Altimeter Data Research Articles

Calibration and verification of century based wave climate data record along the Turkish coasts using satellite altimeter data

In order to produce consistent reanalysis of the climate system, ECMWF (The European Centre for Medium-Range Weather Forecasts) has produced firstly an uncoupled atmospheric reanalysis ERA-20C, and then a coupled climate reanalysis, called CERA-20C, which covers the period January 1900 to December 2010. Both data sets are available at 3-hour time increments. Such a century long data can be an alternative to calculate the extreme waves corresponding to low probability of occurrences without extrapolation of extreme value statistics’ results which may contribute to the error in the estimation of design waves in case of small number of wave data. In this study, main purpose is to calibrate and verify the century-based wave data in order to derive the longest and the consistent wave data along the Turkish coasts as a first time to be used in the extreme wave analysis. For this purpose, first of all, significant wave height data of ERA-20C and CERA-20C are compared by using ENVISAT data over the whole Black Sea for 2007–2008 as a pilot study. Comparison results show that both datasets give similar results but CERA-20C seems to be better in terms of statistical error measures. Then CERA-20C significant wave height data are calibrated using satellite Radar Altimeter data set. Jason family of satellites (TOPEX, Jason-1 and 2) and Envisat family of satellites (ERS-2 and Envisat) are inter-calibrated to get the consistent satellite data sets with a total duration of 18 years (1995–2012) for Envisat family and 26 years (1992–2017) for Jason family in order to be used in calibration of CERA-20C wave height. The mean wave period is also estimated from RA backscatter coefficients (Ku and C bands) and the significant wave height by using Artificial Neural Network Method. Then the estimated mean wave periods are used for the calibration of CERA-20C wave period. Calibrated CERA-20C data are compared with in-situ measurements for the verification purposes. Results of verification study show that the calibrated CERA-20C wave data agree well with the in-situ measurements in terms of Quantile-Quantile analysis with lower deviations from y = x line and capture the largest sea states. In fact, CERA-20C, century-based wave data become appropriate to determine the extreme waves to be used in the design of coastal structures along the Turkish coasts.

Assessment of wave power in the South China Sea based on 26-year high-resolution hindcast data

Analyzing wave power is important for exploiting renewable energy in the South China Sea, particularly near reef islands. This study analyzes the distribution and evolution of wave power and its harnessing potential in the South China Sea based on 26 years of high-resolution hindcast data. A hindcast simulation of wave characteristics is conducted using the WAVEWATCH III model, which has a 3-hour temporal resolution and a high spatial resolution of 0.067°, and the results are used to evaluate the blocking effect of the island groups. The hindcast data are validated using altimeter data from nine satellites for the entire computing period. The mean values over 26 years, seasonal evolutions, and trends are evaluated to determine the wave power distribution, and variabilities in wave power near coastlines and offshore islands are studied at ten reference points. The results show that the South China Sea has considerable wave power and areas that are rich in power originate from the Luzon Strait. Wave energy near the offshore islands is more stable than that at locations near the Chinese mainland, which indicates that installing a wave farm near the islands would be an economically viable and environmentally friendly way to supply power.

Development of an Image De-Noising Method in Preparation for the Surface Water and Ocean Topography Satellite Mission

In the near future, the Surface Water Ocean Topography (SWOT) mission will provide images of altimetric data at kilometric resolution. This unprecedented 2-dimensional data structure will allow the estimation of geostrophy-related quantities that are essential for studying the ocean surface dynamics and for data assimilation uses. To estimate these quantities, i.e., to compute spatial derivatives of the Sea Surface Height (SSH) measurements, the uncorrelated, small-scale noise and errors expected to affect the SWOT data must be smoothed out while minimizing the loss of relevant, physical SSH information. This paper introduces a new technique for de-noising the future SWOT SSH images. The de-noising model is formulated as a regularized least-square problem with a Tikhonov regularization based on the first-, second-, and third-order derivatives of SSH. The method is implemented and compared to other, convolution-based filtering methods with boxcar and Gaussian kernels. This is performed using a large set of pseudo-SWOT data generated in the western Mediterranean Sea from a 1/60 ∘ simulation and the SWOT simulator. Based on root mean square error and spectral diagnostics, our de-noising method shows a better performance than the convolution-based methods. We find the optimal parametrization to be when only the second-order SSH derivative is penalized. This de-noising reduces the spatial scale resolved by SWOT by a factor of 2, and at 10 km wavelengths, the noise level is reduced by factors of 10 4 and 10 3 for summer and winter, respectively. This is encouraging for the processing of the future SWOT data.

