Global Tide Models Research Articles

Assessment of Low-Cost Tide Gauges to Meet GLOSS 1-cm Precision and Accuracy Standards: A Case Study on Pramuka Island, Indonesia

The expansion of the tide gauge network along the coasts is essential for better monitoring of sea-level dynamics. Owing to climate change, the urgency has been exacerbated, especially during the last two decades. However, densification a challenging task because of the lack of affordability of the sensor, especially in the Global South. Further, the precision and accuracy requirements of 1-cm imposed by the Global Sea Level Observing System (GLOSS) is too restrictive, particularly for low-cost tide gauge sensors. Here, we evaluated the performance of a low-cost DIY tide gauge in meeting these standards. Three sets of sea level observations from IR-TIDES, a DIY tide gauge sensor observed in 2016 and 2018, were subjected to a performance test in terms of precision and accuracy in comparison with a global tide model and two neighbouring established tide gauges. All three datasets were estimated to have an 8- cm standard deviation as a metric for the precision level. In terms of accuracy, the IR-TIDES datasets had a standard deviation of 25 cm and a correlation coefficient of 0.616. Overall, IR-TIDES demonstrated sufficient precision while still lacking accuracy, partially meeting the GLOSS quality standard. These findings could strengthen the confidence level of a low-cost DIY tide gauge, especially for use as a back-up and redundant sensor for an established tide gauge station after ad-dressing the limitations.

Global application of a regional frequency analysis to extreme sea levels

Abstract. Coastal regions face increasing threats from rising sea levels and extreme weather events, highlighting the urgent need for accurate assessments of coastal flood risk. This study presents a novel approach to estimating global extreme sea level (ESL) exceedance probabilities using a regional frequency analysis (RFA) approach. The research combines observed and modelled hindcast data to produce a high-resolution (∼1 km) dataset of ESL exceedance probabilities, including wave setup, along the entire global coastline (excluding Antarctica). The methodology presented in this paper is an extension of the regional framework of Sweet et al. (2022), with innovations introduced to incorporate wave setup and apply the method globally. Water level records from tide gauges and a global reanalysis of tide and surge levels are integrated with a global ocean wave reanalysis. Subsequently, these data are regionalised, normalised, and aggregated and then fit with a generalised Pareto distribution. The regional distributions are downscaled to the local scale using the tidal range at every location along the global coastline obtained from a global tide model. The results show 8 cm of positive bias at the 1-in-10-year return level when compared to individual tide gauges. The RFA approach offers several advantages over traditional methods, particularly in regions with limited observational data. It overcomes the challenge of short and incomplete observational records by substituting long historical records with a collection of shorter but spatially distributed records. These spatially distributed data not only retain the volume of information but also address the issue of sparse tide gauge coverage in less populated areas and developing nations. The RFA process is illustrated using Cyclone Yasi (2011) as a case study, demonstrating how the approach can improve the characterisation of ESLs in regions prone to tropical cyclone activity. In conclusion, this study provides a valuable resource for quantifying the global coastal flood risk, offering an innovative global methodology that can contribute to preparing for – and mitigating against – coastal flooding.

Tidal modeling based on satellite altimetry observations of TOPEX/Poseidon, Jason1, Jason2, and Jason3 with high prediction capability: A case study of the Baltic Sea

This research aims to optimize the utilization of long-term sea level data from the TOPEX/Poseidon, Jason1, Jason2, and Jason3 altimetry missions for tidal modeling. We generate a time series of along-track observations and apply a developed method to produce tidal models with specific tidal constituents for each location. Our tidal modeling methodology follows an iterative process: partitioning sea surface height (SSH) observations into analysis/training and prediction/validation parts and ultimately identifying the set of tidal constituents that provide the best predictions at each time series location. The study focuses on developing 1256 time series along the altimetry tracks over the Baltic Sea, each with its own set of tidal constituents. Verification of the developed tidal models against the SSH observations within the prediction/validation part reveals mean absolute error (MAE) values ranging from 0.0334 m to 0.1349 m, with an average MAE of 0.089 m. The same validation process is conducted on the FES2014 and EOT20 global tidal models, demonstrating that our tidal model, referred to as BT23 (short for Baltic Tide 2023), outperforms both models with an average MAE improvement of 0.0417 m and 0.0346 m, respectively. In addition to providing details on the development of the time series and the tidal modeling procedure, we offer the 1256 along-track time series and their associated tidal models as supplementary materials. We encourage the satellite altimetry community to utilize these resources for further research and applications.

