Why do I observe a bias between Rio05 and RioMed around each side of Gibraltar Strait ?-
The difference in mean dynamic topograpy between the Mediterranean Sea and the Atlantic Ocean in the two different products of MDT that we distribute (RioMed and Rio05) is due to the fact that both surfaces have been computed starting from different large scale first guess :
In the Rio05 MDT, the first guess is a merging between the Levitus climatology at low and mid-latitudes and, at higher latitude, a large scale MDT based on a Mean Sea Surface (CLS01) minus a geoid model (EIGEN-GRACE3S) filtered at 400km resolution. The average of the RIO05 MDT over the latitude range between 40°N and 40°S is equal to the average over the same area of the Levitus climatology. On the other hand, the RioMed MDT was constructed starting from the mean of the MFSTEP Mediterranean forecasting model . The average over the Mediterranean Sea of the RioMed MDT is thus equal to the average of the MFSTEP MDT.
There is no reason why these two constants should be the same, and that is why you observe a bias between the Rio05 MDT and the RioMed MDT.
To merge the two fields, you might consider that the two consants just east of Gibraltar are about the same that the seal level just west of Gibraltar, but I have no clue of how best to deal with this.
A solution will be given in the future by the use of Goce geoid data : then the MDT may be computed for the Atlantic Ocean and for the Mediterranean Sea starting from the same first guess (MSS-Goce, filtered at some spatial wavelength).
- What are the differences between Jason-1 GDR-b and GDR-c?
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The GDR-c has a different file naming convention and has improved corrections.
- The file extension .NASA and .CNES are not added to the GDR-c file names,
- There are two new parameters, pseudo_datation_bias_corr_ku and mdt,
- Better orbit, sea stat bias, ionospheric range, inverse barometer, and tidal corrections,
- Improved rain and ice flags.
An updated user handbook is provided:
- SMM-MU-M5-OP-13184-CN, Edition 4.1, October 2008
Aviso and PoDaac User Handbook - IGDR and GDR Jason-1 Products, pdf version (2 Mo).
- What tide models are used in the merged sea level anomaly data?
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For Jason-1&2 as well as for Envisat data, the tide models are
- ocean tide is deduced from GOT00.V2 model (R. Ray code) (load tide is included)
- earth tide is deduced from the Cartwright tide potential
- polar tide is deduced from equilibrium model (IERS conventions)
Each tide data is interpolated on the altimeter data position (time/space interpolation) and subtracted to the altimeter measurement. The same tidal solutions are applied to each mission in order to minimize inter-mission biases. Independently, multi-mission inter-calibration process (crossover adjustment using Jason-2 as reference) is done considering the data corrected from all geophysical corrections (tides, tropospheric and ionospheric effects, high frequency signal aliasing effect, ...).
- How is Mean Dynamic Topography calculated ?
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The MDT used for Aviso products is called Rio05 and is computed starting from a first guess based on:
- Grace data (altimetric Mean Sea Surface minus Grace geoid, further spatially filtered using a 400km gaussian filter)
- and the Levitus climatology referenced to 1500m at low and mid latitudes.
By construction, the average of the Rio05 MDT values over the global oceans equatorward 40° corresponds to the average over the same region of the Levitus climatology referenced to 1500m. As a result, values of the Rio05 MDT for the global oceans vary from -60cm (Antarctic circumpolar current) to 280 cm, and in most areas the ADT is positive.
The MDT is used to calculate the Absolute Dynamic Topography (ADT) according to ADT = MDT + SLA.
From 2009, a new MDT (MDT_CNES-CLS09) is delivered by Aviso. This new version will be used in all Ssalto/Duacs products by 2010. Further information: The MDT_CNES-CLS09 Combined Mean Dynamic Topography (CMDT) .
- Mean Dynamic Topography, Mean Sea Surface, Mean Sea Level or mean profile??
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There are several means used in altimetry processing or altimetry-based studies.
The MSS (Mean Sea Surface) is a mean of several years of altimetry data, eventually using several satellites). It is a reference surface (thus you have values on a lon/lat grid).
The MDT (Mean Dynamic Topography) is the previous surface minus the geoid. It is the ocean topography due to the mean currents. It is also a reference surface.
The Mean profile is the mean of an altimeter measurements along the satellite's tracks. It can be used as reference for computing SLA for the same satellite. It is an along-track data (not distributed)
The MSL (Mean Sea Level) is a trend (a number of mm/year), often given with respect to time as a curve.
