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

2. Stefan Buehler, Sofia, May 17, 20062 Overview Ice clouds in the earths radiation balance Existing ice cloud observations CIWSIR mission idea Summary (Picture by Claudia Emde)

3. Stefan Buehler, Sofia, May 17, 20063 Overview Ice clouds in the earths radiation balance Existing ice cloud observations CIWSIR mission idea Summary (Picture by Claudia Emde)

4. Stefan Buehler, Sofia, May 17, 20064 Earths Radiation Balance Outgoing Longwave Radiation OLR Incoming Shortwave Radiation Sun Earth

5. Stefan Buehler, Sofia, May 17, 20065 Earths Radiation Balance Wavelength [μm] λE λ [normalized] (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. Stefan Buehler, Sofia, May 17, 20066 Clear-Sky OLR Spectrum Water vapor and CO 2 are the most important greenhouse gases.

7. Stefan Buehler, Sofia, May 17, 20067 But what about Clouds?

8. Stefan Buehler, Sofia, May 17, 20068 OLR-Spectrum with Cirrus Single scattering calculation. Ice water content 0.01 g/m 3 (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. Stefan Buehler, Sofia, May 17, 20069 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. Stefan Buehler, Sofia, May 17, 200610 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. Stefan Buehler, Sofia, May 17, 200611 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 CO 2 (Ramanathan et al., Science, 243, 1989). How will the net effect change for a changing surface temperature? No good answer at the moment.

12. Stefan Buehler, Sofia, May 17, 200612 Cirrus Particle Sizes and Shapes (Miloshevich et al., J. Atmos. Oceanic. Tech., 2001) 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).

13. Stefan Buehler, Sofia, May 17, 200613 Ice Clouds in Weather Prediction Models In models: Ice Water Content (IWC) (Met Office, UK, mesoscale model, Image: Sreerekha T.R.)

14. Stefan Buehler, Sofia, May 17, 200614 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. Stefan Buehler, Sofia, May 17, 200615 Overview Ice clouds in the earths radiation balance Existing ice cloud observations CIWSIR mission idea Summary (Picture by Claudia Emde)

16. Stefan Buehler, Sofia, May 17, 200616 Aircraft Campaign Locations (Heymsfield and McFarquhar [2002].)

17. Stefan Buehler, Sofia, May 17, 200617 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. Stefan Buehler, Sofia, May 17, 200618 Overview Ice clouds in the earths radiation balance Existing ice cloud observations CIWSIR mission idea Summary (Picture by Claudia Emde)

19. Stefan Buehler, Sofia, May 17, 200619 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. Stefan Buehler, Sofia, May 17, 200620 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. Stefan Buehler, Sofia, May 17, 200621 Cirrus Measurement with Microwave Sensors ARTS Simulation (CIWSIR Mission Proposal) (Buehler et al., CIWSIR Mission Proposal, 2005, simulation by Sreerekha Ravi)

22. Stefan Buehler, Sofia, May 17, 200622 Frequency Dependence of Ice Signal (Figure: Sreerekha T. R., IWP = 80 g/m 2, randomly oriented cylindrical ice particles, aspect ratio 4, r = 100 µm)

23. Stefan Buehler, Sofia, May 17, 200623 Influence of Cirrus Clouds on AMSU-B Strong ice clouds are detectable at AMSU frequencies (183±7 GHz) (25.1.2002, 1330 UTC Figure: Sreerekha Ravi)

24. Stefan Buehler, Sofia, May 17, 200624

25. Stefan Buehler, Sofia, May 17, 200625 190 GHz664 GHz (ARTS Simulation: Sreerekha T.R.)

26. Stefan Buehler, Sofia, May 17, 200626 CIWSIR Channels (Buehler et al., CIWSIR Mission Proposal, 2005, figure by Viju O. John)

27. Stefan Buehler, Sofia, May 17, 200627 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. Stefan Buehler, Sofia, May 17, 200628 The CIWSIR Instrument (Antenna diameter: 30 cm Picture: Mark Jarrett)

29. Stefan Buehler, Sofia, May 17, 200629 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. Stefan Buehler, Sofia, May 17, 200630 Retrieval by Bayesian Interpolation Create training dataset that covers the atmospheric variability (atmospheric states plus simulated radiances). Bayes’ theorem quantifies the notion that if the measurement looks similar then the underlying state is likely to be also similar. Retrieved IWP is the mean of all IWP in the training dataset, weighted with the “closeness” of the simulated radiances to the measured radiances. For Gaussian statistics same result as 1D-Var.

31. Stefan Buehler, Sofia, May 17, 200631 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)

32. Stefan Buehler, Sofia, May 17, 200632 ESA Earth Explorers CIWSIR current call (Adapted from R. Münzenmayer, EADS Astrium GmbH) Humidity Clouds

33. Stefan Buehler, Sofia, May 17, 200633 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

34. Stefan Buehler, Sofia, May 17, 200634 Figure by Andy Heymsfield For strong clouds the lidar covers only a part of the IWP.

35. Stefan Buehler, Sofia, May 17, 200635 Overview Ice clouds in the earths radiation balance Existing ice cloud observations CIWSIR mission idea Summary (Picture by Claudia Emde)

36. Stefan Buehler, Sofia, May 17, 200636 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.