Altimeter Missions Research Articles

Fenofibrate and Atorvastatin Have Opposite Effects on the Vascularization and Characteristics of Visceral Adipose Tissue in New Zealand White Rabbits

Unlike previous altimetric missions, the CryoSat-2 altimeter features a novel Synthetic Aperture Radar (SAR) mode that allows higher resolution and more accurate altimeter-derived parameters in the coastal zone, thanks to the reduced along-track footprint. The scope of this study is to quantify regionally the skills of CryoSat-2 SAR altimetry for distances to coast smaller than 10 km, during the mission lifetime and at different time scales. The validated geophysical altimeter parameters are the sea surface height above the ellipsoid, the significant sea wave height and wind speed, all computed at 20 Hz. These have been compared to in situ and regional model data along the coasts of German Bight and West Baltic Sea during a time interval of almost six years, from July 2010 to March 2016, to investigate both instantaneous and seasonal behaviour.From CryoSat-2 FBR (Full Bit Rate) data, a Delay-Doppler processing and waveform retracking tailored specifically to the coastal zone has been carried out, by applying a Hamming window and zero-padding, using an extended vertical swath window in order to mitigate tracker errors. Moreover, a dedicated SAMOSA-based coastal retracker (here referred to as SAMOSA+) has also been implemented.Since one of the highest remaining uncertainties in the altimeter parameters estimated in coastal shallow waters arises from residual errors in the applied range and geophysical corrections, innovative and high resolution solutions for ocean tide model, geoid, mean sea surface and wet tropospheric correction have been selected. As CryoSat-2 SAR and LRM (Low Rate Mode) modes are not collocated in time, in order to quantify the improvement in the coastal zone with respect to pulse-limited altimetry, 20 Hz PLRM (pseudo-LRM) data from CryoSat-2 FBR were built and retracked, adopting the ALES adaptive sub-waveform approach, with a numerical Brown-based retracker, here referred to as TALES.The cross-validation proves the good consistency between PLRM and SAR sea level anomaly in the coastal zone. The regional ocean model (BSH) shows the highest agreement with the SAR sea level anomaly, with a standard deviation of the differences (stdd) of 24 cm, whereas the corresponding value with respect to PLRM is 55 cm. Distance to coast plots show that land contamination begins to affect sea level and wave measurements at 2 km from the coast in SAR and at 3.5 km in PLRM TALES. The analysis of monthly mean time-series shows that SAR Altimetry is able to measure the sea level monthly mean in the coastal zone of the region of interest, during the entire mission, more precisely than PLRM. The cross-validation against in situ data also proves the higher accuracy of SAR SAMOSA+ compared to PLRM TALES in the coastal zone, with average SLA stdd of 4.4 cm and 8.4 cm respectively.

Investigating the 59-Day Error Signal in the Mean Sea Level Derived From TOPEX/Poseidon, Jason-1, and Jason-2 Data With FES and GOT Ocean Tide Models

Since the beginning of the altimeter mission TOPEX/Poseidon (T/P), followed by Jason-1 and Jason-2 on similar orbits, and many other missions on different orbits (ERS, EnviSat, etc.), mean sea level (MSL) products became essential for the comprehension of global ocean circulation. Since early in the T/P mission, a suspicious signal, having a period of near 59 days and amplitude of roughly 5 mm, was apparent in the Global MSL record. Compared with the 4–5-mm amplitude of the annual signal, the 59-day signal has understandably attracted attention. Moreover, the same signal has been subsequently detected in Jason-1 and later in Jason-2 MSLs. In 2010, the Ocean Surface Topography Science Team (OSTST) concluded this signal as the aliasing of a higher frequency error inherited from the tide model correction: the semi-diurnal wave S2. The source of this error was mainly attributed to T/P measurements, which were assimilated in ocean tide models. When these models are used in the computation of T/P MSL, most of the error cancels. However, this error is communicated to Jason-1 and Jason-2 MSLs. In order to gather and publish the OSTST analyses on this matter, this paper first attempts to list the myriad possibilities for the puzzling 59-day error in MSL. Then, this paper goes deeper into the description of the main contributor to this list: the tide models error. Indeed, since 2010, considerable efforts have been undertaken within the ocean tide community in order to correct ocean tide S2-waves from this error, particularly in the Goddard Ocean Tide (GOT) and finite element solution (FES) latest versions. Comparing several GOT and FES versions and a pure hydrodynamic tide model, this paper assesses, quantifies, and describes a reduction of the MSL 59-day error thanks to the latest releases. These analyses also confirm that a large part of this error has its origins in the T/P mission and has contaminated ocean tide solutions and Jason-1 and Jason-2 MSLs. They also suggest that ocean tide is not the only possible vector. Jason-1 and Jason-2 MSLs contain additional 59-day error—though to a lesser extent—that may either come from the measurements themselves or from another vector.

