2.1.4 The North Western Mediterranean Sea
by using coastal dedicated products

Satellite altimetry dedicated to coastal ocean.

Satellite altimetry is limited near the coastlines due to a loss of quality in the measurements. These errors are caused by the land contamination in the altimetry and radiometric footprints (10 km and 50 km respectively) but also by inaccurate geophysical corrections. Despite this, the altimetric measurements are present and may contain useful information for coastal studies.
This data use case proposes to give some instructions on how to use the Coastal experimental products distributed by Aviso (PISTACH) products over the Northwestern Mediterranean coasts.

Data used

Jason-2 along-track experimental Coastal products produced by the PISTACH project, a coastal (and hydrology)-dedicated processing applied to the Jason-2 mission.
These products derive from the Jason-2 S-IGDR products and include new retracking solutions, several state-of-the-art geophysical corrections as well as higher resolution global/local models. Numerous extra fields derived from the various PISTACH processes are then added to build the products. These products are based on a 20-Hz along-track sampling rate (vs 1-Hz for the official Jason-2 IGDR) but the nomenclature of their variables and files is similar to Jason-2 IGDRs one.
Download freely coastal data files on the FTP server. Files are in the sub-directories named cycle_xxx/. Each cycle lasts about 10 days, the first one (in the cycle_001/ directory) was acquired early July 2008.

A data selection based on the latitudes (36.5°N, 44°N) enables to extract the Mediterranean Sea.

Methodology

We use the Basic Radar Altimetry Toolbox to observe the data and do some computation.

Data chosen

To limit the volume of data to download, it is better to determine the ground tracks numbers over the area of interest (here, the western Mediterranean Sea). These ground track numbers are available in the pass locator on Aviso web site (download the .kml file for the Jason-2 referenced orbit to visualize it on Google Earth). Here, the Jason-2 interesting passes are: #009, #146, #187 and #222.

The time series used for this data use case streches out from the cycle_003 (August 2008) to cycle_084 (September 2010).

Data editing

to represent the SSHA as a function of latitude

Once the relevant files are all downloaded, create a dedicated workspace and then, a new dataset. In a first Dataset, named dataset_raw_cy5_tr222, we have added one file corresponding to the coastal PISTACH product for the cycle_005 and the pass 222.
In the "Operations" tab, we have created an operation for the SSHA (Sea Surface Height Anomaly) computation, named Operations_raw_cy5_tr222. For a Y=F(X) representation of the SSHA, select latitude for "X" and for the "Data expression", type the SSHA formula with the following combination:

SSHA= satellite altitude – Range (from one retracking) –Sea Surface Bias – Atmospheric corrections (iono, wet , dry) – tides (pole, land and ocean) – HF dynamics - Mean Sea Surface

fig 1: Bathymetry (in meters) below four Jason-2 tracks in the North Western Mediterranean Sea. This map is plotted using the Basic Radar Altimetry Toolbox from the PISTACH coastal products (variable bathymetry_topography).

In practice, for coastal oceans, several combinations are interesting. One of them is (with the full names of the NetCDF product variables):

SSHA1 = alt - range_red3_ku - iono_corr_gim_ku - model_dry_tropo_corr - decontaminated_wet_tropo_corr - solid_earth_tide - ocean_tide_sol3 - pole_tide - inv_bar_corr - hf_fluctuations_corr - mss1

The “RED3” range is actually estimated with a retracking algorithm dedicated to signal affected by the land above water (distance to coast < 10 km). Nevertheless, its drawback is to be biased compared to the classical open ocean computation (range_ku) and to have no sea state bias estimate. The sea state bias in Ku-band is not coherent with the RED3 retracking. But for areas very close to the coastline it could improve the SSHA.
The wet tropospheric “decontaminated” solution has been developed for the coastal ocean. The effect of lands is retrieved using the quantity of land seen by the radiometer. Another wet tropospheric correction is also useful near the coast: the composite_wet_tropo_corr which mixes modeled and radiometer corrections.
No particularly editing is computed, only a selection on the extreme values between the alt - range_red3_ku is done: is_bounded(-130,(alt-range_red3_ku),100).

Netherveless, this raw 20 Hz PISTACH SSHA appears too noisy. To study the variation of the ocean surface, a post-processing (filtering + editing) is needed.
To filter and select data at the same time, an iterative strategy has been tested. Both a median filter and a Low-Pass filter (cut-off length L=7 km), associated with a 3-sigma data selection on the difference (filtered-non filtered), have been applied on the raw 20 Hz PISTACH SSHA data.
Applying this method provides high-resolution SSH anomalies along the tracks without instrumental noise nor erroneous data but with more pronounced meso-scale signals than its classical products (see fig. 2).
The filtered data represented here cannot be directly computed with the Basic Radar Altimetry Toolbox (yet). After its computation, they were inserted in an another Dataset named dataset_filtered_cy5_tr222. Another operation, operations_filtered_cy5_tr222, is created to compute the SSHA (on the base of the SSHA1 formula above).

The two operations are displayed on the same graph. On the Views menu, a view name is created with both operations, by ticking "group expressions in the same plot" (NB: in the Operations menu, both operations must have exactly the same name as "X" to be used on the same plot).

fig 2: Along-track SSHA - Cycle 005 - Track 222 near the coast, made from the raw 20 Hz PISTACH coastal products (in red) and from the filtered SSHA (blue) computed from the coastal products.

to represent the SSHA on a latitude-time diagram

Two new datasets are created, one per track: named dataset_filtered_cy1_cy55_tr146 and dataset_filtered_cy1_cy55_tr222, they contain all the coastal filtered data between cycle 3 (August 2008) and cycle 55 (January 2010). The operations (one per track) are defined as above for the field Data expression (see SSHA1). The "X" is the latitude and the "Y" is the time. Here in our filtered reprocessed data, the variable time is the cycle (for the raw PISTACH product, the cycle is not a variable; but its variable "time" in seconds can be chosen. In this case, an appropriate expression can change the time in seconds in days: round(time/24/60/60)).
In "Set Resolution/Filter" (still in the Operations menu), the X resolution sets to 1/15°.

2.1.4 The North Western Mediterranean Sea by using coastal dedicated products