Altimetry for the future: Building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Mass balance of the Greenland and Antarctic ice sheets from 1992 to 2020

Ice losses from the Greenland and Antarctic ice sheets have accelerated since the 1990s, accounting for a significant increase in the global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume, and in Earth's gravity field. Between 1992 and 2020, the ice sheets contributed 21.0±1.9g€¯mm to global mean sea level, with the rate of mass loss rising from 105g€¯Gtg€¯yr-1 between 1992 and 1996 to 372g€¯Gtg€¯yr-1 between 2016 and 2020. In Greenland, the rate of mass loss is 169±9g€¯Gtg€¯yr-1 between 1992 and 2020, but there are large inter-annual variations in mass balance, with mass loss ranging from 86g€¯Gtg€¯yr-1 in 2017 to 444g€¯Gtg€¯yr-1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (82±9g€¯Gtg€¯yr-1) and, to a lesser extent, from the Antarctic Peninsula (13±5g€¯Gtg€¯yr-1). East Antarctica remains close to a state of balance, with a small gain of 3±15g€¯Gtg€¯yr-1, but is the most uncertain component of Antarctica's mass balance. The dataset is publicly available at 10.5285/77B64C55-7166-4A06-9DEF-2E400398E452 (IMBIE Team, 2021)

Altimetry for the future: building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Airborne Radar and Firn Density Profiles in West Central Greenland from 2006 to 2017

This dataset is constituted of radar profiles acquired using Ku-band radar ASIRAS and Ka-band radar KAREN along the EGIG line in West Central Greenland as part of the ESA CryoVEx campaigns in 2006, 2008, 2011, 2012, 2014, 2016 and 2017. This dataset consists of NetCDF files with the radar backscatter profiles along the EGIG line and various parameters describing the degree of radar penetration (depth at which power falls below 1% of maximum surface return, width of the OCOG retracking algorithm and number of layers recorded with a power higher than 10% of the maximum surface return). The two-way travel time of the radar was converted to depth below the ice sheet surface using the firn density outputs from MAR. The MAR firn density profiles used to perform the radar travel time to depth conversion are included in this dataset. In addition, the IMAU-FDM firn density profiles from 2016 and 2017 are also included, with the chronology of the firn column given for the 2017 profile. Density measured from in-situ firn cores and used to validate the two firn density models are included in this dataset. The radar data and the firn cores were collected as part of the ESA CryoVEx campaigns

A Historic Record of Sea Ice Extents from Scatterometer Data

Sea ice is a vital component of the cryosphere and does not only influence the polar regions but has a more global influence. Indeed, sea ice plays a major role in the regulation of the global climate system as the sea ice cover reflects the sun radiation back to the atmosphere keeping the polar regions cool. The shrinkage of the sea ice cover entails the warming up of the oceans and as a consequence, a further amplification of the melting of sea ice. Therefore, the polar regions are sensitive to climate change and monitoring the sea ice cover is very important. To assess sea ice change in the polar regions, satellite active microwave sensors, scatterometers, are used to observe the evolution of sea ice extent and sea ice types. Thus, this research aims at creating a historic record of daily global Arctic and Antarctic sea ice extents and analysing the change in sea ice types with scatterometer data. A Bayesian sea ice detection algorithm, developed for the Advanced scatterometer (ASCAT), is applied and tuned to the configurations of the scatterometers on board the European Remote Sensing satellites, ERS\textendash 1 and ERS\textendash 2. The sea ice geophysical model functions (GMFs) of ERS and ASCAT are studied together to validate the use of ASCAT sea ice GMF extrapolated to the lower incidence angles of ERS. The main adaptations from the initial algorithm aim at compensating for the lower observation densities afforded by ERS with a refined spatial filter and time\textendash variable detection thresholds. To further analyse the backscatter response from sea ice and derive information on the different sea ice types, a new model of sea ice backscattering at C\textendash band is proposed in this study. This model has been derived using ERS and ASCAT backscatter data and describes the variation of sea ice backscatter with incidence angle as a function of sea ice type. The improvement of the sea ice detection algorithm for ERS\textendash 1 and ERS\textendash 2, operating between 1992 and 2001, leads to the extension of the existing records of daily global sea ice extents from the Quick scatterometer (QuikSCAT) which operated from 1999 to 2009 and ASCAT operating from 2007 onwards. The sea ice extents from ERS, QuikSCAT and ASCAT show excellent agreement during the overlapping periods, attesting to the consistency and homogeneity of the long\textendash term scatterometer sea ice record. The new climate record is compared against passive microwave derived sea ice extents, revealing consistent differences between spring and summer which are attributed to the lower sensitivity of the passive microwave technique to melting sea ice. The climate record shows that the minimum Arctic summer sea ice extent has been declining, reaching the lowest record of sea ice extent in 2012. The new model for sea ice backscatter is used on ERS and ASCAT backscatter data and provides a more precise normalization of sea ice backscatter than was previously available. An application of this model in sea ice change analysis is performed by classifying sea ice types based on their normalized backscatter values. This analysis reveals that the extent of multi\textendash year Arctic sea ice has been declining remarkably over the period covered by scatterometer observations