An improved approach to monitoring Brahmaputra River water levels using retracked altimetry data

Satellite altimetry is an important tool for monitoring water levels over oceans and inland water bodies, particularly over poorly gauged or ungauged areas. This study uses satellite altimetry (Jason-2/3 and Envisat) to derive water levels of the Great Brahmaputra River (GBR) originating from the Tibetan Plateau. Although the width of the river channels of the Lower Brahmaputra River (LBR) is ~1 km, the Upper Brahmaputra River (UBR) (which is part of the Yarlung Zangbo River of China) and the Middle Brahmaputra River (MBR) located in high-mountain regions have river widths that are generally less than 400 m. This poses considerable challenges for existing retracking algorithms to obtain accurately retrieved water levels. In this study, an improved approach for deriving water levels in high-mountain regions with complex terrain is proposed, comprising (1) an improved footprint selection and (2) an improved waveform retracking, called the 50% Threshold and Ice-1 Combined algorithm (TIC). It was applied to river channels of varying widths, ranging from 200 m in the UBR to more than 1 km in the LBR. Results show an increase in both the accuracy and sampling of water levels. Most of the derived water levels at 13 virtual stations (VSs) along the GBR agree reasonably well with gauged water levels (for VSs in the UBR) or published results (for VSs in the LBR). The standard deviation of the difference between the TIC-derived water levels and gauged data at the VSs ranges from 0.3 m to 0.8 m with the highest improvement percentage relative to the unretracked ranges reaching 80% in the UBR. In addition, the developed approach increases water level sampling by reasonably demarcating the buffer zone for footprint selection, thereby generating more water levels in the time series than the published results for VSs in the LBR. However, 3 out of the 13 virtual stations show poor performance for Envisat, primarily due to the extremely narrow river channels. Furthermore, TIC can potentially be applied to estimate water levels near ground tracks of altimetric missions, even where there is no crossover between the river and the track. It could also be applied to other altimetric missions, which would further contribute to monitoring water levels and potentially river discharge in high-mountain regions with narrow river channels.

Satellite remote sensing estimation of river discharge: Application to the Yukon River Alaska

A methodology based on general hydraulic relations for rivers has been developed to estimate the discharge (flow rate) of rivers using satellite remote sensing observations. The estimates of discharge, flow depth, and flow velocity are derived from remotely observed water surface area, water surface slope, and water surface height, and demonstrated for two reaches of the Yukon River in Alaska, at Eagle (reach length 34.7 km) and near Stevens Village (reach length 38.3 km). The method is based on fundamental equations of hydraulic flow resistance in rivers, including the Manning equation and the Prandtl-von Karman universal velocity distribution equation. The method employs some new hydraulic relations to help define flow resistance and height of the zero flow boundary in the channel. Estimates are made both with and without calibration. The water surface area of the river reach is measured by using a provisional version of the U.S. Geological Survey (USGS) Landsat based product named Dynamic Surface Water Extent (DSWE). The water surface height and slope measurements require a self-consistent datum, and are derived from observations from the Jason-2 satellite altimeter mission. At both reach locations, the Jason-2 radar altimeter non-winter heights consistently tracked the stage recorded at USGS streamgages with a standard deviation of differences (error) during the non-winter periods of less than 7%. Part of the error may be due to differences in the gage and altimeter crossing locations with respect to the range of stage change and the response to changes in discharge at the upstream and downstream locations. For the non-winter periods, the radar derived slope estimates (mean = 0.0003) were constant over the mission lifetime, and in agreement with previously measured USGS water surface slopes and slopes determined from USGS topographic maps. The accuracy of the mean of the uncalibrated daily estimates of discharge varied between reaches, ranging from 13% near Stevens Village (N = 90) to −21% at Eagle (N = 246) based on the absolute error, and 5% to −6% based on the error of the log of the estimates. Calibrating to the mean of USGS daily discharge estimates from the streamflow rating for the same period of record at each streamgage resulted in mean absolute errors ranging from 1% to 2%, and log errors ranging from 1% or less. The error pattern of the estimates shows that without calibration, even though the mean is well simulated, the high and low end values over the range of estimates may have significant bias.

