too far from the model background.

An assessment of the effects of the assimilation of CryoSat-2 sea ice freeboard observations on short-timescale forecasts of ice extent, concentration and thickness will be presented. Future plans include assessment of seasonal forecasts of these sea ice variables, and the assimilation of thin sea ice thickness observations from SMOS.

Fully-focused Synthetic Aperture Radar (FF-SAR) is a novel SAR altimetry data processing technique that accounts for the phase evolution of the targets in the scene, making it possible to focus the complex echoes along the aperture. This process, similar to SAR imaging systems, can potentially reduce the along-track resolution to its theoretical limit equal to L/2, where L is the antenna length. The improved resolution allows better representation of radar backscatter changes on the surface and therefore detection of more sea-ice leads, and a better discrimination of waveform returns from floes and leads, which in turn could lead to better freeboard estimates when compared to the standard delay/Doppler processing.

Launched in February 2016, the European Sentinel-3A mission, and its companion satellite, Sentinel-3B, launched in April 2018, enable critical measurements of sea ice freeboard and sea surface height, providing coverage to a latitudinal limit of 81.5 °N. In addition to that, since 2002, the NOAA Laboratory for Satellite Altimetry has cooperated with ESA, EUMETSAT, and NASA to conduct airborne validation experiments over the Arctic Ocean to assess the use of satellite altimetry for measuring sea ice freeboard.

During its Spring Arctic campaigns of 2016, 2017, and 2018, NASA’s Operation IceBridge (OIB) mission, utilizing multi-instrumented aircraft, conducted surveys over Arctic sea ice timed to coincide with overpasses of the Sentinel-3A satellite. Spatially and temporally coincident data were collected in the eastern Beaufort Sea, in a study area that comprised large sea ice floes, interspersed with open and refrozen leads. The OIB instrument suite, including laser and radar altimeters, and high-resolution visible and infrared camera systems, provides measurements of sea ice freeboard, snow depth, and sea ice morphology.

Here we present an evaluation of Sentinel-3A data over Arctic sea ice with coincident OIB data. In this paper, we verify sea ice lead and floe delineations with the Sentinel-3A data with both delay/Doppler and FFSAR processing methods. We also assess the accuracy of Sentinel-3A surface elevation measurements and evaluate Sentinel-3A sea ice freeboard estimates via comparison with independent OIB freeboard measurements, and we evaluate the performance of delay/Doppler vs FFSAR for freeboard determination over an extended dataset encompassing several cycles of Sentinel-3A data.

Several active satellite altimetry missions have been launched in recent decades, providing the means to estimate sea ice freeboard and thus thickness from space. Obtaining accurate, and consistent, retrievals of freeboard from space is extremely challenging, while poor knowledge of the snow loading provides a key source of uncertainty in the conversion of freeboard to thickness. In an attempt to improve our knowledge of the snow depth and density, a new (open-source) snow budget model has been developed that includes several parameterizations to represent various sources and sinks of snow on sea ice through the accumulation season (September through April). The model is forced with daily reanalysis-derived snowfall and winds, and satellite-derived ice concentration and drifts. As the model resolution is relatively coarse, we use high-resolution snow depth retrievals from NASA's Operation IceBridge (2009 to present) to 'distribute the snow' from the snow model resolution (~100 km) to the satellite footprint resolution (hundreds of meters).

We have applied these new daily snow depth estimates to the freeboard measurements obtained by the newly launched NASA ICESat-2 laser altimeter mission along with historical, and updated, freeboard estimates from NASA's ICESat-1 (2003 to 2008) and ESA's CryoSat-2 (2010 onwards) to produce a reconciled (~15 year) time series of sea ice thickness. We validate these products with consistent data from undersea moorings deployed in the Beaufort Sea, the EM-Bird, and Operation Ice Bridge. We also include a detailed uncertainty analysis, attempting to fully account for the uncertainties introduced across the data processing chain.

We have made these data publicly available and seek to continue producing near real-time thickness estimates from the on-going CryoSat-2 and ICESat-2 missions. Our hope is that active engagement with the sea ice community can enable continued improvements to these data products and increase their potential utility and reliability.

Sentinel-3B was launched by the European Space Agency in April 2018, joining Sentinel-3A (launched February 2016) in a mission devoted to long-term operational measurements of the global ocean. The two-satellite constellation provides optimum global coverage and short revisit times (less than two days) at the equator. The Sentinel-3 satellites are equipped with SRAL, a dual-frequency Ku / C band SAR altimeter, allowing surface elevation retrievals up to 81.5º latitude. By measuring the elevation difference between the ocean surface in leads and sea ice floes, sea ice freeboard can be retrieved, and in turn sea ice thickness and volume can be calculated. ESA’s CryoSat-2 satellite, which has been successfully providing sea ice thickness estimates since 2010, is now nearly 5 years beyond its planned mission duration. Provided sea ice thickness results from Sentinel-3 are shown to be consistent with CryoSat-2, the mission ensures the continuation of the Arctic sea ice thickness record (up to 81.5º latitude) for the next 7-12 years. Furthermore, consistency between measurements permits the combination of observations from the three satellites, offering an unprecedented opportunity for high temporal/spatial resolution observations of Arctic sea ice from radar altimetry.

Here, we demonstrate that sea ice freeboard can be retrieved from Sentinel-3A data following similar processing as is employed for CryoSat-2, and we show that the results are consistent across the period of overlap between the two satellites. Sentinel-3B is currently nearing the end of its 9-month commissioning phase during which it will be manoeuvred into the same orbit as Sentinel-3A but with a phase difference of 140º. We quantify the increase in spatial and temporal resolution afforded by the combination of CryoSat-2, Sentinel-3A, and -3B (in its final orbit) and showcase some preliminary 10-day time series of Arctic sea ice thickness in the Beaufort and Greenland seas.

Sea level has been poorly observed in the polar oceans due to a lack of tide gauge records, poor coverage by conventional altimeters, and because large areas are seasonally or perennially covered by sea ice, where conventional altimeter processing fails. Recently, this has changed, as specialized processing techniques to extract sea level in openings in the ice cover have become more commonplace. Recently, we presented a 6-year record of sea level in the ice-covered Southern Ocean using data from CryoSat-2 [Armitage et al. (2018)]. It was found that the Antarctic Slope Current has a significant seasonal cycle and that sea level exhibits several wind-driven modes of variability, in particular, it was found that there was a significant response of sea level to both the Southern Annular Mode and El Niño Southern Oscillation climate modes. Here, we extend this record back in time using historical data from the Envisat radar altimeter, and by cross-calibrating with CryoSat-2 we produce a sea level record spanning 2003 to present. This longer time series allows us to investigate aspects of variability in the ice-covered Southern Ocean in more detail, and to draw links between changing ocean conditions and the increased ocean-driven mass loss of the West Antarctic ice sheet.

Armitage, T. W. K., R. Kwok, A. Thompson, and G. Cunningham (2018), “Dynamic Topography and Sea level Anomalies of the Southern Ocean: Variability and Teleconnections”, J. Geophysical. Res., 121.