Improving estimates of ice sheet elevation change derived from AltiKa and CryoSat-2 satellite radar altimetry

While satellite Ku-band (13.5 GHz) radar altimetry has been used since the 1990s to track changes in the Greenland and Antarctic ice sheets' shape, the launch of AltiKa in 2013 provided the first opportunity to use data from higher frequency Ka-band (36 GHz) and compare it to contemporaneous Ku-band CryoSat-2 data. In this thesis, I develop novel methods and datasets, based on the processing of Ku-band CryoSat-2 and Ka-band AltiKa data, to improve our ability to detect and interpret trends in elevation change from satellite radar altimetry. First, I produced an assessment of higher-frequency Ka-band AltiKa data in West Antarctica. By developing a new slope correction algorithm and applying a least-square model fit to AltiKa surface elevation measurements, I demonstrated that AltiKa detects trends in surface elevation in good agreement with coincident Ku-band CryoSat-2 and airborne laser data within 0.6 ± 2.4 cm/yr and 0.1 ± 0.1 cm/yr, respectively, showing that trends in penetration are minor in this region. Using this new dataset, I showed that surface lowering at Pine Island Glacier has fallen by 9% since the 2000s, while at Thwaites Glacier it has risen by 43%. Next, I examined the impact of surface melting on firn stratigraphy and radar penetration in West Central Greenland by using a combination of airborne radar data, in-situ firn density measurements, and firn densification models. I showed that surface melt strongly affects the degree of radar penetration into the firn, with the largest fluctuations recorded after the extreme melt event of 2012, which caused a 6.2 ± 2.4 m reduction in Ku-band radar penetration. I further assessed different methods to mitigate the effect of fluctuations in radar penetration on surface heights and showed that using threshold retracking algorithms results in surface heights to within 14 cm from coincident airborne laser data. In addition, I showed that over this transect, Ka-band radar penetration is half that of coincident Ku-band data. Finally, I used a decade of CryoSat-2 data to study the imbalance of the Northwest sector of the Greenland Ice Sheet and showed that the margins of this region are rapidly thinning at an average rate of 42.7 ± 0.9 cm/yr. I derived mass balance within 73 individual glacier drainage basins of this region, showing that the Northwest sector lost a total of 386.0 ± 3.7 Gt of ice between July 2010 and July 2019 with all glacier basins losing mass. I compared this new altimetry-based mass balance estimate to independent estimates from the gravimetry and mass budget techniques and found that, while the altimetry estimate is the least negative, differences between techniques vary regionally, with the mass budget and gravimetry exhibiting higher and lower ice losses, respectively

Ice sheet height retrievals from Spire grazing angle GNSS-R

peer reviewedGlobal Navigation Satellite System-Reflectometry (GNSS-R) leverages signals of opportunity for remote sensing, such as for mapping sea surface topography. This study investigates Grazing angle observations of GNSS-R (GG-R) to systematically retrieve ice sheet surface height in Greenland and Antarctica using Spire Global, Inc. CubeSats. Over a nine-month period, approximately 7800 coherent segments have been processed over the polar regions to compute relative height of the ice sheet above the WGS84 reference ellipsoid under grazing angle geometries. The retrieval of surface height is based on using ionosphere-free phase measurements from both the direct and reflected signals. Digital Elevation Models (DEMs) were used for the relative height adjustment and the validation of the GG-R retrievals, which presented a root mean square error (RMSE) of ∼1.7 m for both regions. Furthermore, the collocated GG-R profiles manifest good agreement among each other and reveal systematic behavior in the height residuals that might be caused by the DEMs. The results presented in this study demonstrate the potential of dual-frequency GG-R for measuring ice sheet surface heights.13. Climate actio