Satellite altimetry has proven to be a useful tool to study oceanic processes in the deep ocean; however, its use is still limited in shallow waters near the coast where two main issues still need a more detailed analysis. On one side, the local characteristics of each coastal region imply that certain corrections applied to the altimetry measurements need to be reanalysed. On the other side, the radar signal retracking algorithms need to be improved because the waveforms do not follow the Brown's model, which is designed for deep waters. The ESA mission Envisat was launched in March 2002 with a dual-frequency radar altimetry (RA-2). The satellite was operative until the end of the mission in May 2012. The ESA mission Cryosat-2 was launched in April 2010 being still in operation. The radar instrument on-board Cryosat-2 improves the capabilities of previous pulse-limited altimeters, such as Envisat RA-2. The Spanish-funded ALCOVA project aims at analyzing and improving the altimetry measurements obtained from these two altimetry missions. Regarding the RA-2 data a new prototype retracker -ALES- has been developed under the frame of the ESA-DUE eSurge project. Two pilot regions are proposed, namely, the Gulf of Cadiz and the Strait of Gibraltar in the Southwestern Iberian Peninsula. Cryosat-2 data (in SAR mode), the newly corrected RA-2 data (based on ALES) and the standard RA- 2 product (based on Brown's model) are being validated with available in-situ data (sea level height) to ensure their correct performance in the selected coastal areas.
Sea level anomaly maps from altimeter (AVISO) were retrieved for the Gulf of Cadiz (GoC) for the period 1997-2008, along with maps of Dynamic Atmospheric Correction (DAC), atmospheric pressure at sea level and satellite Sea Surface Temperature (SST). Data were averaged in time to obtain maps of monthly mean time series in order to analyze the seasonal variability of sea level and its main forcing agents along the GoC. Moreover, a very high resolution climatology for the region was combined with the SST maps to explore the steric contribution with enough spatial resolution near the coast. The AVISO sea level anomaly monthly maps were initially de-corrected using the DAC product and then corrected using the inverted barometer method. Atmospheric pressure explained more than 55% of the sea level variance offshore and between 35-45% within the continental shelf. The amplitude of the pressureadjusted sea level semiannual signal was considerably reduced, confirming its meteorological origin. The steric contribution on the pressure-adjusted sea level was addressed by considering local, open ocean, basin-wide and continental shelf steric effects. The open ocean contribution explained the highest percentage of variance all over the basin with the exception of the western shelf, where the best results were obtained with the local contribution. After correcting for the best steric contribution, the amplitude of the remaining offshore annual signal was negligible (0.5-1.0 ± 1 cm). As for the continental shelves, 2- 3 cm (± 0.5-1 cm) of the annual signal remained unexplained, probably due to local effects related with the shelves dynamics.
Time series (1997-2008) of near-shore altimetry data and in-situ tide gauge records have been analyzed to investigate the
seasonal variability of sea level along the Gulf of Cadiz. A high level of agreement was obtained between altimeter and
in-situ observations, indicating that altimeter data can be a valuable tool to study the sea level seasonal cycle near the
coast. Harmonic analysis showed that more than 95% of the average seasonal cycle is explained by the annual and, to a
lesser extent, semiannual components. The average seasonal cycle of sea level anomalies is very similar at the four
coastal stations, with minimum values during winter and maximum during autumn. Atmospheric pressure accounts for
20-38% of the sea level variability, its effect diminishing toward the Strait of Gibraltar. The steric contribution is notable
at the westernmost stations (32-37%) and it also decreases eastward (9-17%). River discharge explains about 15% of the
sea level variability, indicating that its effects should be taken into account. The contribution of direct atmospheric
forcing for a section of the sea level time series (1997-2001) has also been explored using the output of a barotropic
oceanographic model (HIPOCAS project) forced with wind and atmospheric pressure, revealing that the contribution of
wind is generally small (6-12%) at seasonal time scales. Small but significant correlations are found between the time
series of winter-autumn mean sea level and the winter-autumn North Atlantic Oscillation (NAO) indices. Analysis show
that the effect of NAO is mainly reflected on atmospheric pressure, wind and river runoff.
KEYWORDS: Data modeling, Coastal modeling, Atmospheric modeling, Satellites, 3D modeling, Microwave radiation, Radiometry, Climatology, Data processing, Global Positioning System
Satellite altimetry has proved successful as a global tool for monitoring sea surface height, significant wave height and
wind speed. Nevertheless, a global archive of 17 years of raw data from a series of missions is presently unexploited
around the world coastline. This huge amount of unused data can be re-analyzed, improved and more intelligently
exploited, possibly promoting coastal altimetry to the rank of operational service. Operational users interested in
monitoring sea level change and wave conditions in the coastal zone (e.g. for coastal erosion, sediment/pollutant
transport applications) still rely on sparse (and expensive) in situ monitoring stations or poor models. In this work we
present a new approach in the exploitation of altimeter data in the coastal zone (currently impeded by unsuitable
waveform retracking scheme and coarse along-track spatial sampling in the coastal zone, among others). The objective
of this paper is to show how a new, robust, retracking algorithm is able to retrieve with high accuracy physical ocean
parameters from altimeter waveforms in the coastal zone. The main focus lies on retrieving sea surface height in the
coastal zone with the same precision as is achieved in the open ocean. In addition, the retrieval of more accurate
altimeter-derived wave products in the coastal zone is also important as waves are more directly relevant to many
operational applications in the coastal zone.