Accuracy Evaluation of Geoid Heights in the National Control Points of South Korea Using High-Degree Geopotential Model

Precise geoid heights are not as important for understanding Earth’s gravity field, but they are important to geodesy itself, since the vertical datum is defined as geoid in a cm-level accuracy. Several high-degree geopotential models have been derived lately by using satellite tracking data such as those from Gravity Recovery and Climate Experiment (GRACE) and Gravity Field and Steady-State Ocean Circulation Explorer (GOCE), satellite altimeter data, and terrestrial and airborne gravity data. The Korean national geoid (KNGeoid) models of the National Geographic Information Institute (NGII) were developed using the latest global geopotential models (GGMs), which are combinations of gravity data from satellites and land gravity data. In this study, geoid heights calculated from the latest high-degree GGMs were used to evaluate the accuracy of the three GGMs (European Improved Gravity model of Earth by New techniques (EIGEN)-6C4, Earth Gravitational Model 2008 (EGM2008), and GOCE-EGM2008 combined model (GECO)) by comparing them with the geoid heights derived from the Global Navigation Satellite System (GNSS)/leveling of the 1182 unified control points (UCPs) that have been installed by NGII in South Korea since 2008. In addition, the geoid heights derived from the KNGeoid models were compared with the geoid heights derived from the GNSS/leveling of the 1182 UCPs to assess the accuracy of the KNGeoid models in terms of relative geoid heights for further gravimetric geoid determination studies in South Korea. As a result, the EGM2008 model could be selected as the suitable GGM from among the three GGMs for determining a gravimetric geoid model for South Korea.

Observed spatiotemporal variation of three-dimensional structure and heat/salt transport of anticyclonic mesoscale eddy in Northwest Pacific

As in-situ observations are sparse, targeted observations of a specific mesoscale eddy are rare. Therefore, it is difficult to study the three-dimensional structure of moving mesoscale eddies. From April to September 2014, an anticyclonic eddy located at 135°E–155°E, 26°N–42°N was observed using 17 rapid-sampling Argo floats, and the spatiotemporal variations in the three-dimensional structure were studied. The results are as follows: (1) the eddy was identified and tracked using satellite altimeter data. It had a lifetime of 269 days and an average radius of 91.5 km. The lifetime of the eddy can be divided into three phases, i.e., the initiation, maturity, and termination phases. The depth of its influence reached 1 000 m; (2) the Argo profiles were divided into seven periods (approximately 20 days in each) for composite analysis, and the composite Argo profiles and CARS2009 (CSIRO Atlas of Regional Seas) climatology data were merged following the data-interpolating variational analysis (DIVA) method to reconstruct the three-dimensional structure. The temperature and salinity anomaly cores of the anticyclonic mesoscale eddy are located from 500 to 600 m. From 800 to 900 m, there is an area of low salinity at the center of the eddy. A high concentration anomaly of dissolved oxygen was located at approximately 250 m; (3) to better understand the features of the eddy and its interaction with the surroundings, we calculated the anomalous velocity of the geostrophic flow and the heat, salt, dissolved oxygen transport anomaly, and discussed the eddy’s origin and its adjustments to topography. The maximum heat, salt, and oxygen transport caused by eddy were 9.37×1011 W, 3.08×103 kg/s, and 2.70×102 kg/s, which all occurred during the termination phase. This study highlights the applicability of using Argo floats to understand the three-dimensional structure thermohaline features of eddies in the North Pacific.

An extreme sea level event along the northwest coast of the South China sea in 2011–2012

An extreme sea-level (ESL) event in 2011–2012 and its associated mechanisms are investigated using long-term tide gauge records and satellite altimeter data. The composite sea level anomaly (SLA) along the northwest coast of the SCS is up to 85.2 mm in 2012, reaching the highest since 1975. The ESL event in 2011–2012 is a combined effect of the mean sea-level rise, La Niña event and negative Pacific Decadal Oscillation (PDO) phase. The three factors work on different time scales. The long-term rising rate is 3.1 mm/yr from 1975 to 2015, the linear estimate of the mean sea level is 45.1 mm in 2012, accounting for about 52.9% of the total ESL (85.2 mm). After removing the linear trend, La Niña has a 28.2% contribution of the highest SLA, the negative PDO phase has a 51.3% contribution, and they jointly give rise to the ESL event on interannual to decadal time scales. The dynamic sea-level variation is the dominant contributor, explaining 72.8% of the observed sea-level differences between 2010 and 2012. The sea level pressure contributes 34.4%, and the steric effect contributes 7.7% of the observed results.

