Ole Baltazar Andersen

Accuracy assessment of recent ocean tide models

Over 20 global ocean tide models have been developed since 1994, primarily as a consequence of analysis of the precise altimetric measurements from TOPEX/POSEIDON and as a result of parallel developments in numerical tidal modeling and data assimilation. This paper provides an accuracy assessment of 10 such tide models and discusses their benefits in many fields including geodesy, oceanography, and geophysics. A variety of tests indicate that all these tide models agree within 2-3 cm in the deep ocean, and they represent a significant improvement over the classical Schwiderski 1980 model by approximately 5 cm rms. As a result, two tide models were selected for the reprocessing of TOPEX/POSEIDON Geophysical Data Records in late 1995. Current ocean tide models allow an improved observation of deep ocean surface dynamic topography using satellite altimetry. Other significant contributions include theft applications in an improved orbit computation for TOPEX/POSEIDON and other geodetic satellites, to yield accurate predictions of Earth rotation excitations and improved estimates of ocean loading corrections for geodetic observatories, and to allow better separation of astronomical tides from phenomena with meteorological and geophysical origins. The largest differences between these tide models occur in shallow waters, indicating that the current models are still problematic in these areas. Future improvement of global tide models is anticipated with additional high-quality altimeter data and with advances in numerical techniques to assimilate data into high-resolution hydrodynamic models.

Global marine gravity field from the ERS-1 and Geosat geodetic mission altimetry

Satellite altimetry from the Geosat and the ERS-I Geodetic Missions provide altimeter data with very dense spatial coverage. Therefore the gravity field may be recovered in great detail. As neighboring ground tracks are very closely distributed, cross-track variations in the sea surface heights are extremely sensitive to sea surface variability. To avoid errors in the gravity field caused by such effects, sea surface variability needs to be carefully eliminated from the observations. Initially, a careful removal of gross errors and outliers was performed, and the tracks were fitted individually to a geoid model and crossover adjusted using bias and tilt. Subsequently, sea surface heights were gridded using local collocation in which residual ocean variability was considered. The conversion of the heights into gravity anomalies was carried out using the fast Fourier transform (FFT). In this process, filtering was done in the spectral domain to avoid the so-called “orange skin” characteristics. Comparison with marine gravity was finally carried out in three different regions of the Earth to evaluate the accuracy of the global marine gravity field from ERS-1 and Geosat.

Accuracy assessment of global barotropic ocean tide models

The accuracy of state-of-the-art global barotropic tide models is assessed using bottom pressure data, coastal tide gauges, satellite altimetry, various geodetic data on Antarctic ice shelves, and independent tracked satellite orbit perturbations. Tide models under review include empirical, purely hydrodynamic (“forward”), and assimilative dynamical, i.e., constrained by observations. Ten dominant tidal constituents in the diurnal, semidiurnal, and quarter-diurnal bands are considered. Since the last major model comparison project in 1997, models have improved markedly, especially in shallow-water regions and also in the deep ocean. The root-sum-square differences between tide observations and the best models for eight major constituents are approximately 0.9, 5.0, and 6.5 cm for pelagic, shelf, and coastal conditions, respectively. Large intermodel discrepancies occur in high latitudes, but testing in those regions is impeded by the paucity of high-quality in situ tide records. Long-wavelength components of models tested by analyzing satellite laser ranging measurements suggest that several models are comparably accurate for use in precise orbit determination, but analyses of GRACE intersatellite ranging data show that all models are still imperfect on basin and subbasin scales, especially near Antarctica. For the M2 constituent, errors in purely hydrodynamic models are now almost comparable to the 1980-era Schwiderski empirical solution, indicating marked advancement in dynamical modeling. Assessing model accuracy using tidal currents remains problematic owing to uncertainties in in situ current meter estimates and the inability to isolate the barotropic mode. Velocity tests against both acoustic tomography and current meters do confirm that assimilative models perform better than purely hydrodynamic models.

