3.4 Look-angle differences in Tb

Before continuing with this section, close all open windows except ATS_TOA_1COLRA20020727_211258_~.N1. (You can do this by selecting Close All Except from the Windows menu). We are now going to investigate the differences between the nadir and forward view image data sets derived from the AATSR.

Activity:

1. Select the Bands folder to display all the bands, hold down the [CTRL] key and select all the following bands from the right frame: btemp_nadir_1200, btemp_nadir_1100, btemp_nadir_3700 btemp_fward_1200, btemp_fward_1100 and btemp_fward_3700.

2. Open all of the image bands as a connected stack image by right clicking on the selected band folders and selecting Open Connected.

3. Accept the default in the Extract Box, enter the same settings as previously in the Redisplay dialog (Null value <=27315, Stretch Min = 28615 and Max 30000, and click the All button to apply the settings to all the images in the stack. (If you click Apply, you have to set the Redisplay options for all images in the stack one by one).

4. Re-orient the stack image data set to show the northern part to the top of your screen by reflecting the image data in X and Y directions, and use the [TAB] key to look at each of the images in turn.

Differences between AATSR channels

You will see how the brightness temperature, Tb, changes from band to band. You may also notice the smoother texture of the AATSR forward view images; this is due to the larger FoV inherent when viewing the sea surface at angle of 55° from nadir. These larger pixels from the forward FoV have been re-mapped into a 1km GBTR array that matches that of the nadir data. If you take a transect vertically across a feature such as the large eddy in the Gulf of Lions, you can see the differences between the bands more clearly.

1. Select the line button on the tool bar, and use the mouse to draw a vertical line across a feature with large variations in Tb.

2. Select New from the File menu ([CTRL+N]), choose TRANSECT document, making sure the Apply stretches checkbox is unchecked before clicking OK.

3. To see the legend corresponding to the coloured lines, click the down arrows in the selector.

If you want to examine the Tb values at a particular point along the transect, place the cursor in that position and [TAB] through. The vertical line representing the cursor will change colour, telling you what band is currently displayed on the Status Bar. When studying the transect in this way, it may help you to be aware that red green and blue (RGB) are always the colours of the first 3 data sets (in this case the 3 nadir bands), and that these are followed by magenta, yellow and cyan (CMY) representing data sets 4, 5 and 6. Thus red and magenta represent the 1200 µm nadir and forward bands, green and yellow the 1100 µm bands, while blue and cyan represent the two 3.7 µm bands.)

Activity / Question 1 Take a transect across an area of warmer water known to be free of cloud, and another across the patch of scattered cloud southwest of Mallorca.
a)	What does this tell you about the diffences between the three nadir bands (RGB)? What happens to this relationship in the presence of cloud?
b)	What are the main differences between the nadir and forward data sets?

When you have finished, close the transect window and minimize the stack window to clear your Bilko for Windows workspace ready for the next part of the lesson.

Spatial variation in channel differences

We can investigate the spatial characteristics of the channel differences that form the basis of the multi-spectral and the multi-angle atmospheric correction strategies applied to Tb to retrieve a SSTskin measurement. We shall do this by using a Bilko formula document to compute the difference between the 11µm and 12µm channels both at nadir and in forward view where the effect of the atmosphere will be stronger.

Activity:

1. Open the FORMULA called Compute_AATSR_channel_differences.frm and familiarise yourself with its content.

2. Connect the btemp_nadir_1100, btemp_nadir_1200 and btemp_fward_1200 bands together with 2 blanks that will be used to store the formula output. (You can scroll in the Connect window to see the channel descriptions at the end of the image names; this makes it easier to select the right channels.)

3. You should now have a second set of 5. Open the selector and check that the channels are in the order described in the FORMULA document.

4. Check the Options! to see that the output is set either to Same as @1 or to 16-bit unsigned integer; then copy the formula and paste it onto the stack to carry out the image calculations.

5. Tab to see the result of your caluclations, and save the two new images as ats1_diff_n11_f12_n12.dat (@4) and ats1_diff_n11_f12.dat (@5), then close the two stacks and the FORMULA document.

6. Reopen the two recently created images, setting no Null values and changing the Redisplay stretch so that Min=-20 and Max=600 in both images. Use Windows > Tile Vertical to arrange them side by side. You should spend some time thinking about the content of these images.

Look at the nadir only difference image. A histogram ([CTRL+A] to select the image followed by [CTRL+N] > HISTOGRAM to create the histogram) will reveal a range of -90 to 400, with 99.7% of the pixels falling between -20 and +270 (i.e. -0.2 and 2.7 °K). Apply a suitable manual stretch to enhance the contrast; play with the position of the stretch nodes to reveal different features within this image. You also may find it helpful to compare this image to the btemp_nadir_1100 image when attempting to interpret these features. For example:

• There is a marked banding running in a roughly NNE - SSW direction.
• Clouds are identified in some areas; typically larger clouds are bright spot differences. However, some clouds are not displayed in this way - for example those seen southwest of Mallorca in the btempt_nadir_1100 image.
• Perhaps surprisingly, oceanographic features are also revealed in the difference map. These could correspond to local structures in the atmospheric water content. Note the warmer differences to the north of the Balearic Islands.
• In the centre area of the Balearic Sea, there are some some slightly darker, fairly diffuse linear features. This is evidence of old contrails from aircraft; new contrails would be much narrower and crisper lines.
• If you zoom in (double-click on the image) and push the stretch to its limit (just before saturation), you may see a series of concentric lines. These are instrument noise, corresponding to the curved scan lines of the AATSR system.

Now look at the btemp_nadir_1100 - btemp_fward_1200 difference image. This is quite different in character. A histogram reveals values ranging between -1100 and +1800, with 99.5% of the pixels in the range -300 to +700. Zooming in on the Spanish coastal area to the north of Mallorca reveals a clear stereoscopic effect due to the different viewing geometry in the nadir and forward views. This makes it possible to identify hilly regions on land and detect clouds at different heights above the sea surface. The stereoscopic effect is particularly apparent when looking at the two small clouds to the east of the Gulf of Lions where the nadir and forward views are separated in space. As before, you can zoom in and push the contrast stretch to reveal how the noise characteristics of the AATSR channels are drawn out.