Features of Slope Intrusion Mesoscale Eddies in the Northern South China Sea

AbstractSome mesoscale eddies intrude over the continental slope in the northern South China Sea, supporting cross‐shelf matter transport. We investigated the characteristics, the intruding tracks and formation mechanisms, of slope intrusion mesoscale eddies using satellite altimeter data and model outputs. In total, 36 and 22 slope intrusion anticyclonic and cyclonic eddies (SAEs/SCEs) are found, respectively. Slope intrusion eddies have longer lifetimes (~58 days), smaller size (~110 km), and greater eddy kinetic energy and vorticity compared to ordinary eddies but are more unstable and more easily deformed during their life cycles. The statistical results show that more slope intrusion eddies are generated during winter than other seasons. It is found that slope intrusion eddies mainly propagate westward/northwestward, and southwestward along the continental slope and shelf. Eddy intrusions occur mainly near the Dongsha Islands, east of Hainan, and north of the Xisha Islands. SAEs continue to propagate onshore after arrival at the continental slope, while SCEs dissipate more quickly. Using mooring data, we found that eddy‐ambient flow interaction could cause the differences between SAEs and SCEs around the Dongsha Islands. Energy conversion was analyzed in these three regions using numerical products. During intrusion, eddies lose eddy kinetic energy and ambient flows gain energy.

Large-scale hurricane modeling using domain decomposition parallelization and implicit scheme implemented in WAVEWATCH III wave model

WAVEWATCH III has been equipped with a new parallelization algorithm, domain decomposition and an optional implicit numerical scheme for coastal application at high spatial resolution with triangular unstructured grids, compatible with community-based coupling infrastructure. We performed a validation study for Hurricane Ike (2008) to prove the accuracy of the updated model against satellite altimeter data and buoy observations on various grids, forced by two sophisticated atmospheric models for hurricane simulation, using different solution schemes and parallelization algorithms. The new implementations for triangular grids are computationally efficient and scalable to be run on a large number of computational nodes, which constitutes a major breakthrough in the context of increasing needs for high-resolution nearshore wave modeling, making WAVEWATCH III (WW3) a powerful tool to simulate the sea state in the nearshore at high resolution and study wave-surge interactions in inner shelf regions.

Measuring the Earth's Synchrotron Emission From Radiation Belts With a Lunar Near Side Radio Array

AbstractThe high kinetic energy electrons that populate the Earth's radiation belts emit synchrotron emissions because of their interaction with the planetary magnetic field. A lunar near side array would be uniquely positioned to image this emission and provide a near real time measure of how the Earth's radiation belts are responding to the current solar input. The Salammbô code is a physical model of the dynamics of the three‐dimensional phase‐space electron densities in the radiation belts, allowing the prediction of 1‐keV to 100‐MeV electron distributions trapped in the belts. This information is put into a synchrotron emission simulator that provides the brightness distribution of the emission up to 1 MHz from a given observation point. Using Digital Elevation Models from Lunar Reconnaissance Orbiter Lunar Orbiter Laser Altimeter data, we select a set of locations near the Lunar sub‐Earth point with minimum elevation variation over various‐sized patches where we simulate radio receivers to create a synthetic aperture. We consider all realistic noise sources in the low‐frequency regime. We then use a custom Common Astronomy Software Applications code to image and process the data from our defined array, using SPICE to align the lunar coordinates with the Earth. We find that for a moderate lunar surface electron density of 250/cm , the radiation belts may be detected every 12–24 hr with a 16,384‐element array over a 100‐km‐diameter circle. Changing electron density can make measurements 10 times faster at lunar night and 10 times slower at lunar noon.

