Since different channels represent different features in the underlying earth system, including different synoptic phenomena, it has been found that the use of various channel combinations highlights specific meteorological situations. These channel combinations are therefore a useful tool to enhance the synoptic content of the received images. The following combinations will be used in this manual.

METEOSAT (MOP, MTP)

Combination VISible + InfraRed

This is a tool for a quick and easy discrimination of thick and thin cloudiness in different layers of the tropopshere. For this, the VIS image is displayed in yellow colours according to its intensity, the IR image is displayed in blue colours according to its intensity. Thus, after a superposition of both images yellow grey shades in the VIS + IR image represent low, warm cloud tops, blue grey shades only high, cold cloudiness while white to grey shades appear where signals come from both channels, being a sign for thick and/or multilayered cold cloud tops.

Dark yellow	Land
Blue	Sea
White/grey	Thick, multilayered high clouds
Yellow	Low clouds or snow
Blue	Thin, high cirrus clouds

The description of the VIS + IR image should be based on the description of the single channels prepared in the chapter about the Basic Channels: Meteosat (compare Basic channels ).

A1	The easternmost part of the frontal cloud band A located over the Ukraine VIS + IR: yellow, some white spots within cloudiness, some blue areas at the northern boundary Conclusion: In general a frontal cloud band with low cloud tops but some higher small scale areas embedded and some high cloud fibres at the northern boundary
A2	The wave area over north Italy - Switzerland - west Austria and south Germany VIS + IR: grey area north Italy - Switzerland - west Austria; transition to blue areas over Germany and the Czech Republic Conclusion: The grey area represents the thick part of the Wave bulge which can be identified much more easily in the combined image than in the separate channels; blue area is high cloud shield above
A3	Two mesoscale cells MCSs over the western Mediterranean (40.5N/03E and 41N/06E) VIS + IR: grey areas in oval shape, small bluish areas downstream; cells embedded within yellow cloud fields Conclusion: Grey areas represent the thick part of the MCSs (thunderstorms), the blue areas the cirrus shield; both cells are embedded in low level cloud fields behind the frontal cloud band
B1	Cloud shield over the Atlantic VIS + IR: Extended grey areas becoming more blue to the south-east Conclusion: Grey areas show thick mulitlevel cloudiness which changes to mostly high cloudiness downstream
B2	Cloud band over the Atlantic (36N/35W - 50N/30W) VIS + IR: grey areas in the leading part of the cloud band; some small yellow areas and blue parts in the rearward half Conclusion: Rising cloud tops from the leading to the rearward edge and change from thick multilayered parts to only thin high cloudiness at the rear
B3	Cloud spiral west of the cloud shield (around 57N/36W) VIS + IR: grey cloud spiral becoming yellow to the spiral centre Conclusion: Thick multilayered cloudiness in the eastern part of the Occlusion spiral, transition to only low cloudiness at the rearward edge and in the spiral centre
D1	Cloud cells west of the Comma feature and the Norwegian coast: VIS + IR: yellow area with some white spots and lines, and bluish lines Conclusion: Widespread area of low cloudiness with some thick cells and lines of cells at the boundaries of these lines; some high cloudiness is indicated by the blue colour
D2	Cloud cells west, north-west of Scotland and north Ireland VIS + IR: yellow area with cellular structure Conclusion: Area with cold air cells with only low vertical extension; especially compared to D1

NOAA AVHRR

Combination InfraRed 4 - InfraRed 5

The difference in brightness temperatures of the IR 4 and IR 5 channels is mainly caused by differences in emissivity of the underlying surfaces. The differences due to radiative transfer in the atmosphere are of only minor importance.

This implies that IR 4 - IR 5 differences allow the detection of objects for which the emissivity is significantly different at the wavelengths of these channels. Since for both land and sea surfaces these emissivities have little dependence on wavelength, we can look at objects with emissivity differences for these channels. It has been shown that ice displays a significant wavelength dependence on its emissivity in this range of the spectrum.

