Satellite altimetry dedicated to coastal ocean.

 

Satellite altimetry is limited near the coastlines due to land contamination in the altimetry and radiometric footprints (10 km and 50 km of footprint diameter, respectively) but also to 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 along-track Delayed Time Sea Level Anomaly (CoastalDT-SLA) products distributed by Aviso in the Florida Keys area, a chain of little islands and reefs in South of the Florida Strait. We will obtain a map with the filtered across-track geostrophic velocities anomalies for a given date.

Data used

The Jason-2 Coastal along-track Delayed Time Sea Level Anomaly (CoastalDT-SLA) product over the Florida Keys area is used here.

The CoastalDT-SLA are Level-3 products, conceived as an evolution of the Jason-2 Level-2 coastal products, commonly called Pistach products. The Level-2 products rather address altimetry experts and are quite difficult to use for non expert users.

The CoastalDT-SLA products are easier-to-use and offer high resolution (20-Hz sampling rate) sea surface height on reference tracks. Differently from the Level-2 products, the CoastalDT-SLA products are only computed over specific ocean areas (see details on the Aviso website and in the dedicated User Manual)..

Download CoastalDT-SLA data file on the Aviso FTP server for the Florida Keys area. An authentication is required to access it. It can be obtained by filling in the form on Aviso and by selecting the CoastalDT-SLA product on the second page. Once connected to the server, files are in the sub-directories named /experimental/coastal_dt_sla_j2.

Ssalto/Duacs Along-Track Sea level anomalies are used here for comparison with the CoastalDT-SLA products. They are also distributed by Aviso and can be downloaded from the same authenticated FTP server with the same login/password (see details on the Aviso website and in the dedicated User Manual). Once connected to the server, files are in sub-directories named /global/dt/upd/sla/j2_cf and files are tipically named dt_upd_global_j2_sla_vxxc_20090805_20090812_20110329.nc.gz. We choose here to plot the cycle 42 which corresponds to 2009/08/26 (use the tool conversion on the Aviso website to recover the cycle at a given date and vice versa). Download the corresponding file included the cycle 42: dt_upd_global_j2_sla_vxxc_20090826_20090902_20110329.nc

 

Methodology

We use the Broadview Radar Altimetry Toolbox to observe the data, do some computation and plot the accross-track geostrophic velocities anomalies.

Data chosen

  • For the CoastalDT-SLA product, the Jason-2 interesting pass over the Florida Keys area is descending pass: #102. The time series used for this data use case streches out from the cycle_001 (July 2008) to the cycle_110 (July 2011) (all the cycles are included in the file).
  • Ssalto/Duacs Along-track Sea level anomalies used for comparison with the CoastalDT-SLA products contain all the available passes during a 7-days period. A selection based on the pass number (#102) is shown in BRAT (in Operations tab, see the following step).
flokeys_area_tracks_sm

fig 1: Jason-2 ground track over the Florida Keys area included in the CoastalDT-SLA product.

More altimetry data close to the coasts

Dataset

Once the relevant files are all downloaded, create a dedicated workspace in BRAT and then, create two datasets in the “Dataset” tab:

  • a first one, named “flokeys_coastal_102” includes the CoastalDT-SLA product with only one file: coastal_dt_sla_j2_agulhas_102.nc. It contains all the time series between 2008-07-17 and 2011-07-07.
  • the second, named “flokeys_duacs_102” includes the Ssalto/Duacs SLA file between 2009-08-26 and 2009-09-02.

Operations

On “Operations” tab, we create one operation for each dataset, CoastalDT-SLA and Duacs-SLA, in order to plot the map of filtered accross-track geostrophic velocities . These operations are respectively named “Op_flokeys_coastal_102” and “Op_flokeys_duacs_102”.

