CIWSIR, a Mission to Study Cirrus Clouds in the Sub-mm Spectral Range

1. CIWSIR, a Mission to Study Cirrus Clouds in the Sub-mm Spectral Range Sofia, May 17, 2006 S. A. Buehler Institute of Environmental Physics University of Bremen www.sat.uni-bremen.de

2. Overview • Ice clouds in the earths radiation balance • Existing ice cloud observations • CIWSIR mission idea • Summary (Picture by Claudia Emde)

3. Overview • Ice clouds in the earths radiation balance • Existing ice cloud observations • CIWSIR mission idea • Summary (Picture by Claudia Emde)

4. Earths Radiation Balance Sun Earth Incoming Shortwave Radiation Outgoing Longwave Radiation OLR

5. Earths Radiation Balance λEλ [normalized] Wavelength [μm] (Wallace und Hobbs, `Atmospheric Science', Academic Press, 1977.) • Radiative equilibrium temperature: -18°C • Global mean surface temperature: +15°C • 34 K natural greenhouse effect

6. Clear-Sky OLR Spectrum • Water vapor and CO2 are the most important greenhouse gases.

7. But what about Clouds?

8. OLR-Spectrum with Cirrus • Single scattering calculation. • Ice water content 0.01 g/m3 (contrail-cirrus), altitude 6-7 km. • Cloud reduces OLR. • Not the whole story: Clouds are active in the shortwave and in the longwave. (Calculation: Claudia Emde)

9. The Role of Cirrus Clouds: Shortwave • Cirrus clouds reflect sunlight and thus increase the planetary albedo. (AVHRR, Channel 1, 580-680nm, 25.1.2002, 13:30 UTC, Data Source: Met Office / Dundee Receiving Station)

10. The Role of Cirrus Clouds: Longwave • Cirrus clouds are radiatively cold and thus reduce the OLR. • Attention: grayscale is normally reversed for IR images so that clouds look white. (AVHRR, Channel 4, 10.3-11.3μm, 25.1.2002, 13:30 UTC, Data source: Met Office / Dundee Receiving Station)

11. The Net Effect of Cirrus Clouds • For high and optically thin clouds the longwave warming effect dominates. • For lower and optically thicker clouds the shortwave cooling effect dominates. • Global net effect of all clouds is cooling. Magnitude: 4 times double CO2 (Ramanathan et al., Science, 243, 1989). • How will the net effect change for a changing surface temperature? • No good answer at the moment.

12. Cirrus Particle Sizes and Shapes • Many different particle types • For cirrus clouds the net effect depends on the size (and shape) of the ice particles. • Feedback direction unclear. (Stephens et al., J. Atmos. Sci., 47(14), 1742-1754, 1990). (Miloshevich et al., J. Atmos. Oceanic. Tech., 2001)

13. Ice Clouds in Weather Prediction Models • In models: Ice Water Content (IWC) (Met Office, UK, mesoscale model, Image: Sreerekha T.R.)

14. Ice Clouds in Climate Models • Climatology of zonal, annual mean IWP from various models in the IPCC AR4 data archive shows difference up to an order of magnitude. • Delta-IWP after a CO2 doubling shows also vast differences. • IWP observations are needed to resolve model differences. (Figure by Brian Soden, University of Miami)

15. Overview • Ice clouds in the earths radiation balance • Existing ice cloud observations • CIWSIR mission idea • Summary (Picture by Claudia Emde)

16. Aircraft Campaign Locations (Heymsfield and McFarquhar [2002].)

17. Existing Satellite Observations • Cloud emission (IR radiometry):Retrieval of ice water path (IWP) and size (D) only for thin (semitransparent) ice clouds(ATSR-2, HIRS, Meteosat, ...) • Solar reflectance (UV/Vis):Retrieval of D and gross habit classification for particles near cloud top(POLDER, Meteosat, ...) • Cloud transmission (mm-wave):Retrieval of IWP only for thick (deep convective) ice clouds(AMSU-B, SSM-T2, ...)