Thus is it possible to link IR 4 - IR 5 differences exceeding a certain threshold (different over sea or land) to the presence of cirrus clouds, even if these are not optically dense (semi-transparent).

Combination InfraRed 4 - Near InfraRed 3,7

The spectral variation of the emissivity of small liquid water droplets results in an emissivity difference between these two channels, whereas land and sea surfaces display no spectral variation of their emissivities between these channels.

Under the condition that no reflected sunlight is present in the NIR 3,7 channel, i.e., during night-time, it is possible to detect the presence of clouds with small water droplets, i.e., low clouds.

Combination RGB display of channels VISible 1 + VISible 2 + InfraRed 4

When during daytime the information of the VIS 1 channel is displayed on the red channel of a colour display, the VIS 2 channel in the green and the IR 4 channel in the blue, one gets an image where the various colours have the follow meaning:

Green/yellow	Land
Deep blue	Sea
White	Thick, high clouds
Yellow	Low clouds or snow
Purple/blue	Cirrus clouds

Combination RGB display of channels Near InfraRed 3,7 + InfraRed 4 + InfraRed 5

When during nigh-ttime the information of the NIR 3,7 channel is displayed on the red channel of a colour display, the IR 4 channel in the green and the IR 5 channel in the blue, one gets an image where the various colours have the follow meaning:

Brown/grey	Land
Deep blue	Sea
White	Thick, high clouds
Brown	Low clouds
Blue	Cirrus clouds

Examples of the AVHRR channel combinations

Area to be examined:

In western upper corner	Great Britain
In eastern upper corner	Baltic Sea
Below in western corner	North-eastern Spain
Below in eastern corner	Balkans

In this image thin partly translucent Ci clouds should be seen white. In the image there are white fibres at the northern edge of the large cloud band but the same thing can be seen even more easily from images of other channels. Light areas are found also from some other objects, for instance probably from the middle level clouds over southern France and in the centre of Germany. In Denmark and in southern Sweden there are some small white spots, where in the 1, 2 and 4 image are found dark blue translucent clouds. It seems that this combination gives very little new information compared to other channels or combinations.

IR 4 - NIR 3,7	Differences can be found in the upper part of the picture: Cu cloudiness over Scotland and sea areas is ragged without differentiating higher and low clouds. Cloud band from the Netherlands northwards translucent or ragged. Over southern Norway near Oslo can be found two different cloud levels with a sharp rear edge. Probably connected to orography.

Colour pattern in RGB composites with AVHRR channels 1, 2 and 4 during daytime:

Clouds or Surface	Colour
Thick and cold clouds	White or bluish white
Thin and cold clouds	Light blue
Middle level clouds	White or yellowish white
Low level clouds	Yellow to sand brown
Land surface	Dark green
Ocean surface	Dark blue
Ice and snow	Yellowish white

Channel 1 gives red, channel 2 green and IR channel 4 gives blue colour components of the image. Because of visible channels, the use of this image is restricted to daytime. Also during the period of winter darkness in the Nordic countries this combination is useless. In the situation of no sun illumination there is information only in blue component. Also for lowest sun elevation colours become more and more bluish.
Multiple channel image gives the best three dimensional impression from the cloud fields and gives more accurate details about the structure of cloud surface. Coldest clouds are seen bluish white. Middle level clouds are white or sometimes yellowish white. Low clouds have yellow or yellowish white shades. Small separate Cu cells arranged in streets have sand brown colour.

Advantage compared to the VIS 1 image, that in 1, 2 and 4 image coastlines are better detected and use of different colours is helpful when differentiating cloudiness at different altitudes.