  • Operation “Op_flokeys_coastal_102”: in “Operations” tab, click on “New” button to define a new operation, give the choosen operation name in the “Operation name” box.
  • In Data expression, drag and drop the variables “longitude” in “X”, the latitude in “Y” and the “SLA_Filtered_41pts_RED3” in “Data”.
  • With a right-mouse click on Data, select ”Insert empty expression”, and rename this new expression with “U” (for East component). Click on the grey box “Expression” and then on the “Insert algorithm” button. A pop-up window opens. Choose the BratAlgoGeosVelAtp algorithm. A function-like expression is inserted in the Expression box. You have to check that the variable “Height” matches the existing variables within your dataset (here, you have to drag and drop the “SLA_Filtered_41pts_RED3” from “Fields” up to the “Height” field).
  • Then, a Lanczos filter is appplied to “U”. Always by hovering your mouse cursor on the grey box “Expression”, click on the “Insert algorithm” button. The same pop-up window opens. Define the Lanczos filter computation from the proposed algorithms: BratAlgoFilterLanczosAtp. Click on “OK” to close the pop-up window. A function-like expression is inserted in the Expression box. The Window Length, the Cut-Off and the minimum of valid points have to be defined. We respectively set 41,30 and 1. Finally, the filtered geostrophic velocity “U” computation is :
  • exec(“BratAlgoFilterLanczosAtp”,exec(“BratAlgoGeosVelAtp”, %{lat}, %{lon}, SLA_Filtered_41pts_RED3),41,30,1, 0)
  • The “Y” spatial resolution is set to 1/99° in latitude in “Set Resolution/Filter” (it corresponds to the inverse of the difference between two consecutive latitudes).
  • Do the same for the North component. Set the “X” spatial resolution to 1/211°.
  • A selection criteria is applied to only select a given date by choosing the cycle 42. By clicking on “Selection criteria”, move your mouse cursor on the grey box “Expressions” and write: cycle==42.
    Execute the operation “Op_flokeys_coastal_102”.
  • Operation “Op_flokeys_duacs_102”: The previous Operation can be copied to create the operation corresponding to the “flokeys_duacs_102” dataset. Then, it will adapted to this new operation. To do that, select the previous operation (“Op_flokeys_coastal_102”) and click on the “Duplicate” button. Change the name to refer to the new operation name (“Op_flokeys_duacs_102”) and be aware to well select the corresponding Dataset with the corresponding Operation.
  • In Data expression, we have the variables “longitude” in “X”, the latitude in “Y”. Delete the field under “Data” corresponding to the previous Operation (field “SLA_Filtered_41pts_RED3”) and drag and drop the “SLA” variable in “Data”. For the “U” and “V” expressions, only change the SLA field. Since the file resolution is different, change the “Step” in “X” resolution at 1/11° and the “Y” resolution at 1/19°. Select the unique pass #102 in Selection criteria by deleting the previous criteria (cycle==42) and by writing: track==102.
  • Once the Operation has been defined, click on the “Execute” button.

Views

Once the Operations finishes, define one View per Operation in the “Views” tab:

  • View “Vi_flokeys_coastal_102”: drag and drop the U and V of the operation “Op_flokeys_coastal_102” from the list in the “Available” box to the “Selected” box on the right. Click on the “Selected” U expression and below, click on the box “East component”. Then click on the “Selected” V expression and below, click on the box “North component”. Finally, click on “Execute”.
  • View “Vi_flokeys_duacs_102”: Do the same for the operation “Op_flokeys_duacs_102”.

Displays

Once the Views are ready, define one Display per View in the “Display” window.

  • Display for CoastalDT-SLA product: choose your projection, the vector scale (here we have taken 1) and the range (0 to 2). Then, you can save the figure.
  • Display for Duacs-vxxc product: choose your projection, the vector scale (here we have taken 1) and the range (0 to 0.6). Then, you can save the figure.

Results

flokeys_coastal_JA2_tr102_cy42_UV_ATP_sm flokeys_duacs_vxxc_JA2_tr102_cy42_UV_ATP_sm
fig 2: Map of filtered accross-track geostrophic velocities anomalies for the CoastalDT-SLA product (left) and the Ssalto/Duacs Along-Track “vxxc” product (right), along the Jason-2 pass #102, for the cycle 42 (2009/08/26), around the Florida Keys. Note that the vectors scale are not the same n the two panels.

Thanks to the CoastalDT-SLA processing associated to 20Hz processing, a better along-track sampling is obtained, more data are recovered near the coasts, on both sides of the islands, compared to Ssalto/Duacs along-track “vxxc” products.

Further information:

  • Other Data Use Case using CoastalDT-SLA products: The Agulhas current by using coastal dedicated products
  • User Handbook: CoastalDT-SLA User Manual
  • M.Cancet et al.: Anomalies Derived from the PISTACH Products: the Loop Current Case (pdf, Presentation at the San Diego Coastalt meeting, 2011)
  • S.Labroue et al.: Level-3 PISTACH Products for Coastal Studies (pdf, Presentation at the San Diego Coastalt meeting, 2011)
  • V.Kourafalou et al.: Modeling and in Situ Observations around the Florida Keys Coral Reefs: Potential Applications of Coastal Altimetry (pdf, Presentation at the San Diego Coastalt meeting, 2011)
  • Y.Liu et al.: Altimetry on the West Florida Shelf (pdf, Presentation at the San Diego Coastalt meeting, 2011)