Global ocean tides from ERS 1 and TOPEX/POSEIDON altimetry

Ocean tide models representing all major diurnal and semidiurnal tidal constituents with a spatial resolution of 0.75°×0.75° have been estimated using the first 1.5 years of ERS 1 and TOPEX/POSEIDON altimetry. The ocean tide model was derived from the combined use of ERS 1 and TOPEX/POSEIDON data by using a modified orthotide formulation that simultaneously solves for all diurnal and semidiurnal constituents as well as the annual signal. An additional adjustment of the solar semidiurnal harmonic of the gravitational potential was applied in order to account for radiational forcing, particularly in the S2 constituent. TOPEX/POSEIDON provides excellent ocean tide estimates in the open ocean. However, especially in coastal regions, the track spacing of TOPEX/POSEIDON (315 km at the equator) is too coarse to determine large parts of the ocean tide signal. For these regions the inclusion of data from the ERS 1 35-day repeat mission provides a valuable supplement, as the ERS 1 satellite has a track spacing which is around 3.6 times better than that of the TOPEX/POSEIDON satellite. The combined ERS 1 and TOPEX/POSEIDON ocean tide solution exhibits distinct sectoral geographical pattern of highs and lows when compared with the Cartwright and Ray (1990, 1991) ocean tide model. This indicates the presence of small but fundamental orbit errors present in the Cartwright and Ray ocean tide solution. Compared with a new set of 104 tide gauge readings compiled by Le Provost, the RMS differences of the combined ERS 1 and TOPEX solution are 2.51, 1.67, 1.58, and 1.13 cm for the M2, S2, K1, and O1 constituents, respectively. The increased spatial resolution of the combined ERS 1 and TOPEX model as compared to a TOPEX-alone model is seen to reduce RMS differences from 37 to 22 cm for the M2 constituent, when compared to a selection of 90 pelagic and coastal tide gauges in the northwest European shelf region.

Shallow water tides in the northwest European shelf region from TOPEX/POSEIDON altimetry

During recent years the accurate measurements from the TOPEX/POSEIDON satellite have improved global ocean tide models considerably. However, these global ocean tide models are still not accurate enough on the continental shelves for detailed oceanographic studies. The need for increasing accuracy in shelf regions calls for inclusion of more than just semidiurnal and diurnal constituents in future global ocean tide models. This is because a considerable part of the tidal variability on the shelves is caused by shallow water constituents. One example is the M4 constituent, which exceeds 50 cm at several locations on the northwest European shelf. So far, TOPEX/POSEIDON has not been considered for resolving these constituents, mainly owing to its coarse ground track resolution. Reliable empirical estimates of several major shallow water constituents can be obtained from TOPEX/POSEIDON by combining along-track and crossover observations. Estimates of the major shallow water constituents for the northwest European shelf, M4, M6, and MS4, are presented in this study and compared with tide gauges and existing hydrodynamic models.

SAR observation and model tracking of an oil spill event in coastal waters.

Oil spills are a major contributor to marine pollution. The objective of this work is to simulate the oil spill trajectory of oil released from a pipeline leaking in the Gulf of Mexico with the GNOME (General NOAA Operational Modeling Environment) model. The model was developed by NOAA (National Oceanic and Atmospheric Administration) to investigate the effects of different pollutants and environmental conditions on trajectory results. Also, a Texture-Classifying Neural Network Algorithm (TCNNA) was used to delineate ocean oil slicks from synthetic aperture radar (SAR) observations. During the simulation, ocean currents from NCOM (Navy Coastal Ocean Model) outputs and surface wind data measured by an NDBC (National Data Buoy Center) buoy are used to drive the GNOME model. The results show good agreement between the simulated trajectory of the oil spill and synchronous observations from the European ENVISAT ASAR (Advanced Synthetic Aperture Radar) and the Japanese ALOS (Advanced Land Observing Satellite) PALSAR (Phased Array L-band Synthetic Aperture Radar) images. Based on experience with past marine oil spills, about 63.0% of the oil will float and 18.5% of the oil will evaporate and disperse. In addition, the effects from uncertainty of ocean currents and the diffusion coefficient on the trajectory results are also studied.

