14:05 - 14:30
ARCOM, Enhancing the Link between Altimetry and Coastal Models
1CLS, France; 2Rutgers University, USA; 3SOCIB, Spain; 4RSMAS, USA; COSS-TT co-chair; 5LEGOS, France; COSS-TT co-chair
In 2015, a pilot workshop called Altimetry for Regional and COastal Models (ARCOM) was organized within the 4th International Coordination Workshop of the GODAE Coastal Oceans and Shelf Seas Task Team (GOV COSS-TT). Its main objective was to foster the use for validation and/or assimilation of altimetry data in coastal models. ARCOM was a successful event, breaking new ground in the potential for long-lasting synergy between the CAW and COSS communities. In particular, ARCOM helped explain the data and products that CAW has been advancing, while addressing practical aspects for data use and data value for COSS user applications. The tutorial session was the foundation for fruitful discussions about the quality of altimeter measurements and the applied geophysical corrections, the choice and computation of reference surfaces, the capability of the different altimeter sensors in different areas and circulation regimes. Some examples of use of altimetry data in COSS systems were shown, which gave confidence in the value of altimeter data for studying COSS dynamics.
This paper, as an introduction to the “synergies of coastal altimetry and modeling” session, will provide information on the available coastal altimetry datasets and propose concrete actions towards synergistic advances of nadir and wide-swath coastal altimetry on one side, and regional/coastal modeling and prediction on the other side, involving the COSS community and the regional altimetry groups.
14:30 - 14:50
Velocity and Sea Level Anomaly Wavenumber Spectra in the Coastal Ocean: Observations from HF-Radar and Altimetry Compared with Nested High-Resolution Models
Rutgers University, United States of America
A CODAR HF-radar network has been observing surface currents in the Mid Atlantic Bight (MAB) continental shelf ocean for several years. CODAR observes the component of velocity along a radial view direction from a single antenna, geo-located by range and azimuth. Vector velocity is computed by combining radials observed by multiple sites. The concave geometry of the MAB coastline enables us to select radial view transects that are substantially along or across isobaths, and compute wavenumber spectra for both along-shelf and across-shelf components of velocity. Comparing radial view spectra to vector component spectra reveals that the optimal interpolation vector combiner significantly damps energy for wavenumbers exceeding 0.03 km-1.
We further computed SSHA wavenumber spectra using coastal altimeter data from CryoSat-2 for ensembles of tracks in the same region that were predominantly across- or along-shelf. While CODAR-derived velocity spectra exhibit power law dependence close to k-5/3 down to the limit of resolution, the SSHA spectra are somewhat steeper.
Wavenumber spectra from observations are compared to results from hydrodynamic model simulations with increasing resolution achieved by 2-way synchronous nesting (for two and three levels of nested grids). Spectral shapes generally agree well, but with comparable energy levels not achieved until model horizontal grid resolution approaches ~700 m.
The results have implications for specifying observational error and error-of-representation in data-assimilative modeling systems that exploit CODAR and altimeter observations in the coastal ocean.
14:50 - 15:10
The Impact of Satellite Altimeter Observations on Estmates of Cross-Shelf Fluxes in the Mid-Atlantic Bight
1University of California Santa Cruz, United States of America; 2Rutgers University, United States of America
Satellite altimeter observations form an important component of ocean observing systems in the coastal ocean, and along with other satellite and in situ measurements, can be used to constrain ocean models using data assimilation methods. We present here a quantitative assessment of the impact that various recent altimeter missions have on estimates of cross-shelf fluxes of mass, heat and salt in the Mid-Atlantic Bight when these data are assimilated into the Regional Ocean Modeling System (ROMS). The data assimilation method used is a 4-dimensional variational (4D-Var) approach, and using techniques employed routinely in numerical weather prediction, we can partition the 4D-Var transport increments into contributions from each observing platform. Particular attention will be paid here to the impact of the coastal satellite altimeter measurements on the cross-shelf transport.
15:10 - 15:30
A Multi-Technique Combination Method for Altimeter, Tide Gauge and Ships Data
Jade University of Applied Sciences Oldenburg, Germany
Reliable information of the instantaneous sea surface height (SSH) and its behavior are important for all geosciences and the human society in itself. The two standard data sources are tide gauge readings and satellite altimetry measurements. Additionally ships, equipped with geodetic GNSS antennas and receivers can be used for SSH measurements. All three techniques have their individual characteristics such as spatial- and temporal resolution. It is most likely that a multi-technique model will profit from each technique.