Elevation Change Derived from SARAL/ALtiKa Altimetric Mission: Quality Assessment and Performance of the Ka-Band

The waveform retracking algorithm is a key factor that affects the accuracy of elevation change from satellite altimetry over an ice sheet. The elevation change results from four waveform retracker algorithms (ICE1/ICE2/Sea Ice/OCEAN) provided by the Satellite with ARgos and ALtiKa (SARAL/ALtiKa) data were compared using repeated SARAL data between March 2013 and April 2016 to determine the optimal retracker in the crossovers of descending and ascending orbits over a Greenland ice sheet (GrIS). The ICE1 provided slightly better results than the three other algorithms with the lowest standard deviation (SD) of 0.30 m year−1. Further comparison was also conducted between the Satellite with ARgos and ALtiKa (SARAL) and Operation ICEBridge laser data, thereby indicating that ICE1 was the best retracker with an Root Mean Square Error (RMSE) of 0.43 m year−1. The distribution of elevation change rate and uncertainties over Greenland from SARAL were presented using the selected ICE1 retracker with a volume loss of 40 ± 12 km3 year−1. This volume loss did not include the fast-changing coastal areas of the GrIS. A large thinning was observed in Jakobshavn Isbræ, and a trend that extended far inland was also found from 2013–2016. Furthermore, a melting ice sheet was observed in the large areas northwest over the GrIS.

Independent Assessment of Sentinel-3A Wet Tropospheric Correction over the Open and Coastal Ocean

Launched on 16 February 2016, Sentinel-3A (S3A) carries a two-band microwave radiometer (MWR) similar to that of Envisat, and is aimed at the precise retrieval of the wet tropospheric correction (WTC) through collocated measurements using the Synthetic Aperture Radar Altimeter (SRAL) instrument. This study aims at presenting an independent assessment of the WTC derived from the S3A MWR over the open and coastal ocean. Comparisons with other four MWRs show Root Mean Square (RMS) differences (cm) of S3A with respect to these sensors of 1.0 (Global Precipitation Measurement (GPM) Microwave Imager, GMI), 1.2 (Jason-2), 1.3 (Jason-3), and 1.5 (Satellite with ARgos and ALtika (SARAL)). The linear fit with respect to these MWR shows scale factors close to 1 and small offsets, indicating a good agreement between all these sensors. In spite of the short analysis period of 10 months, a stable temporal evolution of the S3A WTC has been observed. In line with the similar two-band instruments aboard previous European Space Agency (ESA) altimetric missions, strong ice and land contamination can be observed, the latter mainly found up to 20–25 km from the coast. Comparisons with the European Centre for Medium-Range Weather Forecasts (ECMWF) and an independent WTC derived only from third party data are also shown, indicating good overall performance. However, improvements in both the retrieval algorithm and screening of invalid MWR observations are desirable to achieve the quality of the equivalent WTC from Jason-3. The outcome of this study is a deeper knowledge of the measurement capabilities and limitations of the type of MWR aboard S3A and of the present WTC retrieval algorithms.

