Evaluation of Sentinel-3A Wave Height Observations Near the Coast of Southwest England

DuetothesmallergroundfootprintandhigherspatialresolutionoftheSyntheticAperture Radar (SAR) mode, altimeter observations from the Sentinel-3 satellites are expected to be overall more accurate in coastal areas than conventional nadir altimetry. The performance of Sentinel-3A in the coastal region of southwest England was assessed by comparing SAR mode observations of significant wave height against those of Pseudo Low Resolution Mode (PLRM). Sentinel-3A observations were evaluated against in-situ observations from a network of 17 coastal wave buoys, which provided continuous time-series of hourly values of significant wave height, period and direction. As the buoys are evenly distributed along the coast of southwest England, they are representative of a broad range of morphological configurations and swell conditions against which to assess Sentinel-3 SAR observations. The analysis indicates that SAR observations outperform PLRM within 15 km from the coast. Within that region, regression slopes between SAR and buoy observations are close to the 1:1 relation, and the average root mean square error between the two is 0.46±0.14 m. On the other hand, regression slopes for PLRM observations rapidly deviate from the 1:1 relation, while the average root mean square error increases to 0.84±0.45 m. The analysis did not identify any dependence of the bias between SAR and in-situ observation on the swell period or direction. The validation is based on a synergistic approach which combines satellite and in-situ observations with innovative use of numerical wave model output to help inform the choice of comparison regions. Such an approach could be successfully applied in future studies to assess the performance of SAR observations over other combinations of coastal regions and altimeters

Sensitivity of Altimeter Wave Height Assessment to Data Selection

Thispaperaddressestheissueofhowtheselectionofbuoysandthecalculationofaltimeter averages affect the metrics characterising the errors of altimetric wave height estimates. The use of a 51-point median reduces the sensitivity to occasional outliers, but the quality of this measure can be improved by demanding that there is a minimum number of valid measurements. This had a marked impact in both the open ocean and the coastal zone. It also affected the relative ordering of algorithms’ performances, as some fared poorly when a representative value was gleaned from a single waveform inversion, but had a much better ranking when a minimum of 20 values were used. Validation procedures could also be improved by choosing altimeter-buoy pairings that showed a good consistency. This paper demonstrated an innovative procedure using the median of the differentretrackersanalysed,whichcanbeeasilyextendedtootherdatavalidationexercises. Thisled to improved comparison statistics for all algorithms in the open ocean, with many showing errors less than 0.2 m, but there was only one strong change in the relative performance of the 11 Jason-3 retrackers. For Sentinel-3A, removing the inconsistent coastal buoys showed that all of the new algorithms had similar errors of just over 0.2 m. Thus, although improvements were found in the procedure usedforthe SeaState RoundRobinexercise, the relative rankingsforthe buoycalibrations are mostly unaffected

Metocean Comparisons of Jason-2 and AltiKa—A Method to Develop a New Wind Speed Algorithm

As well as range, the AltiKa altimeter provides estimates of wave height, Hs and normalized backscatter, s0, that need to be assessed prior to statistics based on them being included in climate databases. An analysis of crossovers with the Jason-2 altimeter shows AltiKa Hs values to be biased high by only »0.05m, with a standard deviation (s.d.) of »0.1m for seven-point averages. AltiKa’s s 0 values are 2.5–3 dB less than those from Jason-2, with a s.d. of »0.3 dB, with these relatively large mismatches to be expected as AltiKa measures a different part of the spectrum of sea surface roughness. A new wind speed algorithm is developed through matchinghistogram of s0 values to that for Jason-2 wind speeds. The algorithm is robust to the use of short durations of data, with a consistency at roughly the 0.1 m/s level. Incorporation of Hs as a secondary input reduces the assessed error at crossovers from 0.82 m/s to 0.71 m/s. A comparison across all altimeter frequencies used to date demonstrates that the lowest wind speeds preferentially develop the shortest scales of roughness

Removal of Covariant Errors from Altimetric Wave Height Data

The echo waveforms received by conventional radar altimeters are interpreted by retracking algorithms to give estimates of range, wave height, and backscatter strength. However, in response to fading noise on the waveform leading edge, common retrackers, such as MLE-3 and MLE-4, show correlated errors in wave height and range. This correlation is used to develop a correction to the wave height data that reduces the high-frequency variability by ∼22%, without affecting the global distribution of values. This correction also results in a closer matchup of Jason-2 and Jason-3 data during their tandem phase. Although the correction is quite straightforward in practice, the appropriate conversion factor has to be determined for each combination of altimeter and retracker. There are also remaining open questions concerning the needed low-pass filtering