Global River Radar Altimetry Time Series (GRRATS): new river elevation earth science data records for the hydrologic community

Abstract. The capabilities of radar altimetry to measure inland water bodies are well established, and several river altimetry datasets are available. Here we produced a globally distributed dataset, the Global River Radar Altimeter Time Series (GRRATS), using Envisat and Ocean Surface Topography Mission (OSTM)/Jason-2 radar altimeter data spanning the time period 2002–2016. We developed a method that runs unsupervised, without requiring parameterization at the measurement location, dubbed virtual station (VS) level, and applied it to all altimeter crossings of ocean-draining rivers with widths >900 m (>34 % of the global drainage area). We evaluated every VS, either quantitatively for VS locations where in situ gages are available or qualitatively using a grade system. We processed nearly 1.5 million altimeter measurements from 1478 VSs. After quality control, the final product contained 810 403 measurements distributed over 932 VSs located on 39 rivers. Available in situ data allowed quantitative evaluation of 389 VSs on 12 rivers. The median standard deviation of river elevation error is 0.93 m, Nash–Sutcliffe efficiency is 0.75, and correlation coefficient is 0.9. GRRATS is a consistent, well-documented dataset with a user-friendly data visualization portal, freely available for use by the global scientific community. Data are available at https://doi.org/10.5067/PSGRA-SA2V1 (Coss et al., 2016).

Об определении параметров отражённого сигнала альтиметрии

The authors summarize the principle underlying the modern satellite altimetry. Careful analysis of the shape of the reflected signal enables estimating the flight altitude of the satellite altimeter above sea level, and other important parameters of the sea surface in the area under study quite reliably. Important in doing so is the reflected signal power model used. The Brown-Hayne model seems to be the most common one. The values of these parameters are determined from measurements using certain optimization methods. It is especially noted that the problem in question can be successfully solved by methods based on modern theory of artificial neural networks. Numerical experiments using real altimetric data were carried out in MATLAB environment. In this regard, the basic concepts of this theory are described and the possibilities of its use as an effective approximation of any dependence are emphasized. The Levenberg-Marquardt method and the genetic algorithm of artificial neural networks show the same results, but the latter does not require setting initial values of parameters, only limits of their possible change.

Wide-Swath Altimetric Satellite Data Assimilation With Correlated-Error Reduction

For decades now, satellite altimetric observations have been successfully integrated in numerical oceanographic models using data assimilation (DA). So far, sea surface height (SSH) data were provided by one-dimensional nadir altimeters. The next generation Surface Water and Ocean Topography (SWOT) satellite altimeter will provide two-dimensional wide-swath altimetric information with an unprecedented high resolution. This new type of SSH data is expected to strongly improve altimetric assimilation. However, the SWOT data is also expected to be affected by spatially correlated errors and, hence, can not be assimilated as easily as nadir altimeters. The present paper proposes to embed a state-of-the-art correlated-error reduction (CER) method for the SWOT data into an ensemble-based DA scheme. The DA with the new correlated-error reduced-data (CER-data) is implemented and tested in a simple SSH reconstruction problem using artificial SWOT data and a quasi-geostrophic model. The results show that, in an energetic large scale region, the DA with CER-data—in comparison to the classical DA—reduces the root-mean-square-error (RMSE) of the reconstruction in SSH by approximately 10%, in relative vorticity by 5% and in surface currents by 5–10%, and also slightly improves the noise-to-signal ratio and spectral coherence of the SSH signal at mesoscale (100–200 km) but with a small degradation on the large scales (>300 km). In a less energetic region, the DA with CER-data cuts down the RMSE in SSH by more than 50% on average therefore allowing a significantly more accurate reconstruction of SSH at mesoscale in terms of noise-to-signal ratio, spectral coherence, and power spectral density.

Spectral Analysis of Satellite Altimeter and Tide Gauge Data around the Northern Australian Coast

The north of Australia is known for its complex tidal system, where the highest astronomical tides (HATs) reach 12 m. This paper investigates the tidal behaviour in this region by developing spectral climatology for tide gauge and altimetry data. Power spectral density analysis is applied to detect the magnitude of ocean tides in 20 years of sea-level data from multimission satellite altimeters and tide gauges. The spectra of altimetry sea level anomaly (SLA) time series have their strongest peaks centred at approximately 2.11, 5.88, and 7.99 cycles per year (cpy), corresponding to the diurnal and semidiurnal tidal constituents K1, M2, and O1, respectively. Closer to the coastline, the spectra peak at high-frequency overtide and shallow-water constituents such as M4, MK4, and MK3. There have been many large, high-frequency spectral peaks near the coastline, indicating the difficulty of predicting tidal signals by coastal altimetry. Similar to altimetry observations, there are dominant semidiurnal and diurnal tidal peaks in tide gauge SLA time series accompanying a number of overtides. The semidiurnal and diurnal peaks are mostly higher on the northwest coast of Australia compared with the north and northeast coast. The results from both altimetry and tide gauges indicate that tidal range increases with increasing continental shelf.

