Measuring radiation with different channels

Figure 1: The Sun and Earth's emittance on the MSG channels

The main radiation source for wavelengths 0,4-5 µm is the Sun; the VIS and NIR channels measure radiation reflected from clouds or the Earth. For wavelengths between 5 and 15 µm, the main radiation source is the Earth. In this range the satellite measures thermal radiation emitted by the Earth, clouds and gases, following the Planck Law. The IR channels 05 - 11 operate in this range. Between the visual and infrared ranges there is channel 4, which measures both the reflected sunlight and the emitted thermal radiation in daytime.

Figure 2: The MSG channels and three of the most important absorption gases and their absorption bands: ozone, carbon dioxide and water vapor

The MSG channels can be divided into two groups on the basis of this figure: the window and absorption channels.

The Window Channels

The absorption by atmospheric gases is neglible in window channels. The most important products of window channel measurements are the visual and infrared images.

  1. Window channels in the visible wavelengths
    In the visible and near infrared range the window wavelenghts lie between 0,4 µm and 1,6 µm. The two VIS channels 01 - 02 and the NIR channel 03 fall in this range. The satellite measures the sunlight reflected off the ground or cloud tops in these channels. This means that the information contained in the grey shades of these channels' images is a measure of the reflectance, the albedo. These three channels have been chosen within this window because they have different sensitivities delivering different information to the user - this is described in more detail below. It has to be noted that there are also small WV absorption bands in this range which might lead to slightly erroneous results in interpretation.
  2. Window channels in the infrared wavelengths
    The most prominent infrared window lies between 10 and 12 µm. The IR channels 09 and 10 are within this range. There is also a second window around 9 µm, in which channel 07 falls. This is the area of the IR channels 09 and 10. Within the IR Window the satellite measures thermal radiation emitted by the surface or clouds. The measurement gives the temperature of the emitting object according to the Planck Law, the Wien's Law and the Kirchhoff Law. This means that the information contained in the grey shades of these channels' images gives the temperature of the surface or clouds. As described in more detail below, there are two channels in this range, because the sensitivity is different for different channels, and they provide different information.

The Absorption Channels

In absorption band wavelenghts the radiation emmited by the surface and clouds isattenuated by tropospheric gases. The gases absorb the radiation from below partly or wholly, and emit further they own radiation as per Kirchhoff's second law. Consequently, the grey shades of the images provide information about the temperature at the level of the absorbing gas, as well as the concentration of this gas there.

The most prominent products of absorption channel measurements are the vertical profiles of temperature and humidity as well as gas concentration charts. However, for some absorbing gases, such as water vapor, also the grey-shade image itself is also significant.

  1. Channels in the water vapour absorption band
    The WV absorption band is around 6 - 7 µm. The channels 05 and 06 fall in this range. The radiation in these channels represents the temperature and amount of water vapour in the middle and upper troposphere. The radiation measured in these wavelenghts comes from tropospheric layers, rather than from single levels, as in VIS and IR ranges. The higher up there is moist air, the lower the temperature of the layer is, and the lighter the gray shades of the image are.
    The two channels 05 and 06 differ in the height ranges from which they receive the radiation:
    • Ch05 is close to the centre of the water vapour absorption band. Therefore, in this channel, the water vapour is absorbed more easily than in Ch06, and the satellite receives radiation only from the high layers; the sensitivity is at maximum around 350 hPa.
    • Ch06 is near the edge of the the absorption band, and water vapour is absorbed less easily than in Ch05. Consequently, radiation also reaches the satellite from the middle layers; the sensitivity is at maximum around 500 hPa.
    These statements already suggest that a combination of the two channels (their difference, for instance) can yield information about the WV content from a thick layer reaching from middle to upper layers.
  2. Channel 08 in the ozone absorption band
    Channel 08 is located in the center of the ozone absorption band; besides radiation originating from the lower levels, which is not so significant, these wavelenghts are sensitive also for radiation originating from stratosphere, where the ozone affects it. Consequently, the gray shades of a channel 08 image relieve information not only about the temperature, but also about the ozone content. However, as described later, subjective evaluation of ozone features in these images is very difficult, so this channel is used mostly for quantitative applications of ozone concentration.
  3. Channel 11 in the CO2 absorption band
    The absorption band around 15 µm has been in use since the time of launching the first polar-orbiting satellites for the derivation of temperature profiles. Channel 11 (13,4 µm) lies on the edge of the CO2 absorption band and is most sensitive for radiation from the layer between 900 and 800 hPa. Also this channel is not as important for image interpretation as it is for the quantitative applications.

The Mixed Channel

Channel 04 (3,9 µm) does not fit in the straightforward classification of "window" and "absorption" channels. It is technically a window channel, but as it is very close to the CO2 absorption band, the CO2 concentration has an influence on the measured temperatures. Basically, high CO2 concentration results in cooler temperatures, but it depends on the prevailing conditions. A more detailed explanation of this can be found in literature.
The importance of the Mixed Channel channel is the fact that both the reflected sunlight and the emitted thermal radiation from the Earth and clouds are combined in this channel in daytime. This leads to numerous applications, which are summarized in the following table.