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The Sea Surface KInematics Multiscale monitoring (SKIM) mission proposes to use Doppler-based measurements of velocities to provide global estimates of surface currents and ice drift at spatial scales of 40 km and more, with snapshots at least every day for latitudes 75 to 82, and every few days otherwise. Given the contribution of wave motion to D...
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... on a regular grid. This mapping er- ror is similar to what happens with HF radars (e.g., Lipa and Barrick, 1983;Kim et al., 2008). In particular, SKIM only measures radial components so that on the edges of the swath only the cross-track component is measured, and in the center there are only measurements of the along-track component, as shown in Fig. ...
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... will be discussed in other publications. We only point out here that the sea-state variability at small scales is probably domi- nated by the effect of ocean currents (Ardhuin et al., 2017b). It is thus logical to measure waves and currents together, and possibly further use the measured variability of sea-state pa- rameters to further constrain the magnitude of current gradi- ents. ...
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... results clearly show that Doppler oceanography from space can be a very useful technique for monitoring space- and timescales of the ocean currents that are not well ob- served today. Future altimeter designs should probably con- sider adding off-nadir rotating beams for more effective cov- erage of the ocean. In the present paper we have not dis- cussed much the added benefits of ocean wave measurements with unprecedented spectral and spatial coverage. These will be discussed in other publications. We only point out here that the sea-state variability at small scales is probably domi- nated by the effect of ocean currents (Ardhuin et al., 2017b). It is thus logical to measure waves and currents together, and possibly further use the measured variability of sea-state pa- rameters to further constrain the magnitude of current gradi- ...
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... the last important source of error we have investi- gated is the mapping error, going from Level 2 data at each footprint to Level 3 data on a regular grid. This mapping er- ror is similar to what happens with HF radars (e.g., Lipa and Barrick, 1983;Kim et al., 2008). In particular, SKIM only measures radial components so that on the edges of the swath only the cross-track component is measured, and in the center there are only measurements of the along-track component, as shown in Fig. ...
Citations
... Recent technological advances in observational coastal oceanography promise increasing amounts of data from multiple sources, including drifters (Thomson 2012), satellite imagery (Horstmann et al. 2015;Ardhuin et al. 2018), X-band radar (Holman and Haller 2013;Honegger et al. 2020) and even using marine animals via biologging (Roquet et al. 2017;Chung et al. 2021). ...
An inversion technique was tested for estimating bathymetry from observations of surface currents in a partially-mixed estuary, Mouth of the Columbia River (MCR). The methodology uses an iterative ensemble-based assimilation scheme which is found to have good skill for recovering bathymetry from observations distributed in space and time. However, the inversion skill is highly dependent on the tidal phase, location of the observations, and flow-dependent estuary dynamics. Inversion skill was found to degrade during periods of higher river discharge (up to ~ 12,000 m ³ ), or low tidal amplitude, while inversion of depth-averaged velocities instead of surface velocities caused increased skill throughout the domain. These results point to dynamical limits on inversion skill, caused by changes in estuary dynamics that affect the sensitivity of surface velocities to bathymetry. An adjoint sensitivity analysis is used to visualize these effects and is combined with data-denial experiments to explore the flow-dependent inversion skill.
... Although great advances have been made in the last decades for measuring geostrophy at meso and larger scales or wind stress over the ocean surface, such as satellite scatterometers like QuikSCAT or ASCAT (Bourassa et al., 2019), wave and wind-wave combined measurements are still limited to specific sites (mooring, stations, and buoys) or interpolated from radar radiometers (Ardhuin et al., 2018). However, the availability of global forecasting systems both for wave and surface winds, allows the inclusion of these high frequency velocities in recently developed models of the ocean circulation, by merging the different sources to obtain improved velocity products (Breivik et al., 2016;Onink et al., 2019). ...
... Gridded wave and sea surface wind data can be obtained from remote sensing equipped with a scatterometer (Bourassa et al., 2019) and from model outputs. However, while satellites collect indirect observations of wind and waves (Ardhuin et al., 2018), data are acquired along tracks, generating maps with an effective resolution of ∼40-50 km and 1 week. Since the wavefield changes at high frequency, that is, for periods spanning a few hours, remote-sensed winds are not the most suitable data set in order to study the wave effect on surface circulation. ...
