Analysis and Inter-Calibration of Wet Path Delay Datasets to Compute the Wet Tropospheric Correction for CryoSat-2 over Ocean

Unlike most altimetric missions, CryoSat-2 is not equipped with an onboard microwave radiometer (MWR) to provide wet tropospheric correction (WTC) to radar altimeter measurements, thus, relying on a model-based one provided by the European Center for Medium-range Weather Forecasts (ECMWF). In the ambit of ESA funded project CP4O, an improved WTC for CryoSat-2 data over ocean is under development, based on a data combination algorithm (DComb) through objective analysis of WTC values derived from all existing global-scale data types. The scope of this study is the analysis and inter-calibration of the large dataset of total column water vapor (TCWV) products from scanning MWR aboard Remote Sensing (RS) missions for use in the WTC computation for CryoSat-2. The main issues regarding the computation of the WTC from all TCWV products are discussed. The analysis of the orbital parameters of CryoSat-2 and all other considered RS missions, their sensor characteristics and inter-calibration is presented, providing an insight into the expected impact of these datasets on the WTC estimation. The most suitable approach for calculating the WTC from TCWV is investigated. For this type of application, after calibration with respect to an appropriate reference, two approaches were found to give very similar results, with root mean square differences of 2 mm

Validation of Sentinel-3a Sral Coastal Sea Level Data at High Posting Rate: 80Hz

Altimetry data of two and a half years (June 2016-November 2018) of Sentinel 3A SRAL were validated at the sampling frequency of 80 Hz. The study areas are three coastal sites in Spain: Huelva (HU) (Gulf of Cadiz), Barcelona (BA), and Bilbao (BI). Two tracks were selected in each site: one ascending and one descending. Data were validated using in situ tide gauge (TG) data provided by the Spanish Puertos del Estado. In the 5 to 20 km segment, the results were 6-8 cm (rmse) and 0.7-0.8 (r) for all the tracks. The 0 to 5 km segment was also analyzed in detail to study the land effect on the altimetry data quality. The results showed that the track orientation, the angle of intersection with the coast, and the land topography concur to determine the nearest distance to the coast at which the data retain a similar level of accuracy than in the 5 to 20 km segment. This distance of good quality to shore reaches a minimum of 3 km for the tracks at HU and the descending track at BA

The gulf of cadiz as a natural laboratory for the validation and exploitation of coastal altimetry and model data

