2.2 Internal waves in the South China Sea

Selecting the area of interest IW crest length IWs near Dongsha Atoll

LESSON 2

Internal
Waves I

References:
References for Lesson 2

Images in this lesson:
(Resizable pop-ups)

ASA_WSM_1P~ 20030815~.N1

Bathymetric map of the South China Sea
(18K)

Useful information:
(Resizable pop-ups)

IWs in the South China Sea

Solitons: Solitary internal waves (Theory and equations)

Identification of internal wave trains and their direction of propagation

The second case study is from the South China Sea - another area where you may see large amplitude,long-crested internal solitary waves. The waves are generated by the interaction of the tidal current with shallow bottom topography in the Luzon Strait (between Taiwan and Luzon). Smaller waves may also be seen on the continental shelf, generated by tidal flow over the shelf break.

Selecting and opening the area of interst

Open the file ASA_WSM_1PPIPA20030815_~.N1 and load the data, using a sampling rate of 10 in both X and Y dimensions and an autolinear stretch. You should see an area in the top half of the image, with bright lines running in a roughly vertical direction - the crest of internal waves.

Note: You will probably notice that the intensity of the radar return signal changes across track from relatively low (dark) on the left side to relatively high (bright)on the right. This is because of the change in incidence angle of the radar signal relative to the sea surface. In level 2 data a correction would be applied to adjust for this across-track difference in signal return. This image is level 1 data, so no correction has been made. However, the correction is not essential for the activities you will carry out in this part of the lesson.

Use box selection to select an area that includes the IW region in the top half of the image and stretches beyond the sea-mount features you can see in the bottom part of the image. Avoid the dark left edge and the bright right edge a far as possible. (See example (31K)).
Open the Go-to dialogue ( [CTRL+G] ), uncheck the 'Coords' box and make a note of the top left pixel coordinates and the size (DX and DY) the selection (in pixels). (Example (1K)).
Use the [x,y] coordinates and selection size to calculate the coordinates you should enter in the 'Extract' dialogue to this selection.
Open the selected area at full resolution, using an autolinear stretch. (Extract dialogue for example (1K)).
Select the whole image ( [CTRL+A] ) and Apply a 5x5 mean filter to smooth out speckle.

Calculating IW crest length

The crest length, d, of an internal wave may be measured using the formula:

d = equation

where the pixel length of the ASAR WSM 1P image is 75 m. This formula assumes that the wave crest follows a straight line, an assumption which is reasonably true for the brightest of the internal waves in this image.

From the tool bar choose 'Line selection' (figure (1K)).
Place your cursor at one end of the wave crest of the long, bright wave in top left quarter of the selection (figure (12K)). Holding the mouse down, draw a line along the crest of the wave.
Open the Go-to dialogue ( [CTRL=G] ), uncheck the 'Coords' box and make a note of the values of DX and DY values (see example (5K)).
Calculate the length of this section of the wave-crest using the formula above.


Question 1.
a)	What is the crest length of the part of this wave that is visible in the image?
b)	What are the main sources of error in this way of measuring the crest length?

IWs near Dongsha Atoll

Internal waves in this region tend to interact with Dongsha Atoll, a major coral reef, which is usually submerged except for a small section to the west. The interaction between the shallow atoll and the internal waves, which propagate past it in a westerly direction, can create some interesting effect, which you should be able to see in this ASAR image.

Answers:
(Resizable pop-ups)

Answer 1

Answer 2

Back up to:
Q1

Open the bathymetric map of the South China Sea. Place the map and the Bilko applications window side by side so you can see both at the same time. To be able to compare the to properly it is best to resample the filtered ASAR image to a standard latitude longitude grid.

From the Image menu select Resample.
When the resample dialogue appears, accept the default settings (figure (5K)) and press OK. A new image appear, which is resampled to a standard grid.


Question 2.
a)	Can you find Dongsha Atoll on the image? If in doubt you can check the latitude and longitude of the pixels on the Bilko status bar (figure), and compare it to the position of the Atoll in the bathymetric map.
b)	Internal waves that pass the atoll to the North and South often interfere when they meet to west of the atoll. Can you find this type of intereference pattern in the bands of internal waves in this image?

Next: Internal Waves in the Gulf of Cadiz.