Fifteen years of global altimetry data over the coastal ocean lie, largely unexploited, in the data archives, simply because
intrinsic difficulties in the corrections and issues of land contamination in the footprint. These data would be invaluable
for studies of coastal circulation, sea level change and impact on the coastline. Amongst some initiatives, we describe
here the COASTALT Project, funded by ESA. The main objective of the COASTALT Project is to contribute towards
making the status of pulse-limited coastal altimetry operational. In this paper we will first illustrate the first project
phase, based on the assessment of user requirements, and summarize those requirements. Then we will describe the
COASTALT methodology and objectives. Finally, we will illustrate and discuss the various options for coastal
waveform retracking, and present a plan for the validation of the retracked data. The first results in the radar altimeter
waveform analysis show the complexity of the coastal signals due to land contamination and calm/rough waters.
The surface extent of a lake reflects its water storage variations. This information has important hydrological and
operational applications. However, there is a lack of information regarding this subject because the traditional
methodologies for this purposes (ground surveys, aerial photos) requires high resources investments. Remote sensing
techniques (optical/radar sensors) permit a low cost, constant and accurate monitoring of this parameter. The objective of
this study was to determine the surface variations of Lake Izabal, the largest one in Guatemala. The lake is located close
to the Caribbean Sea coastline. The climate in the region is predominantly cloudy and rainy, being the Synthetic
Aperture Radar (SAR) the best suited sensor for this purpose. Although several studies have successfully used SAR
products in detecting land-water boundaries, all of them highlighted some sensor limitations. These limitations are
mainly caused by roughened water surfaces caused by strong winds which are frequent in Lake Izabal. The ESA's
ASAR data products were used. From the set of 9 ASAR images used, all of them have wind-roughened ashore waters in
several levels. Here, a chain of image processing steps were applied in order to extract a reliable shoreline. The shoreline
detection is the key task for the surface estimation. After the shoreline extraction, the inundated area of the lake was
estimated. In-situ lake level measurements were used for validation. The results showed good agreement between the
inundated areas estimations and the lake level gauges.
It is well known that ocean-atmosphere dynamic affects the weather conditions over the continents and the ocean itself.
The hydrologic cycle is driven by climatic parameters like precipitation, temperature, evaporation, winds and humidity.
Hence, the river's water discharges and lake water level variations are impelled by climatic conditions also. Lake Izabal
is the largest one in Guatemala; its main tributary is the Polochic River. Its level is related to the Polochic Rivers runoff
and therefore to the precipitation/evaporation over its catchment area. The Lake Izabal water level fluctuations are driven
by the annual cycle of rainy and dry seasons. In this study the ENVISAT RA-2 Geophysical Data Records orbits over the
lake, coupled with in-situ measurements are used in order to determine and characterize the lake level fluctuations. The
precipitation records over the lake's catchment area are also analyzed. In addition, some relationships of the lake level
interannual variations with the climate indexes of Southern Oscillation Index SOI and the Tropical North Atlantic NATL
were investigated. The main result is that the abrupt lake level rise in July 2006 is correlated to an abnormal precipitation
in June 2006. Theoretically, this was forced by "La Nina" Southern Oscillation events during early 2006.
The Cramer-Rao lower bound (CRLB) for the estimations of the cosine and sine amplitudes of multi-tone
sinusoidal model is derived and applied on TOPEX/Poseidon satellite altimetry data sets covering the Indian Ocean. The
CRLB depends on the variance of the White Gaussian Noise that it is computed by Modern Parametric Autoregressive
Adaptative Spectral Analysis. Determining CRLB parameters it is possible to establish the minimal error associated to
any model built to work in the study area what improves the intrinsic bias of the generated time series. The noise that
appears in the altimetric data depends strongly on the errors from the atmospheric and geophysical corrections, so the
role of the inverted barometer and tidal corrections are also analyzed. The results is summarized as: a) the spatial
structure of the order of the parametric model considering the application or not of the above corrections and its
relationship to the surface dynamical system of currents in Indian Ocean; b) the spatial structure of the variance of the
WGN in the area and its meaning; c) the CRLB for the Geoid's estimators and the CRLB for the estimators of the semiannual
and annual waves.