An initial estimate of the North Atlantic steady-state geostrophic circulation from GOCE

The GOCE satellite mission was launched in 2009 and the first gravity models were released in July 2010. Here we present an initial assessment of the GOCE data in terms of the mean circulation of the North Atlantic. We show that with just two months of data, the estimated circulation from GOCE is already superior to a similar estimate based on 8 years of GRACE observations. This result primarily depends on the fact that the GOCE mean dynamic topography (MDT) is generally less noisy than that obtained from the GRACE data. It therefore requires less smoothing and so there is less attenuation of the oceanographic signal. Our results provide a strong validation of the GOCE mission concept, and we anticipate further substantial improvements as the mission progresses.

The role of satellite altimetry in gravity field modelling in coastal areas

Abstract During recent years altimetry from the two geodetic missions of GEOSAT and ERS-1 has enabled the derivation of high resolution near global gravity field from altimetry [Andersen and Knudsen, 1995, 1996; Sandwell and Smith, 1997]. Altimetric gravity fields are unique in the sense that they provide global uniform gravity information with very high resolution, and these global marine gravity fields are registered on a two by two minute grid corresponding to 4 by 4 kilometres at the equator. In this presentation several coastal complications in deriving the marine gravity field from satellite altimetry will be investigated using the KMS98 gravity field. Comparison with other sources of gravity field information like airborne and marine gravity observations will be carried out and two fundamentally different test areas (Azores and Skagerak) will be studied to investigated the different role of these different sources of gravity information.

GEROS-ISS: GNSS REflectometry, Radio Occultation, and Scatterometry Onboard the International Space Station

GEROS-ISS stands for GNSS REflectometry, radio occultation, and scatterometry onboard the International Space Station (ISS). It is a scientific experiment, successfully proposed to the European Space Agency in 2011. The experiment as the name indicates will be conducted on the ISS. The main focus of GEROS-ISS is the dedicated use of signals from the currently available Global Navigation Satellite Systems (GNSS) in L-band for remote sensing of the Earth with a focus to study climate change. Prime mission objectives are the determination of the altimetric sea surface height of the oceans and of the ocean surface mean square slope, which is related to sea roughness and wind speed. These geophysical parameters are derived using reflected GNSS signals (GNSS reflectometry, GNSS-R). Secondary mission goals include atmosphere/ionosphere sounding using refracted GNSS signals (radio occultation, GNSS-RO) and remote sensing of land surfaces using GNSS-R. The GEROS-ISS mission objectives and its design, the current status, and ongoing activities are reviewed and selected scientific and technical results of the GEROS-ISS preparation phase are described.

Ocean tides in the northern North Atlantic and adjacent seas from ERS 1 altimetry

Twenty months of ERS 1 35-day repeat altimeter data containing 18 repeat cycles have been used to estimate the major diurnal and semidiurnal ocean tide signals in the northern parts of the North Atlantic and adjacent seas. ERS 1 provides valuable information when investigating ocean tides, owing to the repeated dense spatial sampling. However, several tidal constituents are extremely difficult to resolve using conventional harmonic analysis with the chosen sun syncronous orbit. Instead, temporal analysis at each crossover location is applied using a modified form of the orthotide formulation, which simultaneously solves for the diurnal and semidiurnal species as well as for the annual signal. The use of the response formalism ensures that the sun syncronous component S2 can be resolved, although this component is “frozen” in the orbit. Maps of the M2, S2 and K1 tidal amplitudes and phases in 0.5°×0.5° grids are presented and are seen to compare favorably with measurements at 68 pelagic tide gauges provided by the International Association for Physical Sciences of the Ocean. The major tidal constituents of the ERS 1 derived model are also in close agreement with the improved Flather (1981) ocean tide model for the northwest European continental shelf area, as well as a numerical model for the Arctic and Nordic Seas by Gjevik and Straume (1989).