The Ice, Cloud and land Elevation Satellite-2 (ICESat-2) mission carries the Advanced Topographic Laser Altimeter System (ATLAS) lidar to measure the changing height of Earth surface. After more than 3 years of science data collection, ATLAS has emitted well over a trillion laser pulses and continues to operate nominally. The ICESat-2 data products were initially released in May 2019, and have been used in over 150 peer-reviewed publications to date. Recent community white papers have noted the importance of ice elevation measurements for the coming decades, and as CryoSat-2 has entered it’s second decade and future missions are uncertain, ICESat-2 will be a critical part of the Earth System Observatory for the 2020s. In the polar regions, ICESat-2 has enabled year-round sea ice freeboard measurements as well as ice sheet elevation changes on seasonal, annual, and decadal time scales when combined with other missions. These data enable measurement of change at fine spatial and temporal scales to help understand or characterize the processes driving these observations.

As of this presentation, Release 005 is the current version of the along-track data products, including surface-specific products for sea ice, land ice, ground and canopy height, ocean, and inland water heights. In addition, first versions of gridded data products for land ice, sea ice, and ocean are all available. Data spans the start of the mission (14 October 2018) through early 2022. The ICESat-2 project has also begun to produce and distribute QuickLook products with a nominal 3-day latency (as opposed to the ~45-day latency of the final data products). Initial versions of these QuickLook products have elevation accuracy of ~3 meters, and geolocation accuracy of ~50 meters. All products are available through the National Snow and Ice Data Center (nsidc.org).

This presentation will summarize the current state and health of the observatory, current state of the ICESat-2 data products, data product quality assessment, and outlook for the future.

For more than 10 years, the ESA Earth Explorer mission CryoSat-2 (CS2) has been observing and monitoring the polar regions, providing unprecedented spatial and temporal coverage. Satellite altimetry allows the measurement of sea ice thickness, a key variable for understanding sea ice dynamics. Over the 10 years of service, many sea-ice products developed by the community have already demonstrated the capabilities of CryoSat-2 to estimate sea ice thickness. Nevertheless, several questions remain to better assess the relevance and quality of the measurements according to the ice type. These include the effects of ice roughness in the footprint and Ku frequency penetration levels in the snow cover.

In July 2020, the orbit of CS2 was raised, as part of the Cryo2Ice project, to have tracks co located with the high-resolution IceSat-2 altimeter of NASA over the Arctic Ocean. These coincident measurements between the Ku-band SAR altimeter for CS2 and the LIDAR altimeter for IceSat-2 provide a unique means of assessing the impact of radar footprints and snow properties in the measurements. It is a unique opportunity to shed new light on existing CS2 products and processing methodologies.

Here we present a methodology for comparing IceSat-2 and CryoSat-2 along coincident trajectories and show the potential of using a dual-sensor approach to understand the measurement. Comparisons between surface roughness estimates and snow products show strong correlations, highlighting the cross-effects of penetration and roughness. We then introduce comparisons with SARAL Ka-band data, which does not penetrate snow, to better understand these effects. We also study the impact of CryoSat-2 waveform processing methods, or retrackers, using various sea ice products proposed by the scientific community.

The ultimate objective is to head toward an optimal processing methodology to monitor snow-depth using a bi(multi)-sensor approach. This ESA-supported study should help prepare the future Copernicus CRISTAL mission, which will include a Ka/Ku dual-frequency altimeter for the first time.

Dual-frequency snow depth on Arctic sea ice from calibrated Ku-band radar and laser satellite altimetry
Isobel R Lawrence, Andrew Shepherd, Michel Tsamados, Jack Landy

Snow on sea ice is a key component of the polar climate system. During the freeze-up season, snow insulates sea ice from the cold atmosphere and limits its thermodynamic growth which keeps it thin, and in summer snow melts to form ponds on the sea ice surface which reduce the surface albedo and exacerbate melting. Knowing the depth of snow on Arctic sea ice is also an essential parameter for deriving sea ice thickness from satellite altimetry.