Hydrodynamic models and satellite altimetry can be complementary tools for the study of sea level variations. Nowadays, users of these tools demand high quality products in coastal zones. In this sense, this doctoral dissertation focusses on the validation of innovative products that entail an advance in the understanding of sea level variation in coastal areas. The study was carried out in the Gulf of Cadiz (GoC), an important region that connects the Atlantic Ocean and the Mediterranean Sea, although other study areas were also used to strengthen the analysis. The research was performed using: in-situ data, sea level altimetry measurements from Sentinel-3 SRAL, and observations from a hydrodynamic model implemented in the main study area. The in-situ data were used to validate the altimetry measurements, as well as to calibrate and validate the numerical model. Different specific objectives were proposed. The first was to determine the quality of altimetric data in coastal areas from the new Sentinel-3 space mission. Altimetry data of Sentinel-3A SRAL (S3A) were validated at the sampling frequency of 80 Hz. The data were obtained from the European Space Agency (ESA) Grid Processing On Demand (GPOD) service over three coastal sites in Spain: Huelva (GoC), Barcelona (Western Mediterranean Sea), and Bilbao (Bay of Biscay). Two tracks were selected at each site: one ascending and one descending. Data were validated using in-situ tide gauge (TG) data provided by the Spanish Puertos del Estado. The altimetry Sea Level Anomaly (SLA) time series were obtained using the corrections available in GPOD. The validation was performed using two statistical parameters, the Pearson correlation coefficient (r) and the root mean square error (rmse). In the 5–20 km segment with respect to the coastline, the results obtained were 6–8 cm (rmse) and 0.7–0.8 (r) for all of the tracks. The 0–5 km segment was also analysed in detail to study the effect of land on the quality of altimetry data. Results showed that the track orientation, the angle of intersection with the coast, and the land topography concur to determine the nearest distance to the coast at which the data retain a similar level of accuracy than in the 5–20 km segment. This ‗distance of good quality‘ to shore reaches a minimum of 3 km for the tracks at Huelva and the descending track at Barcelona. In addition, altimetry sea level data of Sentinel-3A and Sentinel-3B SRAL (S3A and S3B) were also validated at the sampling frequency of 80 Hz. Two tracks of S3A and two of S3B were selected at seven different coasts around the Iberian Peninsula. The altimetry SLA time series obtained were compared with TG in-situ data using the standard deviation of the difference (sdd) and the normalized sdd (sdd_n). Two tidal models were used: TPXO8 and TPXO9. They were previously validated with in-situ data and then used in the S3 assessment. Contrary to expectations, a more current version of the tide model did not always lead to improvements for all of the coasts studied. The same data availability and accuracy results (mean sdd <9cm) were obtained for both satellites, as the radar altimeter on-board S3A and S3B are identical instruments. The sdd_n results were generally ranged between 0.1 and 0.25 cm, higher values were obtained in coastal areas with a complex hydrodynamic. The second specific objective was to implement the Delft3D model in the estuary of the Guadalquivir River and part of the GoC continental shelf with the aim of studying the influence of its discharges on the sea level variability. Details of the Delft3D FLOW module implementation are given in the manuscript. The model was calibrated and validated along the river estuary comparing the output with in-situ observations of water level and current velocities during normal and high-discharge events. In order to obtain the best possible adjustment, the friction coefficient and bathymetry were used as adjustment parameters. The determination coefficients attained mean values of R2= 0.9 and R2=0.8, for calibration and validation, respectively. Moreover, the model was calibrated for the continental shelf during normal discharge conditions using data from three current meters, obtaining good correlation results (rmse= 3.0 cm·s -1 and R2=0.7, on average). The model simulations were able to reproduce the low salinity plume-induced over-elevations at the mouth of the estuary and its adjacent inner shelf during periods of high river discharge from the head dam (> 400 m3 ·s -1 ). These over-elevations were also identified in a qualitative comparison with altimetry data. Despite the good results obtained, there are some improvements that could be made for future work: including wind, coupling the wave module, updating the bathymetry, etc. The aim of this thesis last section was to apply the new Fully Focused SAR (FF SAR) processing technique for the Sentinel-3 altimetry signal. The accuracy and precision of this novel product were analysed in order to provide the best quality product close to the coast (0-5 km track segment). FF SAR processing is similar to SAR altimetry, but with an unprecedented high along-track resolution which goes up to the theoretical limit equal to half the antenna length (~0.5 m). Two FF SAR algorithms still in development were used in this work: FF SAR Back Projection (BP) (S3 prototype version of Kleinherenbrink et al., 2020), and FF SAR Omega-Kappa (WK) (Guccione et al., 2018), as well as different retracking algorithms. Two tracks from Sentinel-3A and two from Sentinel-3B were processed, at 80 Hz. The products were validated by comparing time series of SLA with those obtained from a tide gauge in the Gulf of Cadiz. The accuracy was analysed using the Percentage of Cycles for High Correlation (PCHC) and the standard deviation of the difference (sdd); and the precision was determined by calculating the along-track noise. FF SAR and unfocused SAR products were compared. The results showed improvements using Adaptative Leading Edge Subwaveform (ALES+) retracker for unfocused SAR, although FF SAR BP with ALES+ was the most precise product for all the tracks. In addition, highly accurate SLA measurements were also obtained with FF SAR products. The greatest advantage of FF SAR is that it produces good quality data closer to the coast (1-2 km) than unfocused SAR (3-4 km). Finally, these results highlight the potential of the implementation of validated altimetry data and hydrodynamic models in sea level studies. Furthermore, the methodology described here can be useful to validate altimetry data, as well as to implement the Delft3D model in other coastal areas

Tendance et variabilité de la vapeur d'eau atmosphérique : un enjeu pour l'étude du niveau moyen océanique

La mesure du niveau de la mer par altimétrie satellitaire est perturbée par la présence de vapeur d'eau dans l'atmosphère. Un radiomètre micro-onde, sur les missions altimétriques, est chargé de corriger les mesures de ces perturbations. Les exigences quant à la qualité de cette correction, appelée correction troposphérique humide, sont particulièrement fortes pour l'étude des changements climatiques. Cette thèse a pour objet l'étude des corrections troposphériques humides utilisées dans le cadre des missions altimétriques Jason-1 et Envisat. L'objectif est de caractériser les incertitudes liées à la correction et d'identifier les potentielles anomalies présentes. L'étude faire ressortir une potentielle dérive dans l'étalonnage du radiomètre de la mission Jason-1 après 2008. Pour la mission Envisat, l'analyse met en avant des biais régionaux à l'approche des côtes. Ces derniers sont probablement liés au traitement de la donnée radiométrique.Measurements of the sea surface height are disturbed by the presence of water vapor in the atmosphere. A microwave radiometer, on altimetric missions, is used to correct the measurements from theses disturbances. Requirements on the quality of this correction, called the wet tropospheric correction, are stringent for the survey of climate changes. This thesis concerns the monitoring of the wet tropospheric correction used in the altimetry missions, Jason-1 and Envisat. The aim is to characterize uncertainties related to this correction and to identify potential anomalies. The analysis brings out a potential drift in the radiometer used on Jason-1, after 2008. For the Envisat missions, the presence of biases near coastlines suggests processing related issues