SARin mode, and a window delay approach, for coastal altimetry

The CryoSat-2 (CS-2) mission is an altimetric mission designed to meet the measurement requirements for ice-sheet elevation and sea-ice ‘freeboard’. In addition to Cryosphere applications, this mission is of great interest to the Hydrosphere and Oceanographic communities since it allows for quantitative assessment of expected enhanced altimetric capabilities in coastal monitoring, ocean floor topography, gravity field and inland water monitoring. The CS-2 main payload is a Synthetic aperture Interferometric Radar ALtimeter (SIRAL) operating in three different modes, including the SAR interferometric mode (SARin). In the work described in this paper, dedicated algorithms are developed for exploiting SARin mode for enhancing the sea level mapping at coastal regions. When operating in SARin mode the altimeter measurements allow for the derivation of the across-track Angle of Arrival (AoA), and so the reflection points at any range bin can be geolocated. In coastal altimetry, land contamination highly degrades the backscattered echo. This is sometimes, overcome with the SARM when tracks are perpendicular to the coast, since this mode provides an along-track resolution enhancement with respect to LRM mode due to the different footprints shapes (LRM is circular, while SARM is a stripe of the same wide across track but only around 300 m along track). However, across-track interferences still remain an issue in coastal altimetry and SARin mode can provide a solution for their mitigation. This paper proposes two different solutions for the improvement of SSH retrievals in coastal zones starting from L1b SARin products, developing post-L1b algorithms. The first solution is based in the use of the AoA. The second solution goes beyond SARin mode, and provides an algorithm for all operational modes, based in the historical window delay within a coastal track section. For both options, a sub-waveform approach is considered for the waveforms cleaning and retracking. In addition, a SAR L2 retracker inherited from the SAMOSA model is adapted to SARin. The results are analyzed and the potential improvements are assessed with respect to SARin data retracker by the CS-2 ground processor. A dedicated CS-2 SARin area (mask) covering the Cuban archipelago has been created for this investigation. A land-sea mask has been created from the Open Street Map high resolution catalogue for this study. A dataset has been produced including both ESA CS2 IPF Baseline B SSH and the SSH series coming from this study.

An improved and homogeneous altimeter sea level record from the ESA Climate Change Initiative

Abstract. Sea level is a very sensitive index of climate change since it integrates the impacts of ocean warming and ice mass loss from glaciers and the ice sheets. Sea level has been listed as an essential climate variable (ECV) by the Global Climate Observing System (GCOS). During the past 25 years, the sea level ECV has been measured from space by different altimetry missions that have provided global and regional observations of sea level variations. As part of the Climate Change Initiative (CCI) program of the European Space Agency (ESA) (established in 2010), the Sea Level project (SL_cci) aimed to provide an accurate and homogeneous long-term satellite-based sea level record. At the end of the first phase of the project (2010–2013), an initial version (v1.1) of the sea level ECV was made available to users (Ablain et al., 2015). During the second phase of the project (2014–2017), improved altimeter standards were selected to produce new sea level products (called SL_cci v2.0) based on nine altimeter missions for the period 1993–2015 (https://doi.org/10.5270/esa-sea_level_cci-1993_2015-v_2.0-201612; Legeais and the ESA SL_cci team, 2016c). Corresponding orbit solutions, geophysical corrections and altimeter standards used in this v2.0 dataset are described in detail in Quartly et al. (2017). The present paper focuses on the description of the SL_cci v2.0 ECV and associated uncertainty and discusses how it has been validated. Various approaches have been used for the quality assessment such as internal validation, comparisons with sea level records from other groups and with in situ measurements, sea level budget closure analyses and comparisons with model outputs. Compared with the previous version of the sea level ECV, we show that use of improved geophysical corrections, careful bias reduction between missions and inclusion of new altimeter missions lead to improved sea level products with reduced uncertainties on different spatial and temporal scales. However, there is still room for improvement since the uncertainties remain larger than the GCOS requirements (GCOS, 2011). Perspectives on subsequent evolution are also discussed.

Satellite Investigation of the M2 Internal Tide in the Tasman Sea

AbstractThe M2 internal tide in the Tasman Sea is investigated using sea surface height measurements made by multiple altimeter missions from 1992 to 2012. Internal tidal waves are extracted by two-dimensional plane wave fits in 180 km by 180 km windows. The results show that the Macquarie Ridge radiates three internal tidal beams into the Tasman Sea. The northern and southern beams propagate respectively into the East Australian Current and the Antarctic Circumpolar Current and become undetectable to satellite altimetry. The central beam propagates across the Tasman Sea, impinges on the Tasmanian continental slope, and partially reflects. The observed propagation speeds agree well with theoretical values determined from climatological ocean stratification. Both the northern and central beams refract about 15° toward the equator because of the beta effect. Following a concave submarine ridge in the source region, the central beam first converges around 45.5°S, 155.5°E and then diverges beyond the focal region. The satellite results reveal two reflected internal tidal beams off the Tasmanian slope, consistent with previous numerical simulations and glider measurements. The total energy flux from the Macquarie Ridge into the Tasman Sea is about 2.2 GW, of which about half is contributed by the central beam. The central beam loses little energy in its first 1000-km propagation, for which the likely reasons include flat bottom topography and weak mesoscale eddies.

