3.2 Surface wind in the study area

This particular region of the Mediterranean Sea is dominated by the surface wind field which drives the surface circulation - especially the cold Mistral and Tramontane winds that blow from the North and North-west respectively. We will use wind field data from the Met Office second-generation European wave model to investigate the wind field on the same day that the AATSR data were acquired.

Note: During this exercise, remember that when a wind direction is quoted, this is the direction from which the wind is blowing i.e., a ÔnortherlyÕ wind is a wind that comes from the north. This is opposite to the convention used for ocean currents and other oceanographic features; for these the given direction is the direction that the feature is moving towards i.e., a northerly current moves in a northerly direction propagating towards the north!

The image data files cdwe020727wind_direction.dat and cdwe020727wind_speed.dat provide a forecast estimate of the wind speed (in m s^-1) at a height of ~20m above the sea surface and the associated wind direction (in degrees true). They are generated by the Met Office second-generation European wave model for T=1:00Z on the 27th July 2002. Each pixel represents an area of ~35 km², so the image is quite small and difficult to use. To make it easier to use the images with the AATSR data, zoom in:

• right-click on the image and select Zoom to open the zoom dialog,
• select the Preserve Shape option under Zoom to Fit Window, and
• drag on the bottom right corner of the image window to change the zoom until you get a suitable display size.

Displaying the wind speed data

If you simply opened the images accepting the defaults in the Redisplay window, you will have two black and white display in front of you, where land areas are white and sea areas are black. This is because land pixels have been given a very number - to distinguish them from the sea pixels with real wind data. To get a more sensible display you therefore have to open the Redisplay and set the Null values. We will start with the Wind speed image, which is the simplest of the two to handle.

1. Right-click on the wind-speed window to open the Redisplay dialog.
2. Set the Null value to greater than ( > ) some unrealistically high wind speed - e.g. 300.
3. Choose an autolinear stretch; this should work well.
   Note: You may also like to check the autolinear stretch options: Open the Stretch menu, and select Options; then in the Stretch Option dialog set Min to 0% and Max to 100%, before applying the autolinear Redisplay stretch again.

Displaying the wind directions

Finding a good way to display the wind direction data is a little trickier. Wind fields are vector data, and should ideally be displayed with arrows, where the arrow points in the wind direction, and the arrow colour gives the strength of the wind. This option is not available in Bilko, so we are left with displaying the two separately and way to display the directions so that the image makes visual sense.

The first step is finding a suitable Redisplay stretch. Wind directions are given in degrees true - with 0° = 360° representing winds from the North, 90° wind from the East, 180° wind from the South, and 270° wind from the West, so the Redisplay stretch has to cover this range. As for the wind speed data, the Null value, given to land pixels, and missing data, is very high.

1. Right-click to open the Redisplay window
2. Set the null value to > 360 (no directions will be more than 360°),
3. and set the Max and Min of the stretch to 0 and 360 respectively.

Question 1
a)	What is the main problems with displaying the data in this way? (Hint: How does this stretch display northerly winds with a small westerly component, and how does it display northerly winds with a small easterly component?)
b)	Try applying the pyramid stretch supplied: Select Open from the file menu (keystroke [CTRL+O]), Change the File type to STRETCHES and select pyramid_stretch.str How does this display compare with the original display?
c)	There are a number of colour palettes provided with this lesson. Try these out to see if you can find a suitable display: First of all clear the manual pyramid stretch from the wind direction image by selecting Clear from the Stretch menu, Select Open from the file menu, change the file type to PALETTES, select one of the palettes from the list, select one of the palettes from the list, and click Open to apply the palette to the image Which of the palettes provided would you say is the most suitable for this image? What are your reasons for this choice?

Before you continue you might like to add a colour palette to the wind-speed data; rainbow.pal or modjet2.pal would both be suitable for this. (Palettes are often a question of preference, so you may prefer working with grey-scale for the wind speeds. There is no strong reason for applying a palette, as in the case of the wind directions.)

