Basic Channels

The MSG channels

MSG uses its SEVIRI instrument to measure electromagnetic radiation on 12 channels, which are summarised in table 1 and later, with some additional content, in table 2. The graphics in figures 1 and 2 show the various channels' basic properties regarding the measurement of radiation from the Earth and the Sun, as well as of the most important absorbing gases in the troposphere. More information about these topics can be found in the Basics chapter "References to General Literature about Satellite Meteorology".

In SatManu "Central wave length" is used as a channel name. Nevertheless, channel numbers are widely used - especially in technical literature - so that both "channel number" and "central wave length" are used in this introductory chapter.

Table 1: MSG channels, Abbreviations: VIS: visible, IR: infrared, NIR: near-IR; WV: Water vapor.

Channel number	Central wave length	Additional characterization
01	0,6 µm	VIS: window
02	0,8 µm	VIS: window
03	1,6 µm	NIR: window
04	3,9 µm	IR
05	6,2 µm	WV Absorption band
06	7,3 µm	WV Absorption band
07	8,7 µm	IR
08	9,7 µm	Ozone Absorption band
09	10,8 µm	IR
10	12 µm	IR
11	13,4 µm	CO2 Absorption band
12	Broad band: 0,3 - 1 µm	High resolution visible

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 negligible 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.

Table of channels, their applications and image examples

Table 2: channel number, central wavelenght, characteristics and typical applications, Abbreviations: VIS: visible, IR: infrared, NIR: near IR, MIR: middle IR, WV: Water Vapour

The fourth column of the table lists the RGB (Red-Green-Blue) combinations of different channels in which the respective channels are used. The RGB's are discussed in detail in the next chapter.

Channel number	Central wave length	Additional characterization	Typical application areas
01	0,6 µm	VIS: window Gray shades represent: different values of albedo	Image interpretation of cloud features: Reflected sunlight → clouds with different albedo Input for Natural Colour and Severe Storm RGBs Channel regularly used in SatManu

Channel number	Central wave length	Additional characterization	Typical application areas
02	0,8 µm	VIS: window Gray shades represent: different values of albedo; high sensitivity for soil and vegetation	Image interpretation of cloud and surface features: better recognition of vegetation features than in Ch01 Input for Natural color and Day Microphysics RGB

Channel number	Central wave length	Additional characterization	Typical application areas
03	1,6 µm	NIR: window Gray shades represent: different values of albedo; different reflectivity of ice and water clouds	Image interpretation of cloud and snow/ice features: possibility of discrimination between ice and water clouds - "cloud phase" (because of higher absorption in ice than in water clouds); easy recognition of snow (because of the low reflectivity of ice and snow) easy recognition of water clouds above snow Input for Natural Colour and Severe Storm RGBs

Channel number	Central wave length	Additional characterization	Typical application areas
04	3,9 µm	IR Gray shades represent: Night: surface/cloud top temperatures Day: albedo + surface/cloud top temperatures	Because reflected sunlight is different from the emitted thermal radiation, the channel's applications vary according to the time of day: Night: sea and land surface temperature urban heat islands Day and Night: detection of low clouds and fog detection of thin Cirrus discrimination of cloud phase and particle size detection of forest fires detection of super-cooled clouds Day: multi-layer clouds cloud top structures (overshooting tops) sun glint Input for Day Microphysics, 24 hours Microphysics, Severe Strom RGBs

Channel number	Central wave length	Additional characterization	Typical application areas
05	6,2 µm	WV Absorption band Gray shades represent: Different WV content in high layers	Image interpretation of WV content in high tropospheric layers (max signal from 350 hPa): Identification of areas with high upper level WV content Identification of dry upper level areas Identification of jet axes and connected synoptic scale processes: tropopause folding (stratospheric air protruding into troposphere) PV anomalies triggering of secondary cyclogenesis Input for Airmass, Severe Storm RGBs Channel regularly used in SatManu

Channel number	Central wave length	Additional characterization	Typical application areas
06	7,3 µm	WV Absorption band Gray shades represent: The amount of WV in high and middle layers	Water vapor content in a lower layer than in channel 06 (max signal from 500 hPa) Together with channel 05 the amount of WV in middle and upper layers Input for Airmass and Severe Storm RGBs

Channel number	Central wave length	Additional characterization	Typical application areas
07	8,7 µm	IR Gray shades represent: Surface/cloud top temperatures; sensitivity to high, thin Cirrus	Image interpretation with a focus on: high, thin Cirrus clouds (best usage within RGBs) discrimination between ice and water cloud Input for Dust RGB

Channel number	Central wave length	Additional characterization	Typical application areas
08	9,7 µm	Ozone Absorption band Gray shades represent: Surface/cloud top temperatures; Sensitivity to ozone content	Image interpretation with a focus on specific synoptic processes: Indication of tropopause folding with white stripes representing high ozone concentration Input for Airmass RGBs

Channel number	Central wave length	Additional characterization	Typical application areas
09	10,8 µm	IR Gray shades represent: Surface/cloud top temperatures;	Image Interpretation of cloud features: Temperature of radiating surface → temperatures of cloud tops Temperature of the surface Input for Day Microphysics, HRV, Airmass, Dust, 24 hours Microphysics, Sever Storm RGBs Channel regularly used in SatManu

Channel number	Central wave length	Additional characterization	Typical application areas
10	12 µm	IR Gray shades represent: Surface/cloud top temperatures; sensitivity to high thin Cirrus	Image Interpretation of cloud features like channel 09 with focus on high thin Cirrus, generated from the difference of 10,8 µm and 12,0 µm (the so called "split window") Input for 24 hour Microphysics RGB

Channel number	Central wave length	Additional characterization	Typical application areas
11	13,4 µm	CO2 absorption band Gray shades represent: Surface/cloud top temperatures; sensitivity to CO2 absorption	Quantitative applications: temperature profiles instability winds

Channel number	Central wave length	Additional characterization	Typical application areas
12	Broad band: 0,3 - 1 µm	High resolution Visible Resolution at nadir = 1 km Gray shades represent: Different values of albedo	For detecting sub-synoptic features when a high resolution is needed: Convection: substructures in convection: overshooting tops cirrus shield Orographic features: lee cloud valley fog Input for High Resolution VIS RGB