Many approaches currently exist for estimating snow depth on Arctic sea ice, including dynamic models, snow climatologies built from in-situ data, and multi-frequency altimetry methods.

As part of the ESA Polar+ Snow on Sea Ice project, we present first estimates of snow depth on Arctic sea ice from combining total snow freeboard from ICESat-2 and radar freeboard from CryoSat-2, calibrated with airborne data from NASA’s operation IceBridge (OIB) campaigns. OIB operated in the western Arctic in spring from 2009-2019 and provides an invaluable dataset, especially given the absence of laser altimetry satellite missions during this period.

Snow depth results are compared to existing dual-frequency products, e.g. the LEGOS Altimetric Snow Depth (ASD, Guerreiro et al. 2016, Garnier et al. 2021) and the DUal-altimetry Snow Thickness (DuST, Lawrence et al. 2018) which both utilise CryoSat-2/SIRAL and SARAL/AltiKa data to retrieve snow depth on Arctic sea ice up to 81.5º latitude.

We also validate our results with coincident along-track CryoSat-2 and ICESat-2 data, available since 2021 thanks to the joint ESA/NASA Cryo2ice orbit manoeuvre. Cryo2ice provides the unique opportunity to compare Ku-band radar and laser data along-track over near-coincident sea ice, enhancing our ability to measure snow depth and better understand Ku-band radar penetration.

Antarctic grounding lines are a sensitive indicator of ice sheet stability, and migration of the grounding line over long time scales (annual-decadal) can indicate changes in local environmental forcing. The location of the grounding line is also modified by the effects of oscillatory ocean tides, which can cause the position to migrate back and forth across a wider grounding zone on hourly-daily timescales. However, the wider implications of this tidal GL migration on upstream ice dynamics and basal melt rates are not well understood and observations of short-term change in position are sparse. NASA’s ICESat-2 laser altimeter satellite mission measures surface heights with high along-track resolution (0.7m), covering the southern hemisphere up to 88°S. Altimetry instruments are able to measure the vertical component of ice displacement, which over the floating ice shelves is dominated by tidal displacement. In this study we present new observations of grounding zone dynamics around Antarctica using ICESat-2 repeat-track analysis in combination with modelled tides from CATS2008a.

Our results show that on the Bungenstockrücken ice plain on the southern Ronne Ice Shelf, there has been long-term change in grounding zone structure between the ICESat (2003-09) and ICESat-2 (2019-21) records. We see little evidence of long-term grounding line migration but observe a widening of the overall flexure zone over this period. Analysis of the change in location over shorter timescales shows the impact of ocean tides, with up to 13 km of grounding line migration observed. The short-term migration is asymmetrical, with ice shelf flexure extending much further inland at the highest tides reaching as far as the break-in-slope. These kinds of ICESat-2 observations also allow us to observe viscoelastic behaviour of ice in the grounding zone, providing a dataset which could be used to validate hypotheses of strain response to stress in grounding-line models.

We present additional case studies of grounding line migration on the Crary Ice Rise, Kamb Ice Stream and Foundation Ice Stream. These examples demonstrate the breadth of new information that ICESat-2 altimetry can provide alongside other grounding line datasets, such as interferometry-based techniques, to improve our understanding of grounding line processes. This improved knowledge is vital to better characterise the magnitude of grounding line migration in different regions of Antarctica, which will be used to inform ice sheet and ocean models in future studies.

Snow depth estimates remain a large uncertainty for constraining the accuracy of sea ice thickness retrievals from polar altimetry. There have been several recent investigations into methods for estimating snow depth from airborne observations over sea ice; this presentation outlines a comparison between a snow depth-estimation technique applied to Operation IceBridge data from the Spring 2016 campaign, and coincident in-situ measurements from landfast sea ice near to Eureka, Canada. The airborne snow depth retrieval technique co-locates green laser scanner data from the Airborne Topographic Mapper with observations from the CReSIS Ku-band radar altimeter, using a fixed threshold maxima algorithm for retracking radar waveforms, with a calibration step to remove the vertical offset between sensors. The technique performance is assessed over both first-year ice and multiyear ice sites. This method represents an airborne proxy for the newly-aligned ICESat-2 and CryoSat-2 orbits of the Cryo2Ice campaign. Mean absolute errors in snow depth estimation versus the in situ observations, as well as the fraction of valid retracked waveforms, are compared to both the air-snow and snow-ice interface roughnesses, as well as other Ku-band SAR waveform characteristics. We will assess the relative contributions of snow salinity and snow/ice surface roughness to the accuracy of laser-Ku snow depth retrievals over both first- and multiyear sea ice.