The Benefits of the Ka-Band as Evidenced from the SARAL/AltiKa Altimetric Mission: Scientific Applications

The India–France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated primarily to oceanography. The mission objectives were firstly the observation of the oceanic mesoscales but also global and regional sea level monitoring, including the coastal zone, data assimilation, and operational oceanography. SARAL/AltiKa proved also to be a great opportunity for inland waters applications, for observing ice sheet or icebergs, as well as for geodetic investigations. The mission ended its nominal phase after three years in orbit and began a new phase (drifting orbit) in July 2016. The objective of this paper is to highlight some of the most remarkable achievements of the SARAL/AltiKa mission in terms of scientific applications. Compared to the standard Ku-band altimetry measurements, the Ka-band provides substantial improvements in terms of spatial resolution and data accuracy. We show here that this leads to remarkable advances in terms of observation of the mesoscale and coastal ocean, waves, river water levels, ice sheets, icebergs, fine scale bathymetry features as well as for the many related applications.

The Benefits of the Ka-Band as Evidenced from the SARAL/AltiKa Altimetric Mission: Quality Assessment and Unique Characteristics of AltiKa Data

The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. The mission ended its nominal phase after 3 years in orbit and began a new phase (drifting orbit) in July 2016. The objective of this paper is to provide a state of the art of the achievements of the SARAL/AltiKa mission in terms of quality assessment and unique characteristics of AltiKa data. It shows that the AltiKa data have similar accuracy at the centimeter level in term of absolute water level whatever the method (from local to global) and the type of water surfaces (ocean and lakes). It shows also that beyond the fact that AltiKa data quality meets the expectations and initial mission requirements, the unique characteristics of the altimeter and the Ka-band offer unique contributions in fields that were previously not fully foreseen.

Response of Pacific-sector Antarctic ice shelves to the El Niño/Southern Oscillation.

Satellite observations over the past two decades have revealed increasing loss of grounded ice in West Antarctica, associated with floating ice shelves that have been thinning. Thinning reduces an ice-shelf’s ability to restrain grounded-ice discharge, yet our understanding of the climate processes that drive mass changes is limited. Here, we use ice-shelf height data from four satellite altimeter missions (1994–2017) to show a direct link between ice-shelf-height variability in the Antarctic Pacific sector and changes in regional atmospheric circulation driven by the El Niño-Southern Oscillation. This link is strongest from Dotson to Ross ice shelves and weaker elsewhere. During intense El Niño years, height increase by accumulation exceeds the height decrease by basal melting, but net ice-shelf mass declines as basal ice loss exceeds lower-density snow gain. Our results demonstrate a substantial response of Amundsen Sea ice shelves to global and regional climate variability, with rates of change in height and mass on interannual timescales that can be comparable to the longer-term trend, and with mass changes from surface accumulation offsetting a significant fraction of the changes in basal melting. This implies that ice-shelf height and mass variability will increase as interannual atmospheric variability increases in a warming climate.

Assessing the impact of multiple altimeter missions and Argo in a global eddy-permitting data assimilation system