Effects of wind and waves on the surface dynamics of the Mediterranean Sea are assessed using a modified Ekman model including a Stokes‐Coriolis force in the momentum equation. Using 25 years of observations, we documented intermittent but recurrent episodes during which Ekman and Stokes currents substantially modulate the total mesoscale dynamics by two nonexclusive mechanisms: (a) by providing a vigorous input of momentum (e.g., where regional winds are stronger) and/or (b) by opposing forces to the main direction of the geostrophic component. To properly characterize the occurrence and variability of these dynamical regimes, we perform an objective classification combining self‐organizing maps and wavelet coherence analyses. It allows proposing a new regional classification of the Mediterranean Sea based on the respective contributions of wind, wave, and geostrophic components to the total mesoscale surface dynamics. We found that the effects of wind and waves are more prominent in the northwestern Mediterranean, while the southwestern and eastern basins are mainly dominated by the geostrophic component. The resulting temporal variability patterns show a strong seasonal signal and cycles of 5–6 years in the total kinetic energy arising from both geostrophic and ageostrophic components. Moreover, the whole basin, specially the regions characterized by strong wind‐ and wave‐induced currents, shows a characteristic period of variability at 5 years. This can be related to climate modes of variability. Regional trends in the geostrophic and ageostrophic currents show an intensification of 0.058 ± 1.43 · 10⁻⁵ cm/s per year.
... The first component (U geo ) is due to the motion of the radar platform relative to the solid rotating Earth [33], [52]. It involves the platform velocity, attitude and antenna pointing. ...
One of the challenges in ocean surface current retrieval from synthetic aperture radar (SAR) data is the estimation and removal of the wave-induced Doppler centroid (DC). This article demonstrates empirically the relationship between the dc derived from spaceborne X-band InSAR data and the ocean surface wind and waves. In this study, we analyzed over 300 TanDEM-X image pairs. It is found that the general characteristics of the estimated dc follow the theoretically expected variation with incidence angle, wind speed, and wind direction. An empirical geophysical model function (GMF) is fit to the estimated dc and compared to existing models and previous experiments. Our GMF is in good agreement (within 0.2 m/s) with other models and data sets. It is found that the wind-induced Doppler velocity contributes to the total Doppler velocity with about 15% of the radial wind speed. This is much larger than the sum of the contributions from the Bragg waves (~0.2 m/s) and the wind-induced drift current (~3% of wind speed). This indicates a significant (dominant) contribution of the long wind waves to the SAR dc. Moreover, analysis of dual-polarized data shows that the backscatter polarization ratio (PR=σ⁰VV/σ⁰HH) and the dc polarization difference (PD=|dcVV|-|dcHH|) are systematically larger than 1 and smaller than 0 Hz, respectively, and both increase in magnitude with incidence angle. The estimated PR and PD are compared to other theoretical and empirical models. The Bragg scattering theory-based (pure Bragg and composite surface) models overestimate both PR and PD, suggesting that other scattering mechanisms, e.g., wave breaking, are involved. In general, it is found that empirical models are more consistent with both backscatter and Doppler data than theory-based models. This motivates a further improvement of SAR dc GMFs.
... Quality of hindcasts heavily depends on the atmospheric forcing usually provided by reanalyses. Various atmospheric datasets differ in time and space resolution and also have different assimilation algorithms and data assimilation input [8,10]. In this way, they may not necessarily demonstrate the same signals in 10-m winds which reflects in the simulated wave fields. ...
Four global wind wave hindcasts based on ERA5, MERRA2, ERA-I and CFSR reanalyses and spectral wave model WAVEWATCH III for the period from 1980 to 2017 have been validated against satellite altimetry and NDBC buoys for 1 year. Hindcast based on newly released ECMWF reanalysis ERA5 demonstrated the best agreement with both satellite altimetery (with normalized bias being up to 5%, and RMSE up to 0.5 m) and buoy measurements, including values of upper percentiles. In general, all hindcasts show good correspondence with the observational data and thus can be used in further wind wave climate studies.
... Quality of hindcasts heavily depends on the atmospheric forcing usually provided by reanalyses. Various atmospheric datasets differ in time and space resolution and also have different assimilation algorithms and data assimilation input [8,10]. In this way, they may not necessarily demonstrate the same signals in 10-m winds which reflects in the simulated wave fields. ...
Four global wind wave hindcasts based on ERA5, MERRA2, ERA-I and CFSR reanalyses and spectral wave model WAVEWATCH III for the period from 1980 to 2017 have been validated against satellite altimetry and NDBC buoys for 1 year. Hindcast based on newly released ECMWF reanalysis ERA5 demonstrated the best agreement with both satellite altimetery (with normalized bias being up to 5%, and RMSE up to 0.5 m) and buoy measurements, including values of upper percentiles. In general, all hindcasts show good correspondence with the observational data and thus can be used in further wind wave climate studies.
