Visualizing Satellite-Measured Sea Surface Salinity in IDV: MIRAS via CoastWatch

Marine Data Literacy

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9.26 Visualizing Satellite-Measured Sea Surface Salinity in IDV: MIRAS via CoastWatch

Exercise Title: Visualizing Satellite-Measured Sea Surface Salinity in Integrated Data Viewer (IDV): Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) via CoastWatch
Abstract: In this exercise you'll go to the US NOAA CoastWatch site (or maybe it's the OceanWatch site; who knows?). There you can find lots of near-real-time products, but the vast majority are regional, not global, and there are very few in the formats we like for operational products (especially NetCDF). We focus on the new MIRAS salinity products, and show how to visualize them in IDV, including subsetting and smoothing options.
Preliminary Reading (in OceanTeacher, unless otherwise indicated):
- Remote Sensing Measurements
- Marine Parameters from Remote Sensors
- Self-Describing Formats - Includes HDF
Required Software:
- Integrated Data Viewer (IDV)
Other Resources:
- CoastWatch - Earlier information that this site was being phased out appear to have been incorrect. It is alive and well, but populated with a less-than-ideal product suite.
- OceanWatch - This site has not replaced CoastWatch, and in fact it appears to have been abandoned. Very messy website now, with dead ends and inactive logins.
- MIRAS Instrument on SMOS - New EU salinity products to rival US Aquarius
Author: Murray Brown
Version: 9-6-2014

1. Open the CoastWatch website, and spend some time reading over the resources provided. [For your information, this site was apparently being converted to a global facility named OceanWatch, but this has been terminated in favor of a complete expansion here. CoastWatch folks are invited to let me know when this situation changes.]

2. Click on the DATA ACCESS link in the left-hand menu. Take a few minutes to read over the materials provided.

You will not use the menu in mid-page, because it contains many regional or specialized products. We only want products that are globally available. So we will use the menu system on the left margin.

3. But at the bottom of this same page you'll find these "ALTERNATIVES". We don't suggest their use, because at least one of them, the all-important THREDDS link, is broken.

4. Before we advise you what to get from this site, please consider these issues:

Most of the CoastWatch data products are not in optimal formats for operational use ... especially the HDF!
Only the SALINITY data are in NetCDF grids, so we limit this exercise to those data
CoastWatch folks are invited to let me know when this situation changes. HDF just won't cut it anymore, and your user community is looking forward to wall-to-wall NetCDF.

So make any selections you want, but for this exercise, please make these choices. since Aquarius is covered in a different exercise we'll focus on SALINITY > MIRAS here.

Then click SEARCH.

5. After a quick search, these products are listed. For your own searches later, try to keep the options limits so you don't get a huge pile of products that are difficult to pick from.

6. For this exercise, we click on the very latest one, SEP 3 (for today, which is SEP 6 for the author).

Look just below the map, and you'll see what digital products can be obtained. The NC (for NetCDF) is our main interest. The others are merely images.

Save the NC file to the folder LIBERIA > DATA > OCEAN > COASTWATCH with the current filename SMOSCW_S2014246_3day_WW00_sss.nc (or whatever you have selected).

7. You can also save the metadata file (TXT) for permanent record of the contents, etc.

8. Save it in the same folder as the data, with the filename SMOSCW_S2014246_3day_WW00_sss.txt

9. Now we can work with the file in IDV. You'll need to do some reading on your own to see more about the derivation and actual contents of the salinity grids. This exercise just shows you how to work with them as digital objects, not their background or importance. In IDV you can make these selections to visualize the grid. Notice there are 3 possible "models" for the calculation of the salinity, and here we've just picked the middle one out of laziness. You need to research this issue, compare the products and decide which you want to use for your own work in the future.

Now click CREATE DISPLAY to see the data map.

10. And here it is, shown using the author's favorite color palette RADAR > DBZ. Please notice a couple things:

The overall map is smoother and more "complete" than the the US Aquarius products
There are numerous "outlier" areas along the African and Asian coasts. These appears in north-south bands that hint at local data issues

11. But if you only want Liberia data, how do you get it? Right-click on the data object in the left-hand menu. Then select PROPERTIES.

12. When this window opens, select the SPATIAL SUBSET tab. Then draw a rectangle anywhere on the map with your cursor. Then you can enter the correct coordinates for Liberia, as you see here.

Then click OK.

13. If you have selected the same DISPLAY settings as above, including the palette, then this is the data map for Liberia subset from the global grid. In general the values are higher in mid-ocean, but the pattern is very messy.

14. To clean up the messy product, you can go back to the dashboard for the subset map, and select SMOOTHING > CRESSMAN WEIGHTED.

15. The grid map automatically changes to this very smooth, easily understood image. You can try other smoothing algorithms on your own to select the one that works best for your data. Or you can always ignore smoothing, and work with the raw extract; it's all up to you.

16. If you want to save this new grid, then use the method in 5.10 Basic Grid Operations, Calculations and Subsetting in IDV

9.26 Visualizing Satellite-Measured Sea Surface Salinity in IDV: MIRAS via CoastWatch