The polar oceans are experiencing some of the most rapid environmental changes on Earth, including changes in ocean circulation and freshwater distribution. These changes can manifest as changes in sea surface height (SSH) and dynamic ocean topography (DOT). Measuring and monitoring basin-scale variability in the SSH of the ice-covered oceans has proven challenging because the surface of these oceans is only exposed within narrow openings in the sea ice (e.g., leads and polynyas), requiring high spatial resolution and bespoke measurement techniques.

Here, we use high-resolution laser altimetry measurements of SSH over the Arctic and the Southern Oceans, routinely collected by NASA’s Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) since its launch in September 2018. These SSH estimates are then used to study DOT of both oceans, which represents the deviation of the SSH from the height of the local geoid (e.g., EGM2008). Spatial gradients in DOT can be used to estimate surface ocean geostrophic circulation.

The ICESat-2 mission is now approaching three years of operation, and offers the opportunity to study SSH/DOT over three full seasonal cycles (2018-2021) across both polar oceans at unprecedented spatial resolution. ICESat-2 has proven to be very successful in measuring and tracking monthly and seasonal SSH variations in the Arctic Ocean (Bagnardi et al., 2021).

With the ultimate goal of extending time-varying DOT estimates to the last decade, we make an attempt at reconciling the ICESat-2 SSH and DOT estimates with those obtained using independent data from ESA’s CryoSat-2 satellite radar altimeter. CryoSat-2 has been acquiring unfocussed synthetic aperture radar (SAR) altimetry data over the polar regions since 2010.
In particular, we focus on semisynchronous along-track measurements from the CRYO2ICE orbit alignment campaign over the Arctic Ocean. Especially during the summer months of 2020 and 2021, the CRYO2ICE data provide overlapping measurements of SSH over hundreds of kilometers and offer the unique opportunity to directly compare high-resolution, along-track SSH profiling from laser and radar altimeters.

For ICESat-2, we use SSH estimates from specular leads recorded in the Level 3A sea ice data products (ATL07 and ATL10). These data provide along-track measurements for six individual ground tracks and up to 16 satellite passes per day over both polar oceans. For CryoSat-2, we use SSH retrievals from both Level 2 data and Level 1b waveform data (both archived at ESA’s CryoSat-2 Science Server) to which we apply a recently updated waveform inversion method to determine the retracking corrections.

In our attempt to best reconcile DOT estimates from the two altimetry missions, we reference heights to the same surface (e.g., mean-tide geoid) and perform an assessment of all time-dependent geophysical corrections (e.g., tides) by estimating differences at orbit cross-overs.
We make use of the most recent ICESat-2 data (release 005), which adopted improved range bias corrections with respect to previous data releases. Using these new data, we re-assess the range biases between the six beams (centimeter-level in previous releases) with the aim of taking full advantage of ICESat-2’s six-beam configuration when estimating SSH and DOT.

Overall, we find strong agreement between the two sensors (centimeter-level differences) in both semisynchronous along-track estimates from the CRYO2ICE overlaps in the Arctic Ocean and basin-scale gridded DOT estimates across both polar oceans.

References
Bagnardi, M., Kurtz, N. T., Petty, A. A., & Kwok, R. (2021). Sea surface height anomalies of the Arctic Ocean from ICESat-2: A first examination and comparisons with CryoSat-2. Geophysical Research Letters, 48, e2021GL093155. https://doi.org/10.1029/2021GL093155