... In addition, joint observations of coupled air-sea variables such as total surface currents, wind speed and direction, and directional wind-wave spectrum could bring further improvements in coastal ocean monitoring and further insights on various processes. In that respect, several proposals for satellite Doppler oceanography missions have been developed (e.g., SKIM, Ardhuin et al. 2018;WaCM, Rodriguez 2019; SEASTAR, Gommenginger 2019), each one targeting specific processes at different scales (e.g., Villas Bôas et al. 2019). Although these missions are not planned yet, they respond to actual requirements of coastal users. ...
Coastal zones have large social, economic and environmental values. They are more densely populated than the hinterland and concentrate large economic assets, critical infrastructures and human activities such as tourism, fisheries, navigation. Furthermore, coastal oceans are home to a wealth of living marine resources and very productive ecosystems. Yet, coastal zones are exposed to various natural and anthropogenic hazards. To reduce the risks associated with marine hazards, sustained coastal zone monitoring programs, forecasting and early warning systems are increasingly needed. Earth observations (EO), and in particular satellite remote sensing, provide invaluable information: satellite-borne sensors allow an effective monitoring of the quasi-global ocean, with synoptic views of large areas, good spatial and temporal resolution, and sustained time-series covering several years to decades. However, satellite observations do not always meet the precision required by users, in particular in dynamic coastal zones, characterized by shorter-scale variability. A variety of sensors are used to directly monitor the coastal zone and their observations can also be integrated into numerical models to provide a full 4D monitoring of the ocean and forecasts. Here, we review how EO, and more particularly satellite observations, can monitor coastal hazards and their drivers. These include coastal flooding, shoreline changes, maritime security, marine pollution, water quality, and marine ecology shifts on the one hand, and several physical characteristics (bathymetry, topography, vertical land motion) of coastal zones, meteorological and oceanic (metocean) variables that can act as forcing factors for coastal hazards on the other hand.
... Pencil-beam Doppler scatterometry has been proposed as a method for making these measurements from space in the Winds and Currents Mission (WaCM) concept [2] and the ESA SKIM [3] mission. As a precursor to these spaceborne instruments, the NASA Jet Propulsion Laboratory (JPL) has developed and flown the Ka-band DopplerScatt instrument [4] under NASA's Instrument Incubator Program (IIP-13) and Airborne Instrument Technology Transition (AITT- 16). ...
Doppler scatterometry is a promising new technique for the simultaneous measurement of ocean surface currents and winds. These measurements have been recommended by the recent US NRC Decadal Review for NASA as being priority variables for the coming decade of Earth observations. In addition, currents and winds are useful for many applications, including assessing the operating conditions for oil platforms or tracking the dispersal of plastic or oil by surface currents and winds. While promising, Doppler scatterometry is relatively new and understanding the measurement characteristics is an important area of research. To this end, Chevron sponsored the deployment of DopplerScatt, a NASA/JPL Ka-band Doppler scatterometer, over instrumented sites located at the edge of a Gulf of Mexico Loop Current Eddy (LCE). In addition to in situ measurements, coincident synoptic maps of surface currents were collected by the Areté ROCIS instrument, an optical current measurement system. Here we report on the results of this experiment for both surface currents and winds. Surface current comparisons show that the Ka-band Current Geophysical Model Function (CGMF) needs to include wind drift currents, which could not be estimated with prior data sets. Once the CGMF is updated, ROCIS and DopplerScatt show good agreement for surface current speeds, but, at times, direction differences on the order of 10 ∘ can occur. Remote sensing optical and radar data agree better among themselves than with ADCP currents measured at 5 m depth, showing that remote sensing is sensitive to the the currents in top 1 m of the ocean. The LCE data provided a unique opportunity to study the effects of surface currents and stability conditions on scatterometer winds. We show that, like Ku-band, Ka-band estimates of winds are related to neutral winds (and wind stress) and are referenced relative to the moving frame provided by the current. This is useful for the study of air-sea interactions, but must be accounted for when using scatterometer winds for weather prediction.
... In addition, joint observations of coupled air-sea variables such as total surface currents, wind speed and direction, and directional wind-wave spectrum could bring further improvements in coastal ocean monitoring and further insights on various processes. In that respect, several proposals for satellite Doppler oceanography missions have been developed (e.g., SKIM, Ardhuin et al. 2018;WaCM, Rodriguez 2019; SEASTAR, Gommenginger 2019), each one targeting specific processes at different scales (e.g., Villas Bôas et al. 2019). Although these missions are not planned yet, they respond to actual requirements of coastal users. ...
