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 of footprint  diameter 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.

Methodology

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

The time series used for this data use case stretches 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

med_bathy_4tracks_sm
fig 1: Bathymetry (in meters) below four Jason-2 tracks in the North Western Mediterranean Sea. This map is plotted using the BRAT 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 range term (range_red3_ku) 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 modelled 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).

Nevertheless, 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 Broadview Radar Altimetry Toolbox (yet). After its computation, they were inserted in 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).

med_sla_BrutClassic2_ResidusClassic2_red3F_cy5_t222_sm

fig 2: Along-track SSHACycle 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°.

Results and comments

med_SLA_hovmoller_tr146_near_coast_sm med_SLA_hovmoller_tr222_near_coast_sm
fig.3: Latitude-time diagram of coastal filtered Sea Surface Height Anomalies, between 42°N and 43.2°N, between August 2008 (cycle 1) and January 2010 (cycle 55).

 

The coastal filtered data are used to try and monitor the variability of the coastal current in the Northwestern Mediterranean Sea. The large scale currents of the Northwestern Mediterranean Sea are dominated by a cyclonic gyre. The Western (WCC) and Eastern Corsica Current (ECC) join to form the Northern Current (also named the Ligurian-Provençal Current, LPC) in the Ligurian Sea.
The LPC flows along the continental slope of the Ligurian Sea and the Gulf of Lion down to the Catalan coasts. If the continental shelf is wide in the Western part (Gulf of Lion, tracks 146 and 187), it is narrow in the Eastern part (Ligurian Sea, tracks 009 and 222). The signature of this LPC current in the altimeter data may be located nearer the coasts (~10 km) in the Eastern part, which is the strong interest for the coastal PISTACH data.

Further information:

  • Coastal and Hydrology Altimetry product (PISTACH) handbook (pdf)
  • Dufau C., et al., 2010, Use of Pistach Products in Coastal Studies. Oral presentation at the 4th Coastal Altimetry Workshop, Porto, Portugal.
  • Mercier F. et al., 2010, The Pistach project for coastal and hydrology altimetry: 2010 project status and activities (pdf). Poster presentation at the OSTST 2010 Lisbon, Portugal