VIS 1 + VIS 2 + IR 4

Large cloud band over lower part of the image: Brightest area same as in the case of VIS 1, is seen bluish white. More white or yellowish white eastwards. Below in the right corner translucent, only partly cloudy or low clouds. The western edge of the brightest cloud area in the centre of the picture has broad dark blue shadow. The cloud band has a wave-like feature. At the northern edge of the wave are found Ci streaks indicating the jet stream. East of the whitest cloud area cloudiness has more lumpy appearance. Westwards and northwards from the large cloud band it is mainly cloudless, only here and there some cloud patches. Grey patches, for instance over France, have distinct shadows at their western side which means that they are higher than the yellow areas which are lowest clouds or fog areas. Sand brown areas seem to follow the outlines of river beds and some valleys, which represent probably thin fog covering the rivers. In the upper part of the image the prevailing colour is blue because of the darker morning. Over British Isles and North Sea there is open Cu cloudiness, biggest and highest over Scotland and west of southern Norway. Another cloud band extending from the Netherlands across North Sea northwards and becoming broader north of Denmark. Cloud band has very lumpy appearance, especially western part of the band north of Denmark containing mainly probably Cb type cloudiness. East of it larger Cu cells embedded in lower clouds. On the right side of the picture is found Cu cloudiness (closed type) arranged in parallel lines and Cu cells becoming larger downwind. Highest Cu cells over southern Baltic and Lithuania. Conclusion: Perhaps Wave or at least Wave - like band over Middle Europe. Over North Sea typical cold air cloudiness of the open cells. Cb type cloudiness at the western edge of the cloud band over Norway and North Sea.

AVHRR - channel combination Near InfraRed 3,7 + InfraRed 4 + InfraRed 5

Colour pattern in RGB composites with AVHRR channels 3, 4 and 5 during night-time:

Clouds or Surface	Colour
Thick and cold clouds	White
Thin and cold clouds	White or bluish white
Middle level clouds	Light red to grey
Low level clouds	Red - grey to dark grey
Land surface	From grey to red - grey (or light grey if very cold)
Ocean surface	Black or dark red
Ice and snow	Light red-grey

When using channels 3, 4 and 5 in RGB composites, the images can be used both during day and night.
Since very thin and cold clouds appear slightly colder in channel 5 than in channel 4, these cloud types have bluish appearance during night-time. Low level clouds change from the blue grey colour during day to dark red during night-time. There are difficulties in the ability to separate thin Cirrus clouds at night. Especially troublesome is the separation of thin Cirrus overlying low clouds from cold land surfaces. Colour pattern in RGB composites with AVHRR channels 3, 4 and 5 during daytime:

Clouds or Surface	Colour
Thick and cold clouds	White or bluish white
Thin and cold clouds	Variable from light red to light cyan
Middle level clouds	Cyan
Low level clouds	Dark cyan to dark blue
Land surface	Dark reddish
Ocean surface	Light to dark reddish
Ice and snow	Light red

When using channels 3, 4 and 5 in RGB composites, the images can be used both during day and night.
During day, the coldest high level clouds composed of ice crystals have mainly bluish white colour but the same appearance can also be shown by lower clouds with large water droplets. Therefore 3 4 and 5 channels composite image is more difficult to interpret than 1, 2 and 4 channel combination. In the case of thin transparent Ci clouds, the colour depends on the underlying surface (or layer). Over land or sea Ci cloud may be light red and over lower clouds light cyan. Almost all low- and mid-level water clouds have a cyan to dark blue grey appearance.

NIR 3.7 + IR 4 + IR 5

Comments from large cloud band same as with channel 3 (compare Basic channels ). The wave-like band and also cloud patch over Hungary white. Otherwise clouds eastwards have cyan colour. Difference from channel 3 is that below in the right corner of the picture the cloudless area is seen as dark red or almost black. In the image of channel 3.7 land as well as clouds were black. In this image the white clouds in middle Europe are clearly fibrous. On the other hand the cyan cloud area has more miscellaneous features. The low clouds and fog areas over Germany and France also have a cyan colour. East of the Netherlands cyan cloud area is more vague and darker which probably means fog or thin stratus. Narrow grey lines over France are probably connected to low fog over rivers and valleys. Cloud band from the Netherlands northwards has distinct Cu form, especially over Norway. High Cu clouds over sea are seen white and lower clouds dark grey or cyan. Conclusion: Sea and land easy to detect as well as high clouds from lower clouds. Lowest clouds and fog are found as grey. Cu form cloudiness with embedded smoother lower clouds can be detected.