Research on the Control Point Extraction Method of Chinese Gaofen 7 Satellite Using Altimeter and Footprint Camera

Satellite laser altimetry is one of the most advanced information acquisition technologies in Earth observation system. It can provide high-accuracy elevation information, however, due to the lack of detail intensity information, its planimetric accuracy is usually worse than the elevation accuracy. Gaofen 7 (GF-7) satellite scheduled for launch in 2019 will be equipped with laser altimeter, footprint camera, stereo mapping camera, etc. The laser altimeter together with the footprint camera was designed to provide high accuracy ground control point of satellite mapping. The laser altimeter can provide the high-accuracy elevation information and the joint processing of footprint camera and stereo mapping camera can provide high-accuracy planimetric information. Therefore, this paper mainly studies the technology of extracting high-accuracy control points based on GF-7 satellite’s altimeter, footprint camera and stereo mapping camera using a simulated dataset extracted from Quickbird image and ICESat altimetric data.

The second-order effect of Earth\u2019s rotation on Cryosat-2 fully focused SAR processing

We investigated the sensitivity of fully focused SAR (FF-SAR) processing of Cryosat-2 altimeter data to Earth rotation. Earth’s rotation causes scatterers at varying cross-track locations to have a different relative velocity with respect to the satellite. This second-order effect of Earth rotation on the phase is currently not corrected for in FF-SAR processing of altimetry data. The difference is largest near the poles, where the satellite flies parallel to the equator. Not correcting for the second-order effect yields a parabolic shape in the counter-rotated phase, which increases with the cross-track distance. Its effect is, however, limited by the time-in-view of the scatterer, which is shorter at the edge of the altimeter footprint, and therefore destructive interference will not occur when using Cryosat-2 data. For Cryosat-2, the only expected effect is a reduction in power and along-track resolution in the waveform tail and in the grating lobes. If the FF-SAR processor focuses on one point, and there is a bright scatterer at another, then there is a residual parabolic phase, whose sign and shape depend on the cross-track distance and whether the signal is left or right of the chosen focal point. In theory, if the viewed scene only has few bright coherent scatterers, then it might be possible to determine the cross-track position of each. In practice, however, natural targets are rarely coherent over the integration time.

An Adaptive Filter Based on Local Slope by Adjusting Elliptic Filter Kernel for ICESat-2 Photon Counting Laser Altimetric Data

The Ice, Cloud and Land Elevation Satellite-2 (ICESat-2) was launched on September 15th, 2018, which continues to perform measurement tasks as the successor to ICESat. Unlike full waveform technology of ICESat, ICESat-2 employs micropulse photon counting technology, which provides higher accuracy, but produces plenty of noise. This paper proposes an adaptive filter based on local slope by adjusting elliptic filter kernel. The general approach is 1) data preprocessing, 2) Gaussian density calculation, 3) OTSU adaptive threshold calculation. This method is seen to be robust in identifying signal points from high background noise points and suitable for low density data caused by slope.

Aspect Ratio of Eddies Inferred From Argo Floats and Satellite Altimeter Data in the Ocean