Abstract. A series of observing system simulation experiments (OSSEs) is carried out with a global data assimilation system at 1∕4° resolution using simulated data derived from a 1∕12° resolution free-run simulation. The objective is to not only quantify how well multiple altimeter missions and Argo profiling floats can constrain the global ocean analysis and 7-day forecast at 1∕4° resolution but also to better understand the sensitivity of results to data assimilation techniques used in Mercator Ocean operational systems. The impact of multiple altimeter data is clearly evidenced even at a 1∕4° resolution. Seven-day forecasts of sea level and ocean currents are significantly improved when moving from one altimeter to two altimeters not only on the sea level, but also on the 3-D thermohaline structure and currents. In high-eddy-energy regions, sea level and surface current 7-day forecast errors when assimilating one altimeter data set are respectively 20 and 45 % of the error of the simulation without assimilation. Seven-day forecasts of sea level and ocean currents continue to be improved when moving from one altimeter to two altimeters with a relative error reduction of almost 30 %. The addition of a third altimeter still improves the 7-day forecasts even at this medium 1∕4° resolution and brings an additional relative error reduction of about 10 %. The error level of the analysis with one altimeter is close to the 7-day forecast error level when two or three altimeter data sets are assimilated. Assimilating altimeter data also improves the representation of the 3-D ocean fields. The addition of Argo has a major impact on improving temperature and demonstrates the essential role of Argo together with altimetry in constraining a global data assimilation system. Salinity fields are only marginally improved. Results derived from these OSSEs are consistent with those derived from experiments with real data (observing system evaluations, OSEs) but they allow for more detailed characterisation of errors on analyses and 7-day forecasts. Both OSEs and OSSEs should be systematically used and intercompared to test data assimilation systems and quantify the impact of existing observing systems.

Coastal SAR and PLRM altimetry in German Bight and West Baltic Sea

Unlike previous altimetric missions, the CryoSat-2 altimeter features a novel Synthetic Aperture Radar (SAR) mode that allows higher resolution and more accurate altimeter-derived parameters in the coastal zone, thanks to the reduced along-track footprint. The scope of this study is to quantify regionally the skills of CryoSat-2 SAR altimetry for distances to coast smaller than 10 km, during the mission lifetime and at different time scales. The validated geophysical altimeter parameters are the sea surface height above the ellipsoid, the significant sea wave height and wind speed, all computed at 20 Hz. These have been compared to in situ and regional model data along the coasts of German Bight and West Baltic Sea during a time interval of almost six years, from July 2010 to March 2016, to investigate both instantaneous and seasonal behaviour.From CryoSat-2 FBR (Full Bit Rate) data, a Delay-Doppler processing and waveform retracking tailored specifically to the coastal zone has been carried out, by applying a Hamming window and zero-padding, using an extended vertical swath window in order to mitigate tracker errors. Moreover, a dedicated SAMOSA-based coastal retracker (here referred to as SAMOSA+) has also been implemented.Since one of the highest remaining uncertainties in the altimeter parameters estimated in coastal shallow waters arises from residual errors in the applied range and geophysical corrections, innovative and high resolution solutions for ocean tide model, geoid, mean sea surface and wet tropospheric correction have been selected. As CryoSat-2 SAR and LRM (Low Rate Mode) modes are not collocated in time, in order to quantify the improvement in the coastal zone with respect to pulse-limited altimetry, 20 Hz PLRM (pseudo-LRM) data from CryoSat-2 FBR were built and retracked, adopting the ALES adaptive sub-waveform approach, with a numerical Brown-based retracker, here referred to as TALES.The cross-validation proves the good consistency between PLRM and SAR sea level anomaly in the coastal zone. The regional ocean model (BSH) shows the highest agreement with the SAR sea level anomaly, with a standard deviation of the differences (stdd) of 24 cm, whereas the corresponding value with respect to PLRM is 55 cm. Distance to coast plots show that land contamination begins to affect sea level and wave measurements at 2 km from the coast in SAR and at 3.5 km in PLRM TALES. The analysis of monthly mean time-series shows that SAR Altimetry is able to measure the sea level monthly mean in the coastal zone of the region of interest, during the entire mission, more precisely than PLRM. The cross-validation against in situ data also proves the higher accuracy of SAR SAMOSA+ compared to PLRM TALES in the coastal zone, with average SLA stdd of 4.4 cm and 8.4 cm respectively.