Internal solitary waves in the central Andaman sea observed by combining mooring data and satellite remote sensing

The behavioral characteristics of internal solitary waves (ISWs) in the central Andaman Sea were revealed by analyzing multisource satellite remote sensing images combined with synchronous mooring data. The relationship between the generation time and tides of ISWs in the central Andaman Sea was analyzed and discussed. The ISWs in the central Andaman Sea exhibit a notable semidiurnal tidal cycle, primarily occurring during the spring tide. Additionally, it was observed that larger currents can generate ISWs with larger amplitudes. The propagation velocity of the ISWs initially increases and then decreases from the source, with no significant overall change in the deep areas. Based on the analysis of satellite remote sensing images that cover the source of ISWs in the central Andaman Sea, the generation time of ISWs was estimated. A comparison was made between the estimated generation time and the tidal flow predicted from the TPXO 9 global tidal model at the location of the ISW source. It was found that ISWs are generated when the westward tidal flow reaches its maximum. The ISWs in the central Andaman Sea are generated between the islands of Car Nicobar and Teresa Island. They propagate eastward into the Andaman Sea, grow in amplitude, split into multiple waves, dissipate, and vanish in the waters near the Danao Islands. The entire life cycle of these waves lasts for approximately 3∼4 days.

On the selection of multi-satellite altimetry for empirical tide modelling over Indonesia

ABSTRACT Multi-satellite altimetry has been employed in tidal modelling as its accuracy has been proven acceptable in constructing good-quality models. However, the modelling process often fails to consider the varying temporal resolution and observation periods among satellites, which can affect the accuracy of the model due to aliasing and Rayleigh effects. In Indonesia, we investigate and propose a satellite selection procedure for tidal modelling utilizing the response method, validated with along-track data obtained from both older and newer generations of ERM (Exact Repeat Mission)-type altimetry satellites, including the TOPEX/Jason series (TOPEX, Jason-1, Jason-2, Jason-3, and SENTINEL-6A), ENVISAT, ERS-2, GFO, SENTINEL-3B, and SENTINEL3A. The most optimal satellite combination identified is the TOPEX/Jason series, GFO, and ENVISAT, as they contribute significantly less error compared to ERS-2, SENTINEL-3A, and SENTINEL-3B, with errors exceeding 50%, primarily due to aliasing and Rayleigh effects. This combination of three satellites with tide gauge stations achieves higher precision (11.31 cm) than the current global tidal model. This study provides empirical evidence that the accuracy of tidal models is not solely dependent on the number of altimetry satellites utilized, but implementing effective satellite selection procedures is also crucial to enhance the precision of a model.

Detecting Shoreline Changes on the Beaches of Hainan Island (China) for the Period 2013–2023 Using Multi-Source Data

