Frontiers in Fine-Scale in situ Studies: Opportunities During the SWOT Fast Sampling Phase

Conceived as a major new tool for climate studies, the Surface Water and Ocean Topography (SWOT) satellite mission will launch in late 2021 and will retrieve the dynamics of the oceans upper layer at an unprecedented resolution of a few kilometers. During the calibration and validation (CalVal) phase in 2022, the satellite will be in a 1-day-repeat fast sampling orbit with enhanced temporal resolution, sacrificing the spatial coverage. This is an ideal opportunity – unique for many years to come – to coordinate in situ experiments during the same period for a focused study of fine scale dynamics and their broader roles in the Earth system. Key questions to be addressed include the role of fine scales on the ocean energy budget, the connection between their surface and internal dynamics, their impact on plankton diversity, and their biophysical dynamics at the ice margin

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

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

Simulations of the altimetric signal intensity from 3D-layered air/snow/sea-ice rough interfaces

International audienceRemote sensing of the sea-ice thickness is one of the main objectives of the Ku-band radar altimeter SIRAL-CRYOSAT II mission. On the one hand, sea-ice thickness is derived from the measurement of the height of the freeboard of the floes, and based on isostasy, assuming that the density of the water, the ice, as well as the snow, are known. On the other hand even if the snow load is known, the penetration of the electromagnetic waves into the snow strongly depends on the electrical and geophysical characteristics of the snow layer (density, temperature, permittivity, roughness). The remote sensing of the snow layer thickness (SLT) remains a real challenge and will be useful to correct the snow load for converting freeboard measurements from satellite altimetry into sea-ice thickness. If the dual frequency radar altimetry data show a good potential for remote sensing of snow and more generally of penetrating media [1], providing the SLT from Ku band data alone is highly motivated by the orbit of CRYOSAT designed to cover the entire Arctic. In this framework, a theoretical study, based on a 3D modelling of the scattering of electromagnetic waves by a stratified medium at normal incidence has been carried out in order to investigate and quantify the capacity of snow and ice penetration of Ku-band waves. The stratified medium is modelled as a snow layer considered as a stack of 2 sub-layers and the boundary layer at the bottom represented by a semi-infinite layer of ice-sea as shown on the figure 1. The roughness of each interface is taken into account and the small slope approximation (SSA) is used to determine the coherent and incoherent components of the scattered intensity [2-4]. It is demonstrated that the coherent intensity is the the specular direction but it depends on the rms-roughness heights and does not depends on the shape of the correlation function. The incoherent intensity depends even on the rms-roughness heights, but also on the shape of the correlati- n function. Several simulations have been conducted by varying the permittivity, thickness and roughness of each interface [5-6]. The 3 interfaces are random processes with Gaussian autocorrelation functions with zero mean values. The main conclusion is that the backscattered signal from the stratified medium is strongly related to the dielectric characteristics. It can vary significantly even if the variations of the stratified medium are small. This is an important result to be kept in mind when attempting the signal inversion. In addition, another similar study based on a 2D modelling of the scattering of electromagnetic waves by the same stratified medium at normal incidence and simulations in the same conditions have been previously conducted [7]. The roughness of each interface was also taken into account and the first-order small perturbation method (SPM) was used to determine the coherent and incoherent components of the scattered intensity. Results from those two studies are also compared at the end

Simulations of the altimetric signal intensity from 2D layered air/snow/sea-ice rough interfaces

Remote monitoring of the sea-ice thickness is one of the main objectives of the Cryosat mission. On the one hand, sea-ice thickness is derived from the measure of the freeboard of the ice, based on isostasy and assuming that the density of water, ice, as well as snow, are known. On the other hand, even if the snow load is known, the penetration of the electromagnetic waves into the snow strongly depends on the electrical and geophysical characteristics of the snow layer (density, temperature, permittivity, roughness). The remote sensing of the snow layer thickness (SLT) remains a real challenge and will be useful to correct for the snow load for converting freeboard measurements from satellite altimetry into sea-ice thickness. If dual frequency radar altimetric data show a good potential for remote sensing of snow and more generally of penetrating media, (Legrésy et al., 2005), providing the SLT from Ku band data alone is highly motivated by the orbit of Cryosat designed to cover the entire Arctic. A theoretical study, based on the 2D modelling of the scattering of electromagnetic waves by rough layered interfaces at normal incidence, has been carried out in order to investigate the capacity of snow penetration of Ku-band waves. The multi-layered model used in this study is based on the first-order small perturbation method (Afifi et al. 2010, 2012). Within its domain of validity, this approximate model allows a fast analysis of the multi-layered structures by means of analytical equations giving the scattered field and intensities. The total backscattered intensity IT is written as a sum of a coherent IC and a fluctuating IF contribution: IT = IC + IF IC is the coherent contribution to the total intensity coming from the scattering of the stack of layers, and IF is the fluctuating contribution which takes into account the first order roughness effects. The medium is considered as a stack of three layers, with two interfaces, air/snow and snow/ice. Several simulations have been conducted by varying the temperature, permittivity, roughness and thickness of each layer and the results are presented. More specifically, the influence of the snow thickness on the backscattering is analysed

High-resolution barotropic modeling and the calving of the Mertz Glacier, East Antarctica

In February 2010, the Mertz Glacier Tongue (MGT) calved, releasing an 80 × 40 km iceberg. We have developed a high-resolution barotropic ocean model of the region to simulate the local circulation in response to tides and atmospheric forcing. We improve

Velocities Field of Mountain Glacier Obtained by Synthetic Aperture Radar Interferometry

The Mer de Glace and Argentière glaciers are located in the Mont Blanc region, French Alps. They are temperate glaciers and their velocity flow is about one hundred meters a year (~270 mm a day). This paper presents a use of Synthetic-Aperture Radar (SAR) interferogram obtained from the two European Remote-Sensing satellites (ERS1-2) to measure the motion of Mer de Glace and Argentière glaciers. We investigate whether the interferometric data are quantitatively consistent with terrestrial velocity measurements along two transverse profiles and two longitudinal profiles. Interferometric and terrestrial velocity are in agreement if a (terrestrially measured) surface-normal velocity component is properly accounted for. This suggest that both the interferometric velocities and the conversions of terrestrial data to the winter period are reliable. Finally we show that the application of repeat-pass SAR interferometry to the glaciers enable precise mapping of ice flow dynamics at a much higher level than usually obtained

Ongoing Development of the Bass Strait GNSS/INS Buoy System for Altimetry Validation in Preparation for SWOT

GNSS equipped buoys remain an important tool in altimetry validation. Progressive advances in altimetry missions require associated development in such validation tools. In this paper, we enhanced an existing buoy approach and gained further understanding of the buoy dynamics based on in situ observations. First, we implemented the capability to separate the ambiguity fixing strategy for different constellations in the processing software TRACK. A comparison between GPS and GNSS solutions suggested up to 3 cm reduction in the root mean square of the buoy minus co-located mooring SSH residuals over the selected sidereal periods. Then, comparison between double differencing and precise point positioning solutions suggested a possible common mode error external to GNSS processing. To assess buoy performance in different ocean conditions and sea states, GNSS and INS observations were used during periods where external forcings (waves, current and wind) were not interacting substantially. For the deployments investigated, no significant relationship was found, noting the maximum significant wave height and current velocity was ~2.3 m and ~0.3 m/s, respectively. In the lead up to the validation required for the SWOT mission, these results place important bounds on the performance of the buoy design under real operating conditions

Consistent patterns of Antarctic ice sheet interannual variations from ENVISAT radar altimetry and GRACE satellite gravimetry

International audienc