Introduction into the
Absorption Channels
Description of characteristics and
content of the WV channels:
Ch05: 6.2 
Ch06: 7.3 
Contact person:
Veronika Zwatz-Meise
zwatz-meise@zamg.ac.at
Version 1.0. 13 July 2004
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All MSG channels
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Channel 01:
Channel 02:
Channel 03:
Channel 04:
Channel 05
Channel 06:
Channel 07:
Channel 08:
Channel 09:
Channel 10:
Channel 11:
Channel 12:
VIS 0.6 
VIS 0.8 
NIR 1.6 
MIR 3.9 
WV 6.2 
WV 7.3 
IR 8.7 
IR 9.7  („Ozon“)
IR 10.8 
IR 12.0 
IR 13.4  „CO2“)
HRV (High Resolution Visible)
2
Sun radiation
Earth radiation
Watt/
m2 and
micron
Ozone
Carbon
dioxid
Water
vapour
Wavelength (micron)
3
Comparison of radiation sources
sun - earth
• For wave length < 5 m solar radiation is dominant
• For wave length > 5 m radiation of earth is dominant
• Ch01, 02, 03, 12: only sun radiation
• Ch04: both: radiation from sun and earth
• Ch 05, 06, 07, 08, 09, 10, 11: only thermal earth radiation
Sun radiation
Earth radiation
Watt/
m2
and
micron
only
VIS
VIS +
IR
only
IR
4
Watervapour channels
Ch05, Ch06
• WV has an absorption band around 6 m
– absorbs radiation from below
– emits radiation according to the 2nd Kirchhoff law
• Greyshades in the WV are indicative of the WV
content in the upper part of the troposphere
5
Ch05 and Ch06 within WV absorption band around 6
6
Energy
spectrum
Source:
EUMETSAT
Ch05
Ch06
• Ch05 is more in the centre of the absorption band with strong absorption;
– consequently radiation only from higher levels comes to the satellite;
• Ch06 is more to the wings of the absorption band with less strong absorption;
– consequently radiation also from lower layers comes to the satellite
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Weighting
functions
Source:
EUMETSAT
Max. signal in Ch05 from approx. 350 hPa
Max signal in Ch 06 from approx. 500 hPa
But: If there is no WV radiation from far below reaches the satellite
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Weighting
functions
Source:
EUMETSAT
Max. signal in Ch05 from approx. 320 hPa
Max signal in Ch 06 from approx. 450 hPa
But: If there is no WV radiation from far below reaches the satellite
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Application areas for the WV channels
• Identification of areas with high upper level WV
– Identification of jet axes
– Cloud intensification areas within jet streaks (left exit
region)
– Wave developments within jet streaks (right entrance
region)
• Identification of dry areas in WV
– special interest are areas of stratospheric air protruding
downward
• tropopause folding
• PV anomalies
• release of secondary cyclogenesis
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Some characteristica: WV 6.2 und 7.3
• WV content in two
different layers
– Differentiation
between WV content in
the lower and higher
middle troposphere
WV
6.2
• AMV computation
• Height determination
of semi-transparent
clouds
Clouds
7.3
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Ch05
12
Ch06
13
WV images in connection with upper level
humidity and the vertical humidity gradient
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Ch05:6.2
Black (dry) in upper levels
1 st example
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W06:7.3
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Orange:
dry:
< 30 %
cyan:
wet:
>60 %
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PV anomaly
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W05:6.2
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W06:7.3
20
Orange:
dry:
< 30 %
cyan:
wet:
>60 %
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No remarkable PV anomaly
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W05:6.2
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W05:7.3
24
Orange:
dry:
< 30 %
cyan:
wet:
>60 %
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Presentation in form of WV differences
• Difference image indicates the amount of
high level moisture
• Connection to downgliding dry air
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Ch05
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Ch06
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Ch05
Distinct black stripe only in upper layers
grey area in both levels; darker in the lower layer
No big difference between the two channels:
cloud edge
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Difference Ch05 - Ch06
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Difference Ch05 - Ch06
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Example for dry air in lower levels below frontal surface and jet axis
dry
wet
wet
dry
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