This study presents an in-depth analysis of the dynamic beach landscapes of Hainan Island, which is located at the southernmost tip of China. Home to over a hundred natural and predominantly sandy beaches, Hainan Island confronts significant challenges posed by frequent marine natural disasters and human activities. Addressing the urgent need for long-term studies of beach dynamics, this research involved the use of CoastSat to extract and analyze shoreline data from 20 representative beaches and calculate the slopes of 119 sandy beaches around the island for the period from 2013 to 2023. The objective was to delineate the patterns of beach evolution that contribute to the prevention of sediment loss, the mitigation of coastal hazards, and the promotion of sustainable coastal zone management. By employing multi-source remote sensing imagery and the CoastSat tool, this investigation validated slope measurements across selected beaches, demonstrating consistency between the calculated and actual distances despite minor anomalies. The effective use of the finite element solution (FES) in the 2014 global tidal model for tidal corrections further aligned the coastlines with the mean shoreline, underscoring CoastSat’s utility in enabling precise coastal studies. The analysis revealed significant seasonal variations in shoreline positions, with approximately half of the monitored sites showing a seaward progression in summer and a retreat in winter, which were linked to variations in wave height. The southern beaches exhibited distinct seasonal variations, which contrasted with the general trend due to differing wave impacts. The western and southern shores showed erosion, while the northern and eastern shores displayed accretion. The calculated slopes across the island indicated that the southern beaches had steeper slopes, while the northern areas exhibited more pronounced slope variations due to wave and tidal impacts. These findings highlight the critical role of integrated coastal management and erosion control strategies in safeguarding Hainan Island’s beaches. By understanding the mechanisms driving seasonal and regional shoreline changes, effective measures can be developed to mitigate the impacts of erosion and enhance the resilience of coastal ecosystems amidst changing environmental conditions. This research provides a foundational basis for future efforts aimed at the sustainable development and utilization of coastal resources on Hainan Island.

Numerical simulation study on hydrodynamics of Xisha Islands and Reefs driven by tide levels, sea surface heights and ocean currents

This study employs the FVCOM numerical model to investigate tide flow and hydrodynamics in the marine region surrounding Yongxing Island. The HYCOM model is utilized to obtain sea surface height and ocean current data at the boundary. Additionally, the astronomical tide level predicted by the TPXO9 global tide model is incorporated into the analysis. Through numerical simulation research, the paper aims to examine the aforementioned phenomena. The findings indicate a strong concurrence between the outcomes of the FVCOM numerical simulation and the observed data. The winter and summer tidal patterns in the seas around Yongxing Island exhibit a high degree of similarity. In winter, the current speed of rapid flood and ebb process, exhibits somewhat higher than that in summer. The maximum current speed occurs in the northern marine region of Yongxing Island, during the rapid flood process, whereas it occurs in the northwest marine region of Yongxing Island during the rapid ebb process.

Modeling of Barotropic Tide off the Southeastern Coast of the Kamchatka Peninsula in View of the Accuracy of Global Tidal Models in the Northwest Pacific Ocean

This study introduces the development and implementation of a regional numerical finite-volume model FESOM–C, specifically designed to accurately compute barotropic tidal dynamics in the Pacific waters adjacent to the southeastern region of the Kamchatka Peninsula. The dynamics of principal harmonics of the semidiurnal M2 and diurnal K1 tidal constituents are replicated, as well as the total tide, which encompasses 12 constituents. The computed results, obtained using a detailed unstructured grid, are interpreted through the Long-wave approach. The FESOM–C regional model revealed the variability of harmonic constants of tide and current characteristics within the shelf and canyon-cut continental slope, due to topographic scattering of tidal waves. The assessment includes the estimation of maximum currents and eddy structures associated with residual tidal circulation on the shelf and continental slope. To investigate the influence of varying open boundary conditions, sensitivity experiments have been conducted using data from two state-of-the-art global tidal models FES2014 and TPXO9. The findings reveal that the regional model’s solution exhibits only minimal dependency on this choice, and it aligns well with the limited available tidal data. Interestingly, the global models themselves demonstrate significant disparities in the tidal currents. Furthermore, we assess the accuracy of global tidal model solutions in a broader region encompassing the Sea of Okhotsk, as well as the Pacific waters along the Kuril Islands and the Kamchatka Peninsula. This assessment utilizes a verified database of tidal harmonic constants derived from the Soviet and British tide tables. While the average errors in tidal heights calculations remain minor and closely approximate officially declared values, certain areas within the region exhibit notable discrepancies in the outputs of the global models. These discrepancies are site-specific and vary depending on the particular model and tidal harmonic under consideration. This underscores the need for caution when applying results from global tidal models at the regional scale. Meanwhile, the importance of advancing regional tidal dynamics modeling remains evident.