AVHRR - channel combination VISIBLE 1 - (- Near InfraRed 3,7)

VIS 1- (- NIR 3,7)

Large anticyclonically curved cloud band or Wave on lower part of the picture: Fibrous texture Whitest area extending from south-eastern France across Switzerland to Bavaria. over Austria and downstream the colour of the cloudiness varying from white to grey having more ragged and lumpy appearance. Partly translucent. From Bavaria eastwards the northern edge of the cloud band grey, fibrous and indistinct. Western edge sharper. Low and middle clouds over France and Germany: dark grey scattered, ragged cloudiness Land marks, rivers and coastlines: Hardly seen. Coastline of British Isles and Baltic coastlines difficult to detect. River beds over western Europe can be seen partly. Cu cloudiness over Scotland and southern Norway with open cells and with different vertical extension can be seen as well as Cu clouds over Baltic Sea and Baltic countries. Highest cloud cells are seen as white spots and lower clouds have grey colour. Another cloud band extending from the Netherlands to southern Norway: Becoming broader northwards. In the southern part of the band the centre of the band whiter than the edges. In the northern part of the band larger white cloud patches or cells. Brightest cloud cells between Denmark and southern Norway as well as over southern Baltic.

Artificial Channels

Meteosat: InfraRed development image

Deviation images: These are motion-corrected difference images; several steps are necessary for their computation:

• Two consecutive images are used for deriving the atmospheric motion field (AMVs: Atmospheric Motion Vectors)
• The second image is traced backward by means of this AMV field; this computation leads to a motion-corrected image
• The first and the motion-corrected image are subtracted: if the changes in cloudiness between two consecutive images were a consequence of advection only, the difference signals would be zero;
  But: in reality the difference signals are not zero, which means that additional processes are also responsible for the cloud changes: new development or decay of cloudiness
• The motion corrected difference image is called: development image

White signals	New cloud development
Black signals	Cloud decay
Grey signals	No non-advective changes

This is demonstrated with the following example from 29 July 1997.

The images above show two consecutive (1 hour difference) IR images presented with an enhancement for cold cloud tops. The images contain several convective cells over the west Alps, central Italy and Croatia; cloud fibres with less cold tops than the convective systems extend from Slovenia across the east Alps northward to Slovakia and Poland. A visual inspection of the two images identifies an eastward movement of most of the cells as well as a northward movement of the cloud fibres. But additional to these movements a new development can be noticed in the area of the three main cells over Italy, and decay can be noticed for instance over north Italy and Slovenia. The image above represents the development image between 14 and 15 UTC. Grey, white and black colours can be noticed in the area under consideration. The most impressive features are the white circular areas enclosing grey areas over middle and north-west Italy. This is a typical appearance for growing convective cells close to the mature stage; a smaller but similar feature can be seen over Croatia. Black areas over Slovenia and the west Alps represent decay of cloudiness. This image is a different presentation of the same content. The satellite signals are now the usual IR image from 15.00 UTC instead of the development image but the red isolines superimposed are mean values of deviation computed from the development image above. It is very clear now that the southernmost cell over Italy has its most intensive growth in the boundary areas while the centre shows moderate rising of the cloud tops there. The cell in the north-west, close to Genoa, shows growth in the whole cloud area which is characteristic for an intensifying stage. Negative values representing decay can be found for instance over Slovenia and the coastal areas of Croatia.

The use of such development images is relatively new and has to be investigated in much more detail in the near future, but it has already turned out to contain important signals within the life cycle of some conceptual models like Wave developments (compare Wave ) and, as described above, MCCs and MCSs (Mesoscale Convective Complexes and Systems respectively) (compare Cumulonimbus (Cb) and Mesoscale Convective System (MCS) ).