An analysis of Synthetic Aperture Radar (SAR) images along with the analysis of velocity, salinity and temperature records taken at Camarinal Sill, the main sill of the Strait of Gibraltar, is intended to explain how the different hydrodynamic variables determine the wavelength of the internal waves generated in this location. The generation and subsequent propagation, toward the Mediterranean, of these waves produces an intense surface signature (surface slicks), which can be seen clearly in SAR images. The analysis procedure includes the application of wavelets analysis to determine the wavelength from the SAR images and an analytical model to compute the theoretical wavelengths as function of the hydrodynamic variables over the sill. Those results will be compared and discussed in this work.
KEYWORDS: Oceanography, Backscatter, Radar, Interference (communication), System on a chip, Satellites, Error analysis, Thermal modeling, Signal generators, Statistical modeling
Present paper summarizes the capabilities of the ocean retracking model developed at the Southampton Oceanography Centre to retrieve geophysical parameters (mainly, significant wave height, backscatter power and range), using the RA-2 Envisat 18-Hz averaged waveforms. This model is based on the use of the Maximum Likelihood Estimation. One of the most important characteristics of the model is that non-linear parameters can be added, in order to derive more information from the ocean waveforms. First results show that the linear model is able to retrieve accurate geophysical parameters from RA2, but the use of the non-linear model has to be considered with care. This work is part of the RAIES project, for the exploitation of the Envisat radar altimeter individual echoes and S-band data for ocean, coastal zone, land and ice/sea-ice altimetry.
The real sea level response to atmospheric pressure variations is quantified and compared with the one deduced from the isostatic assumption, which is normally used to correct the effect of the atmospheric pressure oscillations in the ocean, the so-called Inverse Barometer Correction (IBC). We have analysed the first four years of ERS-2 altimetric records in the Mediterranean Sea. We obtained the regression coefficients at each geographical point of the satellite tracks and in the crossover track points, estimating the relation between the surface atmospheric pressure and the sea level anomalies.
The geographical distribution of the regression coefficients obtained, demonstrates that there are important local deviations from the hypothetical value (-0.998 cm/bar), being more evident when we reduce the time sampling from 35 days to 10.5 days. We have obtained the variance associated to the inverse barometer correction, and the one obtained by using the individual regression coefficients obtained at each geographical position in both, collinear and crossover
method. We observe a variance reduction of approximately 30% in the sea level anomalies series, when barometric corrections are applied, in the case of collinear track method, and around 15% when we use the crossover track method. We have also quantified the difference in the variance reduction of the sea level anomalies when the standard IBC and the atmospheric pressure correction (use of the regression coefficients estimated on each geographical position, instead of
the isostatic value: -0.998 cm/mbar) are applied. We also observe a reduction of approximately 2% in the variance when the atmospheric correction is applied, instead of IBC (in both cases, CM and XM).
KEYWORDS: Atmospheric modeling, Data modeling, Solids, Digital signal processing, Atmospheric corrections, Satellites, Atmospheric physics, Applied physics, Picosecond phenomena, Linear filtering
We have used the first three years of ERS-2 altimeter data (May 1995 - June 1998) in order to analyse the space-temporal ocean response to atmospheric pressure variations in the three main ocean world basins: Atlantic, Indian and Pacific. We have also quantified the magnitude of the departures of this response from the hypothetical barometric factor (-0.995 cm/mbar), commonly applied to altimeter data records to eliminate the effect of the atmospheric pressure variations in the ocean (Inverse Barometer Correction). From the results obtained, we have found a different behaviour in the Atlantic Ocean with respect to the other two basins, as far as the magnitude of the barometric factor is concerned. Considering that we have estimated the meridional response to atmospheric pressure variations by applying the collinear track and the crossover track methods, the Atlantic Ocean response is quite similar to the one deduced from the isostatic assumption at all latitudinal bands. Nonetheless, Indian and Pacific Oceans show important departures from the hypothetical value at low latitudes. In order to understand why the Atlantic Ocean response is different from the one obtained in the others two, we can infer some explanations but it seems that the different climatology in the basins could be explaining the results obtained, especially the effect of winds.
KEYWORDS: Data modeling, Astatine, Satellites, Atmospheric modeling, Error analysis, Radar, Data processing, Factor analysis, Signal to noise ratio, Atmospheric corrections
The ocean response to pressure variations is subtracted from altimeter records using the standard Inverse Barometer Correction (IBC), based on the hypothetical isostatic assumption. Previous analyses have demonstrated that this assumption has to be applied with care when the high frequency pressure variations are considered, as is the case of using the crossover track method. Using ERS-2 radar altimeter data, we study the response of the Atlantic Sea Level (ASL) to pressure forcings at different ranges of frequency, in order to determine the validity of the isostatic assumption. We have also determined this response when using the outputs of a Global Ocean Model (GOM) forced by pressure and wind fields. From the comparison between both results we have observed that data errors could be underestimating our estimations of the response of the ocean to pressure variations; this underestimation could represent more than 20 percent of the values obtained in equatorial and tropical zones, being insignificant out of the latitudinal band.
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