Late Pleistocene Evolution of Tides and Tidal Dissipation

AbstractStudies of the Last Glacial Maximum (LGM; 26.5–19 ka) tides showed strong enhancements in open ocean tidal amplitudes and dissipation rates; however, changes prior to the LGM remain largely unexplored. Using two different ice sheet and sea level reconstructions, we explicitly simulate the evolution of the leading semi‐diurnal and diurnal tidal constituents (M2, S2, K1, and O1) over the last glacial cycle with a global tide model. Both sets of simulations show that global changes, dominated by the Atlantic, take place for the semi‐diurnal constituents, while changes for the diurnal constituents are mainly regional. Irrespective of the reconstruction, open ocean dissipation peaks during the sea level lowstands of MIS 2 (∼20 ka) and MIS 4 (∼60 ka), although dissipation values prior to MIS 2 are sensitive to differences in reconstructed ice sheet extent. Using the statistically significant relationship between global mean sea level and dissipation, we apply regression analysis to infer open ocean and shelf dissipation, respectively, over the last four glacial cycles back to 430 ka. Our analysis shows that open ocean tidal energy was probably increased for most of this period, peaking during glacial maxima, and returning to near‐present‐day values during interglacials. Due to tidal resonance during glacial phases, small changes in bathymetry could have caused large changes in tidal amplitudes and dissipation, emphasizing the need for accurate ice margin reconstructions. During glacial phases, once global mean sea level decreased by more than ∼100 m, the amount of open ocean tidal energy available for ocean mixing approximately doubled.

Verification and intercomparison of global ocean Eulerian near-surface currents

The OceanPredict task team for Intercomparison and Validation (IV-TT) has established the CLASS4 data standard for routine forecast verification against selected reference observing platforms. The set of CLASS4 reference data has been recently extended to include near-surface currents derived from the trajectories of drifting buoys drogued at 15 m. We have applied these data to the Ocean Model, Analysis and Prediction System (OceanMAPS) at the Australian Bureau of Meteorology (ABoM) for verification and inter-comparison with Mercator Océan International ocean forecast system (MOi), the operational models of the Met Office, UK: Forecast Ocean Assimilation Model (FOAM) and Coupled Atmosphere-Land-Ocean-Ice Data Assimilation (CPLDA) systems, and the Global Ice Ocean Prediction System (GDPS-GIOPS) at the Canadian Centre for Meteorological and Environmental Prediction (CCMEP). The aims for this verification analysis are to extend the routine monitoring of the operational system; to assess the OceanMAPS skill against other models; and to inform our stakeholders of the OceanMAPS performance.We have evaluated the impacts of adding Stokes drift and tidal currents from separate global wave and global tide models to the model currents on the verification of currents. Including Stokes drift leads to significant improvements in representation and the metrics for model skill while inclusion of tides has no significant impact on the global statistics. Zonal currents have a stronger signal than the meridional currents as indicated by their high correlation with observations, and lower errors. This indicates zonal currents on average are more predictable and persistent than the meridional currents, resulting in improved representation in the models.Overall, MOi, and the new version of OceanMAPS (4.0i), and FOAM show marginally better performance against the observations relative to other models. Although there are significant differences in the configurations of the eight models under evaluation, all the models are shown to be remarkably statistically equivalent with consistent spatial and temporal patterns indicating the main differences are attributable to unrepresented processes. We therefore conclude that there is significant scope to further improve the representation of the modelled currents with the observations.

Internal Waves Near Half Moon Island, South Shetland Islands

We analyze internal waves based on measurements on cruise 87 of the R/V Akademik Mstislav Keldysh in the Bransfield Strait near Half Moon Island and calculations using a numerical model. The measurements were carried out on January 25, 2022 for four hours using a line of temperature and pressure sensors, along with the measurements with a CTD probe. Temperature fluctuations according to the sensors of the thermal line and the probe showed that the amplitude of internal waves is close to 5 m, sometimes reaching 15 m. According to the results of calculations of the global tidal model TPXO9, irregular semidiurnal tides predominate in the study area. Numerical calculations of the parameters of internal waves show that the baroclinic tide generated on a steep slope breaks up into higher frequency waves.