Interpreting the wind fields

Question 2
a)	What are the maximum and minimum wind speeds forecast for this area? A quick way to check this is to use a Histogram of all the pixels: From the Edit menu choose Select All (keystroke [CTRL+A]), From the File menu choose New (keystroke [CTRL+N]), In the 'New' dialog, make sure the Apply stretches check-box is unchecked, select HISTOGRAM and click OK. This will display a Histogram of pixel values (in m s-1 that give you the wind speed range of the forecast.
b)	Where to you find the stronger winds, and what directions are these winds coming from? (Use the map in figure 6 to help you analyse the wind fields).

The light nortwesterly surface wind over the Gulf of Lions is known as the Tramontane Wind. In the Northern hemisphere winds from the North and West are known to cause upwelling along the south coast of land masses; this may explain the areas of cooler water along the coast in the Gulf of Lions. The wind field may also help explain a number of other features seen in these scences. A comparison of the SST and wind-field data will become easier if the images are in the same map projection.

Resampling the scenes to a Lat/Lon projection

A comparison of the SST and wind-field data will become easier if the images are in the same map projection. This means rectifying the SST data to a Lat/Lon projection, using a window that corresponds to that use in the wind-field data. Here is how:

1. First determine the corner co-ordinates of the wind-field data:

   Place the cursor on the top left corner pixel of the wind image, and read the Lon/Lat coordinates from the Bilk status bar. (Top left is 1° 39' 45" W, 44° N; i.e. approximately 1° 40' W, 44° N;) Read the coordinates of the bottom right pixel the same way Note: Remember to take account of the fact that pixel locations are given from the top left corner, so the bottom right of the image is actually further South and East than the co-ordinates of the bottom right pixel - pixel size for the wind field data is about 24' in the E-W direction and 15' N-S direction, so the bottom right corner is more about 8° 20' E, 35° 45' N.

2. Use the cursor to get a rough idea of the pixel size in the AATSR data - this is about 40" in the E-W direction and 32" in the N-S direction.

3. Having determined these corner co-ordinates you will now resample the Tb images to a Lat/Lon grid with these corner co-ordinates and pixel size:

   Activate the first Tb_1100 image (of the African coast), open the Image menu and select Resample , select the Window tag, type the corner co-ordinates from above (figure), and click the Apply button to set the new window; select the Pixel tag and set the pixel size (figure), rememering to click the Apply button; select the Image tag and check that the image has 901 columns and 929 rows, Finally select the Interpolation tag and check that the default choice is Based on 'Self' and the Method is 'Nearest neighbour, before clicking OK to carry out the resampling.

4. Open the Edit menu and select Coords to open the 'Set Coordinates' dialog, and check that the top left Longitude and Latitude, and the pixel size are as you expected before clicking OK.

5. Don't worry about applying a Redisplay stretch (this will come later), but save the image as ats1_tb_1100_r.dat

6. Repeat (3-5) for the second Tb image (European coast) and save this as ats2_tb_1100_r.dat

The final bit of processing before we can compare the Tb and wind-field data is to put the two scenes together in one image:

1. Open the Bilko FORMULA document merge_aatsr_scenes.frm and familiarise yourself with the content.

2. Right-click on one of the resampled images, select Connect from the pop-up, highlight both the resampled images, add 1 blank, check the Stack check box, and click OK to create a set of three images.

3. Check the Redisplay stretch for both the rectified images to ensure that no null value has been assigned.

4. Set the output image type to 16-bit unsigned integer by

   either	opening the Image menu and selecting Options from the drop down, then change the Output Image Type to what you want;
   or	activate the FORMULA document, and select Options! on the menu bar, then change the Output Image Type that way.

5. Copy and paste the formula document onto the connected set.

6. Save the new merged image (@3) as ats_tb_1100_r.dat, close the set, and re-open the saved image.

7. Right-click on the new image to open the Redisplay dialog, set the null value to 0, and select a linear stretch with min=28615 as before.

8. Right-click on the new image to open the Redisplay dialog, set the null value to 0, and select a linear stretch with min=28615 as before.