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Introduction into the Absorption Channels

Description of characteristics and content of the WV channels:

Ch05: 6.2

Ch06: 7.3

Contact person:Veronika Zwatz-MeiseVersion 1.0

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All MSG channels• Channel 01: VIS 0.6

• Channel 02: VIS 0.8

• Channel 03: NIR 1.6 • Channel 04: MIR 3.9

• Channel 05 WV 6.2

• Channel 06: WV 7.3

• Channel 07: IR 8.7

• Channel 08: IR 9.7 („Ozon“)

• Channel 09: IR 10.8

• Channel 10: IR 12.0

• Channel 11: IR 13.4 „CO2“)

• Channel 12: HRV (High Resolution Visible)

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Sun radiation Earth radiation

Wavelength (micron)

Watt/m2 andmicron

Ozone

Carbondioxid

Watervapour

Sun radiation Earth radiation

W/m2/m

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Comparison of radiation sources sun - earth

• For wavelength < 5 m solar radiation is dominant

• For wavelength > 5 m terrestrial radiation is dominant

only VIS

VIS +IR

only IR

• 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

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Water vapour 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

Sun radiation Earth radiation

Ozone

CO2

H2O

Wavelength (m)

W/m2/m

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Ch05 and Ch06 within WV absorption band around 6 m

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• 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

Ch05 Ch06

EnergyspectrumSource:EUMETSAT

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Max. signal in Ch05 from approx. 350 hPaMax signal in Ch 06 from approx. 500 hPaBut: If there is no WV radiation from far below reaches the satellite

WeightingfunctionsSource:EUMETSAT

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Max. signal in Ch05 from approx. 320 hPaMax signal in Ch 06 from approx. 450 hPaBut: If there is no WV radiation from far below reaches the satellite

WeightingfunctionsSource:EUMETSAT

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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

• Height determination of semi-transparent clouds

WV

Clouds

6.2

7.3

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Ch05

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Ch06

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WV images in connection with upper level humidity and the vertical humidity gradient

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Ch05:6.2

1 st example

Black (dry) in upper levels

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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

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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

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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

drywet

wetdry

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