Accuracy Evaluation of Regional and Global Tidal Models (TPXO9 and Goddard Ocean Tide) at Kabil Tidal Station

Indonesia is an archipelagic country with a total marine area of 5.9 million km², consisting of 3.2 million km² of territorial waters and 2.7 km² of Exclusive Economic Zone waters, not including the continental shelf. With the vast waters in Indonesia, sufficient information about the tides is needed. Limitations of terrestrial tide data amidst the increasing need for marine information can be overcome by using global and regional tide models. This study uses the regional tidal data model released by BIG (Geospatial Information Agency) and the global tide model TPXO9 and GOT (Goddard Ocean Tides). From the two global tidal models, the tidal harmonic constant values are extracted at the tidal observation point in Kabil. Evaluation of global and regional tide models is carried out by comparing the amplitude values of the main harmonic constants of the tide models of global and regional tides with the amplitude values of the harmonic constants of terrestrial tidal measurement data to obtain a comparison of accuracy. The results of this research show the value of main tidal harmonic.

Relative contributions of water-level components to extreme water levels along the US Southeast Atlantic Coast from a regional-scale water-level hindcast

A 38-year hindcast water-level product is developed for the US Southeast Atlantic coastline from the entrance of Chesapeake Bay to the southeast tip of Florida. The water-level modeling framework utilized in this study combines a global-scale hydrodynamic model (Global Tide and Surge Model, GTSM-ERA5), a novel ensemble-based tide model, a parameterized wave setup model, and statistical corrections applied to improve modeled water-level components. Corrected water-level data are found to be skillful, with an RMSE of 13 cm, when compared to observed water-level measurement at tide gauge locations. The largest errors in the hindcast are location-based and typically found in the tidal component of the model. Extreme water levels across the region are driven by compound events, in this case referring to combined surge, tide, and wave forcing. However, the relative importance of water-level components varies spatially, such that tides are found to be more important in the center of the study region, non-tidal residual water levels to the north, and wave setup in the north and south. Hurricanes drive the most extreme water-level events within the study area, but non-hurricane events define the low to mid-level recurrence interval water-level events. This study presents a robust analysis of the complex oceanographic factors that drive coastal flood events. This dataset will support a variety of critical coastal research goals including research related to coastal hazards, landscape change, and community risk assessments.

Monitoring multidecadal coastline change and reconstructing tidal flat topography

Intertidal zones are vulnerable to both landward and seaward pressures, i.e., sea level rise and aquaculture expansion. Thus, the conservation of coastal habitats has reached international prominence, as addressed in Sustainable Development Goal (SDG) target 14.5. Nevertheless, stakeholders often have limited resources to monitor ample intertidal zones and cannot effectively evaluate actual conservation progress. Modern satellite imagery, especially optical data, provides a cost-efficient opportunity. Nonetheless, most past studies relied on noise-included water indices and demanded intensive manual editions. Therefore, this study presents an automatic processing strategy to estimate essential baseline information of coastal dynamics and topography, including shoreline changes, intertidal digital elevation model (DEM), and mean sea level (MSL). The results were statistically validated with bathymetric surveys and indicated that our proposed workflow can identify hotspots of coastal erosion-accretion and topographic features. Methodologically, by incorporating a fine-resolution global tide model, the tidal effect was greatly mitigated. We confirm that images acquired during flood stages can yield better quality tidal flat DEMs. The shoreline extraction approach refined in this study also enhances the positional accuracy via overcoming common biases, including water color variations and breaking wave-induced whitewash. These improvements and the wide applicability of our approach highlight the opportunity of utilizing spaceborne remote sensing data to assess the coastal stability in different spatio-temporal scales.