Within a PhD project at the Jade University in Oldenburg a new combination method is under development. With this method it will be possible to combine altimeter data, tide gauge readings and ship based SSH measurements in one empirical model. The estimation of various parameters like the mean height, the trend and tidal amplitudes and phases will be possible. The calculation is done on a grid. Coastlines and islands were taken into account during grid generation. The introduction of auxiliary constraints ensures an invertible normal equation matrix.
First tests of this approach were done with simulated data in the North Sea region. This conference contribution will explain the combination approach and show the combination results using the simulated observations.
15:30 - 15:50
Impact of Coastal Altimetry Data in the Black Sea Physical Ocean Analysis System
1CMCC (Italy); 2Mercator Ocean (France); 3DGFI-TUM (Germany)
The Black Sea physical ocean assimilation system used in this study implements a three-dimensional variational data assimilation scheme at a nominal resolution of 3 km, which assimilates observations from Argo profiling floats, SST measurements from infrared sensors and along-track altimetry data, usually taken from CMEMS (i.e. CLS/AVISO). Altimetry data are assimilated by means of local hydrostatic adjustments, i.e. the altimetry misfits are covariated onto increments of temperature and salinity profiles. In order to look at the impact of coastal altimetry data in the system, Jason-2 data were reprocessed using the ALES retracking algorithm. ALES is a specialised coastal subwaveform retracker (i.e. an algorithm that fits the received altimetric signal for precise sea level estimation), which analyses only a portion of the received altimetric echo, in order to avoid coastal signal corruption while keeping a precision comparable to the open ocean measurements. After processing Jason-2 data with ALES, a set of four experiments was designed. We consider a control experiment, represented by a free simulation (1) of the Nucleus for European Modelling of the Ocean (NEMO)Ocean General Circulation Model (OGCM) implemented in the Black Sea; a reference (2) experiment obtained using classical 1 Hz along-track altimetry data. Finally, two experiments where coastal altimetry data from Jason-2 satellite mission sampled at 1 Hz (3) and 20 Hz (4) were performed. Preliminary results show how the ocean assimilation scheme used in this work is suitable for coastal altimetry data, giving consistent results with the reference experiment, even when 20 Hz data are considered within the system.
15:50 - 16:10
Understanding Altimetry Signals in Near-Coastal Areas Using Underwater Autonomous Vehicles
NATO STO CMRE, Italy
During the LOGMEC16 (Long-Term Glider Mission for Environmental Characterization) sea trial carried out in the eastern Ligurian Sea (Northwestern Mediterranean Sea), two deep oceanographic gliders (maximum depth up to 1000m) were operating continuously from 3 May to 27 June 2016. Where and when possible, glider tracks were synchronized in space and time with the footprints of spaceborne altimeters crossing the Ligurian Sea during the sea-trial (i.e., Jason 2, Altika and Cryosat 2). Objectives of the sea trial included the definition of new methods for a more adequate projection along the water column of satellite derived SLA (or SSH) and their effective assimilation in coastal/near-coastal ocean models.
Using temperature and salinity measurements from glider tracks that were co-localized with the altimeter passages, we calculated the dynamic height, and then compared it with the CMEMS near-real time absolute sea level using the TAPAS (Tailored Product for Data Assimilation), where the SLA dataset is available with all the terms used in the correction (i.e., Dynamic Atmospheric Correction, tides, long wavelength error) and the associated Mean Dynamic Topography.
A preliminary comparison between the glider-derived dynamic heights and the altimeter products (with and without corrections) shows that the agreement between the compared datasets is variable and seems to depend on the spatial scales considered. As an effective assimilation of altimeter measurements could be extremely beneficial for the quality of current modelling systems in coastal and near-coastal areas, it is important to understand the causes of the disagreements, and how these may be related to the geographical and/or oceanographic conditions.
Sensitivity experiments with an ocean model based on the Nucleus for European Modelling of the Ocean (NEMO), configured to assimilate satellite-derived SLA, have been also carried out using different altimeter datasets.