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