Barotropic tides in MPAS-Ocean (E3SM V2): impact of ice shelf cavities

Abstract. Oceanic tides are seldom represented in Earth system models (ESMs) owing to the need for high horizontal resolution to accurately represent the associated barotropic waves close to coasts. This paper presents results of tides implemented in the Model for Prediction Across Scales–Ocean or MPAS-Ocean, which is the ocean component within the U.S. Department of Energy developed Energy Exascale Earth System Model (E3SM). MPAS-Ocean circumvents the limitation of low resolution using unstructured global meshing. We are at this stage simulating the largest semidiurnal (M2, S2, N2) and diurnal (K1, O1) tidal constituents in a single-layer version of MPAS-O. First, we show that the tidal constituents calculated using MPAS-Ocean closely agree with the results of the global tidal prediction model TPXO8 when suitably tuned topographic wave drag and bottom drag coefficients are employed. Thereafter, we present the sensitivity of global tidal evolution due to the presence of Antarctic ice shelf cavities. The effect of ice shelves on the amplitude and phase of tidal constituents are presented. Lower values of complex errors (with respect to TPXO8 results) for the M2 tidal constituents are observed when the ice shelf is added in the simulations, with particularly strong improvement in the Southern Ocean. Our work points towards future research with varying Antarctic ice shelf geometries and sea ice coupling that might lead to better comparison and prediction of tides and thus better prediction of sea-level rise and also the future climate variability.

Ocean tides in the colombian basin and atmospheric contribution to S2

The tidal behavior in the Colombia basin is described based on the analysis of eighteen tide gauge time series, fourteen in the Colombian coasts and four placed in neighboring countries. Tidal constituents are published for the first time at nine of these stations. Harmonic analysis shows that the main constituents in the Caribbean correspond to three diurnals (K1, O1, P1); three semidiurnals (M2, N2, S2), and one long period harmonic (Mf), showing amplitudes and phase lag that correspond to previous tidal reports in the basin. In Turbo, due to the shallow and extensive continental shelf in the Urabá gulf, M2 is amplified, and shallow water harmonics appear. The amplitude and phase of each observed constituent are compared with global tide models FES2014, TPXO9 and DTU10, showing good agreement. The most significant differences occur with semidiurnal harmonics at stations close to the amphidromic point in the eastern Caribbean. In Mf, considerable interannual variations are found, supporting the need of over one year of sea level data to assess this constituent in the Colombia basin accurately. The radiational component of S2 is assessed using barometric pressure in thirteen stations, confirming its importance when compared to the gravitational contribution to the observed sea level harmonic. A trend in the atmospheric S2 is found in Cartagena, which supports those trends in sea-level S2, previously reported in the Caribbean Sea, are caused by variations in the radiational forcing.

Estudio de la precisión de modelos de marea mediante la varianza de los residuos de alturas satelitales en la plataforma patagónica

This work proposes to compare the astronomical tidal predictions obtained by using the totality of the constituents provided by seven global tide models and by the Center for Topographic studies of the Ocean and Hydrosphere (CTOH). To quantify this comparison, the variance of residuals of satellite sea level heights are computed from 1992 to 2019, for each model and CTOH. As a case study, we focus on the Patagonian shelf. It is found that the most accurate tidal predictions are obtained when using FES2014 model and CTOH harmonic constants. It is also observed that the decrease in variance values is due to the addition of certain minor constituents. As an example, in the case of CTOH, a location yields a reduction of more than 57% in the variance values when 9 minor constituents are added to the nine common ones. Likewise, in the case of FES2014, variance reduction over this same location is more than 56% by incorporating 14 minor constituents. Finally, a comparison of the amplitudes and phases values of the common constituents among models and the CTOH shows that these values are practically the same. However, when comparing predictions, very dissimilar results are obtained among models

Regional Tidal Modelling Using Tide Gauges and Satellite Altimetry Data in South-West Coast of Turkey