AbstractMesoscale eddies dominate the majority of oceanic kinetic energy. The size of eddies is one of the basic properties; however, little is known about it because of lack of observations. Especially, their global vertical extent has still not been well investigated. Here, a ratio relationship of height to length scale for an eddy is derived based on dynamical constraints. Global satellite altimetry data combined with Argo float observations are used to calculate the aspect ratios of mesoscale eddies in the global ocean, and then their vertical scale is inferred. Approximately 25% of the total identified eddies with lifetimes ≥16 weeks from January 2002 to December 2017 trapped at least an Argo float near the eddy center. The result shows that the size of eddies has an obvious latitudinal dependence. Eddies at middle‐low latitudes tend to be “large and shallow” with small aspect ratios, and eddies at high latitudes tend to be “small and deep– reaching” with large aspect ratios. Such results are coarsely considered as a competition between rotation and stratification. Eddy aspect ratios in middle‐high‐latitude regions show enhanced variance compared to those in low‐latitude regions. More than half of eddies have a vertical extent smaller than 400 m. The majority of eddies extend over the mixed layer depth and are instead situated in the pycnocline. Some eddies at the middle‐high latitudes penetrate through the entire pycnocline and reach deep ocean; as such, these eddies may play an important role in linking the deep‐sea circulations and the surface circulations.

Fluvial environmental disasters: risk perception and evaluation of government responses by riverine populations in Cacau Pirêra, Iranduba/AM

This article unveils the perceptions of vulnerable populations of the Amazon regarding the risks of river disasters and highlights their assessment of the effectiveness of government responses. The record of perceptions, assessments and identification of social vulnerabilities were obtained in interviews with focus groups formed by residents of Cacau Pirêra, in the Amazon. To characterize the environmental threats and vulnerabilities, fluviometric and altimetric data of the occupied areas were used. In that District, floods do not have to exceed normal limits to constitute environmental disasters and, in the perception of residents, pose more risks than ebb. Participants point to the low effectiveness of public policies as the main cause of the ineffectiveness of the government's response to water disasters. Natural disaster studies should consider the interactions between the environmental and social aspects of territories to reach an understanding of the complexity of the context in which they occur.

EVALUACIÓN DE LA AMENAZA POR TSUNAMI EN POBLACIONES DEL SUR, CENTRO Y NORTE DEL LITORAL PACÍFICO COLOMBIANO

La geomorfología marina en el Pacífico colombiano, se caracteriza por presentar una zona de convergencia de las placas de Nazca y Sudamérica, generando un proceso de subducción Nazca por debajo de la Sudamericana, la alta actividad sísmica en la zona de subducción Colombo-Ecuatoriana. Se han presentado sismos precursores de tsunami, como lo son, los eventos de 1906 y 1979, que afectaron poblaciones costeras del sur del Pacífico colombiano. En su parte norte se evidencia una alta actividad sísmica que podría generar eventos fuertes precursores de tsunami afectando a las poblaciones costeras. Se evaluó la amenaza por tsunami a lo largo del Litoral Pacífico colombiano, a partir del cálculo de la altura del tsunami en las poblaciones de Tumaco, Curay, Juanchaco, Buenaventura, Ciudad Mutis y Juradó. Se utilizó el método determinista, para lo cual se simularon escenarios de tsunami, empleando dos fuentes sísmicas una para el bloque sur y otra para el bloque norte. Se utilizaron dominios de cálculo, construidos con datos batimétricos y altimétricos de alta resolución espacial. El modelo para la simulación de los escenarios de tsunami fue el modelo TUNAMI N2. Como resultado, se obtuvo el tiempo de llegada de la primera ola de tsunami y la inundación máxima en cada población costera. Se concluye que el área más vulnerable y afectada en caso de un tsunami en el Pacífico colombiano es Tumaco.

Analysis of wave climate and trends in a semi-enclosed basin (Persian Gulf) using a validated SWAN model

Results from a spectral wave model of the Persian Gulf for a long-term period (32-year) are used to describe and quantify the seasonal wave climate and trends. The model is forced by a wind reanalysis dataset and validated against a set of field measurements and altimeter data from various satellite missions. The results revealed the existence of a more energetic wave (higher Hm0 and larger TP) zone located at the southern regions of the Gulf that is linked to the predominant and stronger north-westerly winds. The wave heights and periods are intensified in winter (Hm0>2.7m and TP > 8s for the 99th percentile) followed by spring. The waves conditions are milder in autumn. The analysis of wave climate trends throughout the region clearly show an overall slight positive trend (0–0.2%/year) in mean Hm0, which is associated with long-term positive trends in wind speed. There is generally a positive trend in the mean significant wave height during winter (0–0.8%/year), spring (0–0.6%/year) and autumn (0–1.0%/year), and negative during summer (0–0.8%/year). The results indicate that the more statistically significant results are at the areas with larger magnitude of trends.