6.5 Surface Current Vectors from the Global Drifter Program: ASCII Spreadsheets Option

Exercise Title: Surface Current Vectors from the Global Drifter Program: ASCII Spreadsheets Option Abstract: In this exercise you'll learn an infinitely easier way to manage and visualize the Global Drifter Program climatological surface current vectors. The huge ASCII table of all monthly statistics has been converted by the MDL editor to a set of 12 monthly tables. This allows for direct gridding and display in Saga, which was impossible with the complete table. These smaller ASCII tables import directly into Saga and are immediately ready for conversion to point shapes and quite ordinary gridding, followed by current vector displays. [This exercise is intended to replace completely the older exercise 6.3 Surface Current Vectors from the Global Drifter Program: NetCDF Option, which is kept in MDL only for historical reference.] Preliminary Reading (in OceanTeacher, unless otherwise indicated): Scalars and Vectors Required Software: Saga Other Resources: Global Drifter Program (GDP) - Satellite-tracked surface drifting buoy observations of currents, sea surface temperature, atmospheric pressure, winds and salinity A drifter-derived seasonal climatology of global near-surface currents FTP download files for the monthly climatology (all in ASCII format) frame_grid_liberia_0.5deg - Dummy grid you made in the exercise Creating Grid Templates in Saga Author: Murray Brown & Lilian Krug, using materials published by the GDP Version: 07-Feb-2019
1. The above ASCII data files (whichever ones you want) should be downloaded to DATA > OCEAN > GDP. You can use the file names used below, or the original filenames. They all have months names to identify them correctly. But please use the latest version of the dataset.
2. Run Saga. Use FILE > GRID > LOAD to load the 0.5-degree dummy grid listed above Then use FILE > TABLE > LOAD to load any months you want to work with right now. Here you can see 4 of them (JAN, APR, JUL, OCT).
3. Open any table to see its contents (from the GDP webpage): Lat (1x317): latitude (degrees), negative = south Lon (1x720): longitude (degrees), negative = west. Ubar (317x720): mean eastward speed (m/s) vs. Lat/Lon. Vbar (317x720): mean northward speed (m/s) SSTbar (317x720): mean sea surface temperature (degrees C) eUbar, eVbar, eSSTbar: Not included after Version 2.06 N (317x720): Number of drifter-days per square degree
4. To work with the data, we need to convert the tables to point shapes. Select TOOLS > SHAPES/POINTS > CONVERT TABLE TO POINTS. TABLE - Select the month you want, e.g. January X - Select LONGITUDE Y- Select LATITUDE Z - Leave it alone to select all the other fields Then click OK.
5. Now you have your first point shape, as you see here. Check to make sure.
6. Here you can use ADD TO MAP to see if it resembles a global map. Seems to be ok.
7. To fix the map and make it easier to see, make these settings: OUTLINE - Unchecked COLORS - TYPE > GRADUATED COLORS COLORS - SCALING > 100 COLORS and PRESET PALETTE > RAINBOW VALUE RANGE - MINIMUM > 2 VALUE RANGE - MAXIMUM > 40 VALUE RANGE - ATTRIBUTE > SSTBAR (just as an example) ATTRIBUTE - DEFAULT SIZE > 1 Now click APPLY at the bottom.
8. You should see something like this. It indicates you're working with good data and they are behaving as expected. Remember, these are thousands of actual surface measurements, not satellite data.
9. And here we've zoomed into the Liberia area to confirm that the original GDP data are a regular array of points.
10. Now to work with these data to see the current vectors, we need to grid the point shapes.
11. Select TOOLS > GRID-GRIDDING > INVERSE DISTANCE WEIGHTED (the gridding algorithm we frequently use in Saga): POINTS - Select the new point shape (January in this case) ATTRIBUTE - Select Ubar (the east-west component) TARGET GRID - Select GRID (means we will specify a dummy grid in the next step) SEARCH RANGE - Local MAXIMUM SEARCH DISTANCE - 1 (degree) which should be enough for a 0.5-degree array NUMBER OF POINTS - All points DISTANCE WEIGHTING - Inverse distance to a power POWER - 2 Of course you are invited to experiment with these settings and use anything you like better. Now click OK to continue.
12. Now you're asked to specify the dummy grid for the gridding. Select the 0.5-degree dummy grid. Make certain that GRID is set to CREATE, or you might lose something you want to keep. Now click OK.
13. Now a new grid appears ("Ubar") in the same system as the dummy grid.
14. You can use ADD TO MAP to see the grid. The strong eastward current along the coast of Cote d'Ivoire and the westward flow in the equatorial area are both expected features. The LEGEND confirms the data values must be m/sec.
15. Use the same method to grid the Vbar data. It is again extremely important you set GRID to CREATE, so you don't over-write the previous grid.
16. And here is the Vbar grid. Typically, north-south components are much weaker than east-west, as you see here.
17. Check to make sure that you have these 2 new grids in the proper system.
18. Now that we have the U and V grids, we can make current vectors.
19. Select TOOLS > SHAPES-GRID > GRADIENT VECTORS FROM DIRECTIONAL COMPONENTS: GRID SYSTEM - The dummy system with the new grids X COMPONENT - Ubar Y COMPONENT - Vbar STEP - Distance between vectors in the source grids; usually begin with 1; increase if arrows are too crowded SIZE > MINIMUM - If not sure, then select zero to be safe SIZE > MAXIMUM - Select 1000 to be safe; expenence with Saga shows this item behaves quite erratically from dataset to dataset AGGREGATION - Mean value; if multiple values are given for a single location STYLE - Cell-centered arrow Now click OK to make the vectors.
20. A new line shape appears with a double name indicating the sources of the components.
21. And here you can see the vectors by using ADD TO MAP.
22. Here are some adjustments you can make in the properties panel: STEP - Set to 2 to double the distance between the arrows MAXIMUM - Reduce to 400 Click APPLY in the bottom of the panel.
23. Now the vectors are not so crowded, and much easier to understand.
24. Here are some more adjustments you can make to improve appearance: TYPE - Graduated colors COLORS - PRESETS > RAINBOW ATTRIBUTE - LEN (sets the palette colors to reflect the "length" of the vector arrows) Click APPLY in the bottom of the panel.
25. These are the best vectors you can do in Saga, with 2 kinds of visualization of the currents speeds: arrow size and arrow color.
26. If you want to use these vectors in another system, Integrated Data Viewer (IDV) for example, right-click on the line shape and select SAVE AS.
27. Navigate to PRODUCTS > SAGA > VECTORS and save the shape with the name current_surface_liberia_jan_gdp_ver2.06_saga.shp
28. And here, in IDV, is the same shape, added through DATA CHOOSERS > FILES. Note that this object is a true vector, not a pixilated image, so you can zoom in or out without any distortion.
29. Also in Saga, you can use IMPORT/EXPORT-GDAL/OGR > EXPORT SHAPES TO KML to create vector shapes for Google Earth that work the same way.
30. If, though, you want to display very large areas or even the entire globe, then the initial display might look a little strange. If you want to deal with these vector fields, then go to the older exercise, 6.3