Coastal zones have large social, economic and environmental values. They are more densely populated than the hinterland and concentrate large economic assets, critical infrastructures and human activities such as tourism, fisheries, navigation. Furthermore, coastal oceans are home to a wealth of living marine resources and very productive ecosystems. Yet, coastal zones are exposed to various natural and anthropogenic hazards. To reduce the risks associated with marine hazards, sustained coastal zone monitoring programs, forecasting
and early warning systems are increasingly needed. Earth observations (EO), and in particular satellite remote sensing, provide invaluable information: satellite-borne sensors allow an effective monitoring of the quasi-global ocean, with synoptic views of large areas, good spatial and temporal resolution, and sustained time-series covering several years to decades. However, satellite observations do not always meet the precision required by users, in particular in dynamic coastal zones, characterized by shorter-scale variability. A
variety of sensors are used to directly monitor the coastal zone and their observations can also be integrated into numerical models to provide a full 4D monitoring of the ocean and forecasts. Here, we review how EO, and more particularly satellite observations, can monitor coastal hazards and their drivers. These include coastal flooding, shoreline changes, maritime security, marine pollution, water quality, and marine ecology shifts on the one hand, and several physical characteristics (bathymetry, topography, vertical land motion) of coastal zones, meteorological and oceanic (metocean) variables that can act as forcing factors for coastal hazards on the other hand.
... Under certain conditions, optical observations of the sea surface from space make it possible to reconstruct two-dimensional spatial wave spectra, for example, according to data from the Sentinel-2 satellite [5,6]. The latest satellite missions CFOSAT (a joint project of the French and Chinese space agencies) [7] and SKIM (European Space Agency) [8] for the first time will allow constant monitoring of the angular spectra of sea waves on a global scale. However, the complexity of field measurements, a number of methodological limitations (for example, clouds for optical measurements, the need to take into account the orbital movements of waves and collapses in radar data), the high cost of special instruments, their installation and maintenance, all of them lead to the fact that the scientific community has an extremely small a group of measuring instruments for the surface velocity vector and spectral characteristics of surface waves. ...
Measurement of the parameters of the upper layer of the ocean is one of the tasks of oceanology, on the solution of which the progress of a number of areas of ocean research depends on. So, the data of field measurements are used for verifying and calibrating the processing algorithms of remote meters, oceanographic models, in various research problems and in synoptic/climatic monitoring of the ocean. A continuous increasing in the volume of remote sensing data, an increasing in the complexity of their processing algorithms, and improvement of ocean modeling methods form new requirements for the quality and quantity of experimental measurements. In this paper a small-sized drifter is considered. It is equipped with the necessary measuring channels and software, as an element of the system for measuring the spatiotemporal characteristics of the surface layer of the ocean, namely, the stream vector and the angular spectrum of sea waves. Its structural scheme, the general view, the description of the mechanical part, as well as the scheme for performing the experimental work, all of them are presented in the paper.
... Novels approaches, such as machine learning, could allow to identify oceanic features that imprint the stress by using various data set (such as SST, altimeters) and, thus, to filter out their imprint on scatterometer stress. Future satellite missions (such as SKIM; Ardhuin et al., 2017 andChelton et al., 2018;Rodríguez et al., 2018;WaCM Bourassa et al., 2016) would likely allow measuring 10-m wind, surface stress, and surface currents in a consistent way and, thus, could be another option to construct a data set for forcing ocean models but also to estimate the and coefficients used to predict s ((k) in Figure 11). They would help to better understand and characterize the wind and surface stress responses to both TFB and CFB. ...
The Current Feedback to the atmosphere (CFB) contributes to the oceanic circulation by damping eddies. In an Ocean‐Atmosphere coupled model, CFB can be correctly accounted for by using the wind relative to the oceanic current. However, its implementation in a forced oceanic model is less straightforward as CFB also enhances the 10 m wind. Wind products based on observations have “seen” real currents that will not necessarily correspond to model currents, whereas meteorological reanalyses often neglect surface currents or use surface currents that, again, will differ from the surface currents of the forced oceanic simulation. In this study, we use a set of quasi‐global oceanic simulations, coupled or not with the atmosphere, to (i) quantify the error associated with the different existing strategies of forcing an oceanic model, (ii) test different parameterizations of the CFB, and (iii) propose the best strategy to account for CFB in forced oceanic simulation. We show that scatterometer wind or stress are not suitable to properly represent the CFB in forced oceanic simulation. We furthermore demonstrate that a parameterization of CFB based on a wind‐predicted coupling coefficient between the surface current and the stress allows us to reproduce the main characteristics of a coupled simulation. Such a parameterization can be used with any forcing set, including future coupled reanalyses, assuming that the associated oceanic surface currents are known. A further assessment of the thermal feedback of the surface wind in response to oceanic surface temperature gradients shows a weak forcing effect on oceanic currents.
Open Gold Access: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019MS001715