Improving the precision of sea level data from satellite altimetry with high-frequency and regional Sea State Bias corrections

The sea state bias (SSB) is a large source of uncertainty in the estimation of sea level from satellite altimetry. It is still unclear to what extent it depends on errors in parameter estimations (numerical source) or to the wave physics (physical source). By improving the application of this correction we compute 20-Hz sea level anomalies that are about 30% more precise (i.e. less noisy) than the current standards. The improvement is two-fold: _rst we prove that the SSB correction should be applied directly to the 20-Hz data (12 to 19% noise decrease); secondly, we show that by recomputing a regional SSB model (based on the 20-Hz estimations) even a simple parametric relation is su_cient to further improve the correction (further 15 to 19% noise decrease). We test our methodology using range, wave height and wind speed estimated with two retrackers applied to Jason-1 waveform data: the MLE4 retracked-data available in the Sensor Geophysical Data Records of the mission and the ALES retracked-data available in the OpenADB repository (https://openadb.dg_.tum.de/). The regional SSB models are computed parametrically by means of a crossover analysis in the Mediterranean Sea and North Sea. Correcting the high-rate data for the SSB reduces the correlation between retracked parameters. Regional variations in the proposed models might be due to di_erences in wave climate and remaining sea-state dependent residual errors. The variations in the empirical model with respect to the retracker used recall the need for a speci_c SSB correction for any retracker. This study, while providing a signi_cantly more precise solution to exploit high-rate sea level data, calls for a re-thinking of the SSB correction in both its physical and numerical component, gives robustness to previous theories and provides an immediate improvement for the application of satellite altimetry in the regions of study

Agulhas ring transport efficiency from combined satellite altimetry and Argo profiles

Agulhas rings are one of the main processes contributing to the westward transport of Agulhas leakage water across the South Atlantic basin. Here, we quantified the water transported and exchanged by three Agulhas rings by combining remote‐sensing altimetry and in‐situ Argo observations. Satellite velocities showed that two of the eddies formed within the Cape Basin west of South Africa at the beginning of 2013 and reached the Mid‐Atlantic Ridge by the end of 2014. There, they merged forming the third eddy which dissipated a year later when it approached the Brazilian continental shelf. Eddy structure reconstructed from Argo profiles showed that the eddies were at least 1500‐m deep and that their dynamics was strongly affected by the two open‐ocean ridges encountered along their path. Between the ridges, eddy volumes were mostly conserved, but waters were continuously exchanged. During eddy dissipation, volume losses and water exchanges were more pronounced at depth. These findings highlight the importance of combining surface with in‐situ information to accurately represent Agulhas ring transport and exchanges. Overall, the eddies transported roughly 0.5 × 1013 m3 of water from the Cape Basin to west of 30° W in a 3‐year span. Lagrangian diagnostics indicated that, after an initial period of instability, the surface waters exchanged by the eddies along their tracks dispersed roughly in the same direction as the eddies, albeit at a much slower rate. These results further confirm that Agulhas eddies are the most efficient process for westward transport across the South Atlantic basin

Joint distributions of waves and rain

The transfer of gases between the atmosphere and ocean is affected by a number of processes, of which wave action and rainfall are two of potential significance. Efforts have been made to quantify separately their contributions; however such assessments neglect the interaction of these phenomena. Here we look at the correlation statistics of waves and rain to note which regions display a strong association between rainfall and the local sea state. The conditional probability of rain varies from ~0.5% to ~15%, with most of the equatorial belt (which contains the ITCZ) showing a greater likelihood of rain at the lowest sea states. In contrast the occurrence of rain is independent of wave height in the Southern Ocean. The 1997/98 El Niño enhances the frequency of rain in some Pacific regions, with this change showing some association with wave conditions