- What are the conditions to get Aviso data?
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All Aviso data are free of charge for non-commercial use, whether online (FTP, Opendap) or on media (DVD).
The request must be justified by a short text explaining how the user plan to use the data. This helps Aviso in refining the available data to be better tuned to the users' needs.
For a first level of education, the best is to work with online maps (gif format), e.g. the one available through the Argonautica project, or with the Live Access Server.
- How are geostrophic velocities computed at the Equator?
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In the geostrophic velocity data distributed by Aviso today, current velocities at the equator are computed using equatorial geostrophy [Picaut et al., 1989]. At the Equator, (lat = 0) we use second derivative. This method is quite usual. In a 5° band around the Equator, a connection is computed to ensure continuity with classical geostrophy. This continuity is not completly satisfying just now, so a study was conducted to test the connection proposed by Lagerloef et al. [1999]. The improvement thus obtained is significant, and was validated using TAO measurements.
- What are the differences between ADT and SLA products?
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Absolute Dynamic Topography (ADT) and Sea Level Anomaly (SLA) are defined by:
ADT=MDT+SLA
where MDT is Mean Dynamic Topography.
- Could you send me an explanation of the times associated with your gridded data?
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The gridded data are computed for a given day. Each map represents the situation on the day indicated in its name. It is not a mean over the period between two files.
To generate these maps, computing methods based on objective analysis allow us to interpolate data in time and in space. In delayed time, the processing window used is centered, e.g. 6 weeks of data before and after the given date are taken into account. In near-real time, only the 6 weeks of data preceding the given day are considered.
- In the gridded data, to which point lat/lon refer to?
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The lon/lat given is the SW corner of the box, as specified in the metadata of the NetCDF file
- Where can I find tables showing the correspondance date / cycle and pass number?
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On-line tools on this website make the correspondance between hour and calendar date and cycle and pass number.
- How to convert measurement time in local time ?
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- For T/P, 86400xTim_Moy_1 + 10-3xTim_Moy_2 + 10-6xTim_Moy_3 = time (in seconds)
- For Jason-1&2, 86400xTime_day + 10-3xTime_sec + 10-6xTim_microsec = time (in seconds)
Time is given in UTC (Universal Time Coordinate, linked to Greenwich meridian), date in Julian day, which is the elapsed time between a reference epoch (January 1st 1950, 0h0min0s for Cnes Julian days, January 1st 1958, 0h0min0s for Nasa Julian days) and measurement date. To convert UTC time in local time, you have to convert it in date, hours, minutes, seconds, then in solar time and finally in your local time. We haven't got any conversion procedure.
- We haven't receive T/P cycle 118 (or 431, 432), Jason-1 cycle 178
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There were no data processed for Topex/Poseidon cycle 118 (resp. 431, 432) and for Jason-1 cycle 178, due to an incident.
- What are Jason1&2, Topex/Poseidon data resolution?
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Spatially, along the track, data are available every 7 km (measurements averaged on 1 second, or "1 Hz data"). GDR 10 or 20 Hz data also exists (depending on the satellite), but those are more noisy. Between tracks, for Jason-1&2 and Topex/Poseidon, the distance is up to 315 km at the Equator; for ERS and Envisat, it is about 80 km.
Gridded Aviso data are available at 1/3° on a Mercator grid.
- What are the characteristics of the reference ellipsoid you are using?
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The reference ellipsoid is the first-order definition of the non-spherical shape of the Earth. Concerning Jason-1&2 and Topex/Poseidon, we are using the following ellipsoid:
- radius : 6378136.3
- inverse Earth flattening coefficient : 298.257
For Envisat:
- radius : 6378137
- inverse Earth flattening coefficient : 298.257223563
- On which platform/Operating system can I use Aviso data?
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You can use Aviso data on every platform and Operating system, but the reading software may not be available for your precise configuration (see the available software).
Note that T/P GDR-Ms are coded in little endian format and Jason-1 GDRs in big endian format.
- I'd like data over a precise area (data extraction)
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- for gridded NetCDF data, an online extraction tool is available
- for Topex/Poseidon and Jason-1 GDR, Aviso can perform extraction for a short period of time on a small area
For the other dataset, users have to perform their own extraction from the global or regional datasets
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