This study aims to determine the main tidal parameters and the regional tidal model in the south-west coasts of Turkey using the least-squares harmonic estimation method. For this purpose, the sea level data were analyzed and compared to the tides that affect sea level based on the tidal model using the tide gauge station and satellite altimetry data. High-Frequency tide gauge data were obtained from the TUDES (Turkish National Sea Level Monitoring System) data archive and multi-mission satellite altimetry data were achieved from the AVISO data archive. The tidal components of ten tide gauge stations distributed in the south-west coast of Turkey were computed between the period 1998–2018. The results were compared with DTU10 and FES2014b global tide models and regional X-TRACK tidal constants (for Mediterranean). It was found that the differences of the global and regional model solutions were less than 1.3 cm for the main tide constituents (semidiurnal: M2, S2 and diurnal: K1, O1). Moreover, a regional tidal model for Turkey's south-west coast was determined from tide gauge stations and satellite altimetry. The results indicated that the difference between the two solutions was about 0.8 cm.

Tidal dissipation from free drift sea ice in the Barents Sea assessed using GNSS beacon observations

One of the major challenges facing global hydrodynamic tidal models is the modelling of the interaction between sea ice and tides in high-latitude waters. Recent studies have shown strong seasonal correlation between sea ice and tides. Hence, it is important to accurately model the effect of sea ice in a tidal model. Presence of sea ice leads to a frictional dissipation of tides. Most models either completely ignore sea ice or partly include it by assuming a fixed sea ice cover (landfast ice). However, sea ice can also be drifting and the nature of dissipation between drifting sea ice and tides is partly unknown. We assess the dissipation of tides due to free drift sea ice. In the absence of wind, this is negligible in the deeper and open ocean. For the shallow water regions, however, this dissipation is unknown. Here, we evaluate this dissipation for the Spitzbergen Shelf region using a beacon dataset showing strong free drift subdaily sea ice oscillations and a physics based point ice model. Two analyses were done which compared the model and observed motion. The analyses showed that for winds speeds below 8m/s and with low subdaily signals, the subdaily free drift sea ice motion is strongly connected to the tides and that the frictional dissipation is low. In the context of global tide and storm surge models, the dissipation from free drift sea ice on tides should be evaluated based on the region (deep ocean or shallow water) and existing wind conditions. In the presence of strong winds the dissipation between free drift sea ice and air can be significant on a subdaily scale even if there are no subdaily signals in the wind itself.

Improving the SSH Retrieval Precision of Spaceborne GNSS-R Based on a New Grid Search Multihidden Layer Neural Network Feature Optimization Method

The altimetry precision of conventional spaceborne Global Navigation Satellite Systems Reflectometry (GNSS-R) is limited, and the error models are complicated. To compensate for the shortcomings of conventional methods, we present a new grid search multihidden layer neural network feature optimization method (GSMHLFO) for sea surface height (SSH) retrieval. Firstly, the GSMHLFO is constructed by combining the multihidden layer neural network, feature engineering, and a grid search algorithm. Moreover, the retrieval performance of the GSMHLFO and its sensitivity to various features are analyzed. By analyzing 14 feature sets with different information details, we concluded that the elevation, signal-to-noise ratio (SNR), atmospheric delay, and ocean wind speed can provide essential contributions to the SSH retrieval based on GSMHLFO. Secondly, the Technical University of Denmark 18 mean sea surface (DTU18 MSS), which is corrected by the TPXO8 global tide model, was used to verify the GSMHLFO. The number of hidden layers and neurons was optimized using the grid search algorithm. The experimental results show that the proposed GSMHLFO with four hidden layers and 200 neurons per layer has a better retrieval performance. Compared with DTU18, the mean absolute difference (MAD), the root mean square error (RMSE), and the Pearson correlation coefficient (PCC) equal 4.23 m, 5.94 m, and 0.98, respectively. The retrieval precision obtained is significantly improved compared to that reported in the literature for the TDS-1 SSH retrieval. Finally, the retrieval performance of the GSMHLFO and the traditional HALF single-point retracking method were compared. The precision of GSMHLFO is higher than that of traditional retracking method according to MAD, RMSE, and PCC, which are increased by 32.86, 25.00, and 8.99%. The GSMHLFO will provide innovative theoretical and methodological support for the high-precision SSH retrieval of GNSS-R altimetry satellites in the future.