Initial Examination of AltiKa's Individual Echoes

The AltiKa altimeter records the reflection of Ka-band radar pulses from the Earth’s surface, with the commonly used waveform product involving the summation of 96 returns to provide average echoes at 40 Hz. Occasionally there are one-second recordings of the complex individual echoes (IEs), which facilitate the evaluation of on-board processing and offer the potential for new processing strategies. Our investigation of these IEs over the ocean confirms the on-board operations, whilst noting that data quantization limits the accuracy in the thermal noise region. By constructing average waveforms from 32 IEs at a time, and applying an innovative subwaveform retracker, we demonstrate that accurate height and wave height information can be retrieved from very short sections of data. Early exploration of the complex echoes reveals structure in the phase information similar to that noted for Envisat’s IEs

A sensitivity analysis of the impact of rain on regional and global sea-air fluxes of CO2 (dataset)

Directories containing the results from several different runs of the FluxEngine software (see Shutler et al 2015 http://www.oceanflux-ghg.org). These directories are named according o the parameterisation used to derive the results within. 'SOCAT' or 'takahashi' refers to the source of pCO2 climatology used in the software. 'Nonlinear raink' or 'raink' refer to the parameterisation used for estimating rain enhancement of gas transfer velocity (Harrison et al 2012 & Ho et al. 2004 respectively). 'wetdep' and 'wet deposition' refer to the direct deposition of carbon to the surface ocean by rain. 'reference' data sets do not include the effects of rain. Within the directories, results are in netCDF files within sub-directories for year and month. Net Fluxes and summary statistics have been calculated and are provided as text files. The names are again according to the parameterisation used to derive them. More details are in the associated paper References: Shutler JD, Land PE, Piolle J-F, Woolf DK, Goddijn-Murphy L, Paul F, et al. FluxEngine: A flexible processing system for calculating atmosphere-ocean carbon dioxide gas fluxes and climatologies. Journal of Atmospheric and Oceanic Technology. 2015; (Early release). doi: 10.1175/JTECH-D-14-00204.1. Harrison EL, Vernon F, Ho DT, Reid MR, Orton P, McGillis WR. Nonlinear interaction between rain- and wind-induced air-water gas exchange. Journal of Geophysical Research. 2012;117(C03034). doi: 10.1029/2011JC007693. Ho DT, Zappa CJ, McGillis WR, Bliven LF, Ward B, Dacey JWH, et al. Influence of rain on air-sea gas exchange: Lessons from a model ocean. Journal of Geophysical Research. 2004;109(C08S18). doi: 10.1029/2003JC001806.The article associated with this dataset is available in ORE at http://hdl.handle.net/10871/22888Data sets calculated using the FluxEngine software to examine the sensitivity of global estimates of CO2 exchange between ocean and atmosphere to rainfall. These data contribute to the publication 'A sensitivity analysis of the impact of rain on regional and global sea-air fluxes of CO2', accepted for publication by PlosOneThis work was funded by the European Space Agency (ESA) Support to Science Element (STSE) through the OceanFlux Greenhouse Gases project (contract 4000104762/11/I-AM) and the OceanFlux Greenhouse Gases Evolution project (contract 4000112091/14/I-LG). http://due.esrin.esa.int/stse

Estimation of Ocean Surface Currents from Maximum Cross Correlation applied to GOCI geostationary satellite remote sensing data over the Tsushima (Korea) Straits

Attempts to automatically estimate surface current velocities from satellite-derived thermal or visible imagery face the limitations of data occlusion due to cloud cover, the complex evolution of features and the degradation of their surface signature. The Geostationary Ocean Color Imager (GOCI) provides a chance to reappraise such techniques due to its multi-year record of hourly high-resolution visible spectrum data. Here we present the results of applying a Maximum Cross Correlation (MCC) technique to GOCI data. Using a combination of simulated and real data we derive suitable processing parameters and examine the robustness of different satellite products, those being water-leaving radiance and chlorophyll concentration. These estimates of surface currents are evaluated using High Frequency (HF) radar systems located in the Tsushima (Korea) Strait. We show the performance of the MCC approach varies depending on the amount of missing data and the presence of strong optical contrasts. Using simulated data it was found that patchy cloud cover occupying 25% of the image pair reduces the number of vectors by 20% compared to using perfect images. Root mean square errors between the MCC and HF radar velocities are of the order of 20 cm s−1. Performance varies depending on the wavelength of the data with the blue-green products out-performing the red and near infra-red products. Application of MCC to GOCI chlorophyll data results in similar performance to radiances in the blue-green bands. The technique has been demonstrated using specific examples of an eddy feature and tidal induced features in the region. This article is protected by copyright. All rights reserved