1. ECMWF/EUMETSAT NWP-SAF Satellite data assimilation Training Course
1 to 4 July 2013

2. ECMWF/EUMETSAT NWP-SAF Satellite data assimilation Training Course
The infrared spectrum, measurement, modelling and information content
ECMWF/EUMETSAT NWP-SAF Satellite data assimilation Training Course

3. Sensors

4. High Spectral Resolution IR sounders on Polar Spacecraft
(GEO covered in winds lecture)

5. What do we mean by high spectral resolution ?

6. Why is high spectral resolution important ?
HIRS
19 channels
IASI
8461
HIRS
IASI

7. AIRS
IASI
CrIS
15 um
6 um
4 um

8. IASI v CrIS

9. (CrIS) Infrared Atmospheric Sounding Interferometer (IASI)
Cross-track Infrared Sounder
(CrIS)
Infrared Atmospheric Sounding Interferometer (IASI)

10. IASI has higher spectral resolution compared to CrIS

11. IASI has higher instrument noise compared to CrIS

12. CrIS has stronger inter-channel correlations than IASI
Observation error (K)
Observation error (K)
IASI
CrIS
Inter-channel correlation
Inter-channel correlation

13. The assimilation of IASI
Case study:
The assimilation of IASI

14. The assimilation of IASI results in a significant reduction of forecast errorNH EU SH US

15. (and what do we assimilate)
What does IASI measure
(and what do we assimilate)

16. The infrared spectrum of atmospheric radiation

17. The infrared spectrum of atmospheric radiation
The long-wave temperature sounding band
9.6 um
3.7 um
15 um
6.4 um
4.2um
~ 200 channels assimilated

18. Zoom of long-wave temperature sounding
channel usage for IASI

19. The infrared spectrum of atmospheric radiation
The long-wave ozone sounding band
9.6 um
3.7 um
15 um
6.4 um
4.2um
~ 20 channels assimilated

20. The infrared spectrum of atmospheric radiation
The long-wave water vapour sounding band
9.6 um
3.7 um
15 um
6.4 um
4.2um
~ 10 channels assimilated

21. The infrared spectrum of atmospheric radiation
The short-wave temperature sounding band
9.6 um
3.7 um
15 um
6.4 um
4.2um
~ 0 channels assimilated – IASI noise high

22. The infrared spectrum of atmospheric radiation
The surface sensing channels (window channels)
9.6 um
3.7 um
15 um
6.4 um
4.2um
channels mostly used for cloud detection

23. Issues specific to IR
Ambiguity
Nonlinearity
Clouds
Channel selection (PC)

24. Ambiguity in infrared radiance observations

25. When the absorbing gas is itself a variable

26. AMBIGUITY BETWEEN GEOPHYSICAL VARIABLES
When the primary absorber in a sounding channel is a well mixed gas (e.g. oxygen) the radiance essentially gives information about variations in the atmospheric temperature profile only.
When the primary absorber is not well mixed (e.g. water vapour, ozone or CO2) the radiance gives ambiguous information about the temperature profile and the absorber distribution. This ambiguity must be resolved by :
differential channel sensitivity
synergistic use of well mixed channels (constraining the temperature)
the background error covariance (+ physical constraints)

27. When sounding channels are sensitive to the surface

28. Atmospheric sounding channels
…selecting channels where there is no contribution from the surface….
Surface
reflection/
scattering
Surface
emission
Cloud/rain
contribution + + +
+ ...

29. Surface sensing Channels (passive)
…selecting channels where there is no contribution from the atmosphere….
Surface
reflection/
scattering
Surface
emission
Cloud/rain
contribution + + +
+ ...
Screen data to remove clouds / rain

30. BUT … AMBIGUITY WITH SURFACE AND CLOUDS
By placing sounding channels in parts of the spectrum where the absorption is weak we obtain temperature (and humidity) information from the lower troposphere (low peaking weighting functions).
BUT …
These channels (obviously) become more sensitive to surface emission and the effects of cloud and precipitation.
In most cases surface or cloud contributions will dominate the atmospheric signal in these channels and it is difficult to use the radiance data safely (i.e. we may alias a cloud signal as a temperature adjustment)
K(z)

31. Practical session

32. Metview web pages: https://software.ecmwf.int/metview
Download (Open Source)
Documentation and tutorials available
Metview articles in recent ECMWF newsletters
Metview:
-Several tutorials available: metview, odb, ogc, macro,…
Copyright ECMWF 2010

33. RTTOV Radiative Transfer Model
Y radiances
X (z)

34. Task 1: Overlay Tropical and Polar IASI spectra
View the map of TCWV (visualise PLOT_TCWV)
Select a Tropical profile ( edit / save profile extractor)
Select a Polar profile ( edit / save profile extractor)
Edit / visualise IASI SPECTRUM (with tropical.nc)
Edit / drop IASI SPECTRUM (with polar.nc)

35. Task 1: Overlay Tropical and Polar IASI spectra

36. Task 2: Overlay Tropical and Polar IASI channel 272 weighting function
Edit / visualise IASI JACOBIAN (with tropical.nc)
Edit / drop IASI JACOBIAN (with polar.nc)
Change channels and do similar ?

37. Task 3: Tropical and Polar IASI weighting functions

38. Task 3: Overlay Tropical and Polar IASI matrix of jacobians
Edit / visualise IASI MATRIX JAC (with tropical.nc)
Edit / drop IASI MATRIX JAC (with polar.nc)

39. Task 3: Overlay Tropical and Polar IASI matrix of jacobians

40. End… Questions ?