18. Overview • Ice clouds in the earths radiation balance • Existing ice cloud observations • CIWSIR mission idea • Summary (Picture by Claudia Emde)

19. CIWSIR Mission Proposal • CIWSIR = Cloud Ice Water Sub-millimeter Imaging Radiometer • Proposal community: • Uni Bremen • Uni Bonn • Uni Mainz • Uni München • Uni Kiel • DLR Oberpfaffenhofen • DWD • MPI (Mainz, Hamburg) • Alfred Wegener Institut • Met Office, UK • ECMWF • Uni Wisconsin, US • Uni Rome, IT • Uni Florence, IT • Uni Edinburgh, UK • LMD, FR • Chalmers, SE • Uni Colorado, US • NCAR/NESDIS, US • Uni Bern, CH • RTH Zürich, CH • Uni Paris, FR • Uni Miami, US

20. Cirrus Measurement with Microwave Sensors • Ice cloud reduces the brightness temperature, as a part of the upwelling radiation is scattered away. • Compared to the IR, the measurement „sees“ the inside of the cloud, not just the top. • Sensitivity is strongly frequency dependent. (Buehler et al., CIWSIR Mission Proposal, 2005, Figure by Oliver Lemke)

21. Cirrus Measurement with Microwave Sensors ARTS Simulation (CIWSIR Mission Proposal) (Buehler et al., CIWSIR Mission Proposal, 2005, simulation by Sreerekha Ravi)

22. Frequency Dependence of Ice Signal (Figure: Sreerekha T. R., IWP = 80 g/m2, randomly oriented cylindrical ice particles, aspect ratio 4, r = 100 µm)

23. Influence of Cirrus Clouds on AMSU-B (25.1.2002, 1330 UTC Figure: Sreerekha Ravi) • Strong ice clouds are detectable at AMSU frequencies (183±7 GHz)

25. 190 GHz 664 GHz (ARTS Simulation: Sreerekha T.R.)

26. CIWSIR Channels (Buehler et al., CIWSIR Mission Proposal, 2005, figure by Viju O. John)

27. Different Particle Sizes • Different frequencies sample different parts of the size distribution • IR sees only smallest particles, radar only largest particles (Buehler et al., CIWSIR Mission Proposal, 2005, simulation by Claudia Emde)

28. The CIWSIR Instrument (Antenna diameter: 30 cmPicture: Mark Jarrett)

29. The CIWSIR Instrument • Mission proposal to ESA for current explorer call. • Conical scanner. • Goal: Ice water path and effective ice particle size with 10-20 km horizontal resolution and 20-25% accuracy.

30. Performance Estimate • IWP and D median errors mostly below 25 % • IR radiances complement sub-mm channels • Requirement for CIWSIR to fly tandem with Metop (AVHRR/3, IASI) • Co-registration facilitated by high AVHRR spatial resolution (Buehler et al., CIWSIR Mission Proposal, 2005, simulation by Frank Evans)

31. ESA Earth Explorers current call (Adapted from R. Münzenmayer, EADS Astrium GmbH) Humidity Clouds CIWSIR

32. EarthCARE • Cloud Profiling Radar (CPR)at 94 GHz (similar to the CPR on CLOUDSAT) • Lidar (ATLID) at 355 nm (UV) • + other instruments • Spots of < 1 km diameter • High vertical resolution (CPR < 400 m, ATLID < 100 m) • aerosol and cloud profiles plus radiation fluxes • Point samples along flight track • IWC from CPR to factor of 2 with assumptions on size distribution

33. For strong clouds the lidar covers only a part of the IWP. Figure by Andy Heymsfield

34. Overview • Ice clouds in the earths radiation balance • Existing ice cloud observations • CIWSIR mission idea • Summary (Picture by Claudia Emde)

35. Summary • Cirrus clouds play a crucial role in the earths climate due to their strong interaction with shortwave and longwave radiation. • Climate models and NWP models include cloud ice water content (IWC). There are large variations between models. • Global IWC or ice water path (IWP) data are urgently needed for validation. • IWP can not be directly measured with existing satellite sensors. • CIWSIR can measure IWP directly. • Low scientific and technological risk, moderate cost.