Assessment of CryoSat-2 interferometric and non-interferometric SAR altimetry over ice sheets

The launch of CryoSat-2 heralded a new era of interferometric Synthetic Aperture Radar altimetry over the Polar Ice Sheets. The mission’s novel SAR interferometric (SARIn) mode of operation has enabled monitoring of rapidly changing coastal regions, which had been challenging for previous low resolution altimeters. Given the growing requirement to continue the 25-year altimeter record, there is now a need to assess the differences between existing SAR and SARIn altimeter datasets, with a view to understanding the impact on ice sheet retrievals of the different radar hardware and processing methodologies. Uniquely, CryoSat-2 data can be processed both with and without interferometric information, offering the opportunity to directly compare the SAR and SARIn products generated by the current ground segment. Here, we provide a first comparison of these Level-2 datasets, and evaluate their capacity to measure ice sheet elevation and elevation change. We find that the current interferometric product has substantially improved precision, accuracy and coverage compared to its non-interferometric counterpart, yielding a ∼35% improvement in the root-mean-square-difference (RMSD) of elevations recorded at orbital cross-overs, and a ∼30% lower RMSD of elevation rates relative to Operation IceBridge airborne altimeter measurements. This analysis demonstrates the value that the interferometer adds to the current CryoSat-2 configuration, and highlights the importance for non-interferometric SAR Level-2 processing of the auxiliary data used to identify the location of the echoing point. These results provide a benchmark of the relative performance of the Level-2 interferometric and non-interferometric products currently produced by the ground segment, which will help to inform the design and implementation of a future polar radar altimeter mission.

A data assimilation system combining CryoSat-2 data and hydrodynamic river models

There are numerous hydrologic studies using satellite altimetry data from repeat-orbit missions such as Envisat or Jason over rivers. This study is one of the first examples for the combination of altimetry from drifting-ground track satellite missions, namely CryoSat-2, with a river model. CryoSat-2 SARIn Level 2 data is used to improve a 1D hydrodynamic model of the Brahmaputra River in South Asia, which is based on the Saint-Venant equations for unsteady flow and set up in the MIKE HYDRO River software. After calibration of discharge and water level the hydrodynamic model can accurately and bias-free represent the spatio-temporal variations of water levels. A data assimilation framework has been developed and linked with the model. It is a flexible framework that can assimilate water level data which are arbitrarily distributed in time and space. The setup has been used to assimilate CryoSat-2 water level observations over the Assam valley for the years 2010–2015, using an Ensemble Transform Kalman Filter (ETKF). Performance improvement in terms of discharge forecasting skill was then evaluated. For experiments with synthetic CryoSat-2 data the continuous ranked probability score (CRPS) was improved by up to 32%, whilst for experiments assimilating real data it could be improved by up to 10%. The developed methods are expected to be transferable to other rivers and altimeter missions. The model setup and calibration is based almost entirely on globally available remote sensing data.

Comparison of CryoSat-2 and ENVISAT radar freeboard over Arctic sea ice: toward an improved Envisat freeboard retrieval

Abstract. Over the past decade, sea-ice freeboard has been monitored with various satellite altimetric missions with the aim of producing long-term time series of ice thickness. While recent studies have demonstrated the capacity of the CryoSat-2 mission (2010–present) to provide accurate freeboard measurements, the current estimates obtained with the Envisat mission (2002–2012) still require some large improvements. In this study, we first estimate Envisat and CryoSat-2 radar freeboard by using the exact same processing algorithms. We then analyse the freeboard difference between the two estimates over the common winter periods (November 2010–April 2011 and November 2011–March 2012). The analysis of along-track data and gridded radar freeboard in conjunction with Envisat pulse-peakiness (PP) maps suggests that the discrepancy between the two sensors is related to the surface properties of sea-ice floes and to the use of a threshold retracker. Based on the relation between the Envisat pulse peakiness and the radar freeboard difference between Envisat and CryoSat-2, we produce a monthly CryoSat-2-like version of Envisat freeboard. The improved Envisat data set freeboard displays a similar spatial distribution to CryoSat-2 (RMSD = 1.5 cm) during the two ice growth seasons and for all months of the period of study. The comparison of the altimetric data sets with in situ ice draught measurements during the common flight period shows that the improved Envisat data set (RMSE = 12–28 cm) is as accurate as CryoSat-2 (RMSE = 15–21 cm) and much more accurate than the uncorrected Envisat data set (RMSE = 178–179 cm). The comparison of the improved Envisat radar freeboard data set is then extended to the rest of the Envisat mission to demonstrate the validity of PP correction from the calibration period. The good agreement between the improved Envisat data set and the in situ ice draught data set (RMSE = 13–32 cm) demonstrates the potential of the PP correction to produce accurate freeboard estimates over the entire Envisat mission lifetime.

