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Presentation and delivery of the data -- some notes

Some notes regarding the presentation and delivery of the data:  

The colour scale of the plots

The colour scale of the SO2 vertical column plots shows the colour coding for the concentration in Dobson Units (DU). The regular colour bar is from 0.0 to 2.0 DU. If measurements are between 2.0 and 10.0 DU the color is orange. A special colour (red) is used for really strong SO2 signals -- usually these associated with volcanic eruptions -- for measurements higher than 10 DU.

SO2 vertical column colour scale
Colour bar for SO2 vertical column densities.

The reason to end the main scales at 2 DU is to make sure that low level SO2 concentrations are visible, concentrations that can be great interest too, e.g. because they signify small-scale volcanic eruptions. Furthermore, it is most interesting to show where large concentrations of SO2 appear, without being too specific about the exact values, especially because large SO2 concentrations do not occur that widespread or that often. Furthermore, representing all data using the same colour makes it easy to compare results for different periods and regions.

Note that the scale for the SO2 plots shown on the website varies linearly. For special purposes, however, the scale may have other ranges and/or may be logarithmic.

The colour scale of the (effective) cloud fraction varies linearly from zero (cloud free; dark blue) to one (fully clouded; white). Values below zero and above one cannot occur. The snow/ice mode of the cloud data products is represented by very ligth grey (indicated simply as "snow" in the colour bar).

Cloud fraction colour scale
Colour bar for the effective cloud cover fraction.


Plots of overlapping orbits

SO2 over Iceland on 6 March 2007

At high latitudes, above 65 degrees or so, satellite orbits overlap since the swath width of the instrument has a fixed size at ground level (960 km in normal mode for SCIAMACHY, about 2000 km for OMI and GOME-2). When plotting the daily data at orbit coordinates, this plotting is done with the orbits listing in ascending time. As a result, the values shown in the plot at a given location are those of the latest orbit of that day. This can be seen clearly in the SCIAMACHY-based picture shown here.

For the gridded data this is not a problem: all data falling in a given grid cell is averaged for that grid cell and therefore each grid cell has a unique data value.  

Plots for the website made with IDL

The plots of the SO2 data and the associated maps are all made with IDL (version 6.3). Some notes:
  • The IDL plotting routines include the possibility to draw lines along continents and to colour the continents. IDL appears to have some problems correctly colouring the continent Antarctica. The reason for this is that the information for colouring the continents and for drawing the borders is taken from two different databases, which evidently differ at the border of Antarctica. The line around the continents shows the real border.
  • Unfortunately, the lakes drawn on the maps by IDL do not get the same colour as oceans do: lakes get the land colour, whereas one would expect lakes to have the sea (water) colour. This is especially clear in the plots of Eastern Europe, where the Black and Kaspian Sea and several lakes are given the land colour.
  • On the maps of the regions, boundaries of the countries are drawn. These boundaries are the political boundaries from 1993. IDL's database unfortunately does not have all borders correct; for example, in Arabia boundaries are missing. The IDL developers are aware of this problem.
For plots of the daily data at orbit coordinates, rectangles made up of the ground pixel corners as given in the data product are filled with the appropriate colour. Since these rectangles are made up of straight lines, they do not exactly match those north and south of them: there are little openings between the rectangles, because the earth's surface is curved. This can, for example, be seen in the example image shown above and below. It does not show up in the images of the gridded data.  
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