Altimetry in a Regional Tropical Sea [Space Agencies

The satellite for Argos and AltiKa (SARAL/AltiKa) is the first ocean altimeter mission to operate in the Ka-band frequency. The objective of this article is to investigate the extent to which SARAL/AltiKa sea-level measurements provide valuable information in a complex bathymetric region, i.e., the semienclosed Solomon Sea. The data-editing procedure is revisited, and we propose two new data-editing criteria. The first is based on the detection of erroneous sea-level values after computation, and the second directly analyzes the radar measurements and geophysical corrections. We show that both methods are significantly more efficient than the standard procedure used in operational processing chains.

A new phase in the production of quality-controlled sea level data

Abstract. Sea level is an essential climate variable (ECV) that has a direct effect on many people through inundations of coastal areas, and it is also a clear indicator of climate changes due to external forcing factors and internal climate variability. Regional patterns of sea level change inform us on ocean circulation variations in response to natural climate modes such as El Niño and the Pacific Decadal Oscillation, and anthropogenic forcing. Comparing numerical climate models to a consistent set of observations enables us to assess the performance of these models and help us to understand and predict these phenomena, and thereby alleviate some of the environmental conditions associated with them. All such studies rely on the existence of long-term consistent high-accuracy datasets of sea level. The Climate Change Initiative (CCI) of the European Space Agency was established in 2010 to provide improved time series of some ECVs, including sea level, with the purpose of providing such data openly to all to enable the widest possible utilisation of such data. Now in its second phase, the Sea Level CCI project (SL_cci) merges data from nine different altimeter missions in a clear, consistent and well-documented manner, selecting the most appropriate satellite orbits and geophysical corrections in order to further reduce the error budget. This paper summarises the corrections required, the provenance of corrections and the evaluation of options that have been adopted for the recently released v2.0 dataset (https://doi.org/10.5270/esa-sea_level_cci-1993_2015-v_2.0-201612). This information enables scientists and other users to clearly understand which corrections have been applied and their effects on the sea level dataset. The overall result of these changes is that the rate of rise of global mean sea level (GMSL) still equates to ∼ 3.2 mm yr−1 during 1992–2015, but there is now greater confidence in this result as the errors associated with several of the corrections have been reduced. Compared with v1.1 of the SL_cci dataset, the new rate of change is 0.2 mm yr−1 less during 1993 to 2001 and 0.2 mm yr−1 higher during 2002 to 2014. Application of new correction models brought a reduction of altimeter crossover variances for most corrections.

Hurricane Isaac storm surges off Florida observed by Jason-1 and Jason-2 satellite altimeters

Storm surges can cause damage to properties and loss of life in coastal communities. Thus it is important to enhance our capabilities of observing, understanding and forecasting storm surges for mitigating damage and loss. Previous studies have shown that cross-shelf altimetric sea surface height profiles can be used to determine storm surge features. In this study we combine satellite altimetry with tide-gauge data to study storm surge features off Florida in the Gulf of Mexico during Hurricane Isaac 2012. Satellite observations show a storm surge of about 0.8 m near Cedar Key and Apalachicola decreasing westward and southward in the early morning of August 28, 2012 (UTC), consistent with tide-gauge measurements. Our analysis of satellite data reveals that the storm surge propagates northward from Naples to Cedar Key with a phase speed of 14–16m/s and a cross-shelf decay scale of 190–220km, in approximate agreement with the estimates from tide-gauge data. In contrast, an analysis of tide-gauge data reveals that the storm surge propagates westward from Apalachicola to Pensacola with a phase speed of 6–7m/s and a cross-shelf decay scale of about 85km. It is further shown that the post-storm sea level variations at these stations are associated with first-mode continental shelf waves. The present study shows that along-shelf altimetric sea surface height profiles can be useful to observe and understand storm surge features as cross-shelf ones demonstrated previously, of importance to improvement of storm surge forecasting. It suggests that a constellation of altimeter missions especially with wide-swath altimetry could be suited to monitor storm surges.