1. Millimeter and sub-millimeter observations for Earth cloud hunting Catherine Prigent, LERMA, Observatoire de Paris

2. Interstellar cloud Earth cloud Why hunting Earth clouds??? - Not that far away - Rather abundant Similarities in the techniques used to characterize both cloud types

3. Passive microwave observations from satellites for meteorology Atmospheric temperature profiling (operational) - observations in the O 2 band around 57 GHz Atmospheric water vapor profiling (operational) - analysis of the H 2 O line at 183 GHz Surface characterization Ocean surface wind speed (operational) - measurement of the wind-induced surface roughness (window channels 10-37 GHz) Soil moisture - dielectric properties of water and dry soil very different (window channels 1-19 GHz) Others … Cloud and precipitation monitoring - emission and scattering by hydrometeors (window channels 10-190 GHz)

4. The passive microwave satellite instruments for meteorology and climatology Future Present Many instruments, most of them with very similar characteristics Limited to 190 GHz and all on low orbit satellites

5. Why millimeter and sub-millimeter wave observations for meteorological applications? Two main drivers: 1) Need for real-time observations of the atmosphere, especially for extreme events - high temporal sampling => geostationary satellite - good spatial resolution from high orbits => use of higher frequencies 2) Lack of information on ice cloud

6. Millimeter radiometers onboard geostationary satellites for meteorology The Meteosat 2de Generation project:  20 years ago…  strongly pushed by the DEMIRM  technology transfer to the industry Today, new serious projects?  nothing planed on Meteosat 3d Generation…  Meteosat 4th Generation ????  ‘GOMAS’ submitted to the ESA call for Earth Explorer mission T sounding at 54, 118, and 425 GHz humidity sounding at 183 and 380 GHz 3 m antenna 1/12 of the Earth every 15 mn

7. Why studying ice cloud characteristics? Ice clouds cover ~30% of the globe on average Substantial but unknown impact on global radiative budget depending on their optical properties, -net warming (LW trapping) or -net cooling (SW reflection) Climate simulations very sensitive to cirrus cloud parameterizations derived from regional measurement campaigns and not tested globally Formation and growth of ice particles: key processes in convective systems strength of the cloud activity and stage in the cloud life cycle ‘The most glaring absence of information is any kind of estimate of the global distribution of ice water path', (A. Del Genio, WMO Report on Measurements of Cloud Properties for Forecasts of Weather and Climate, 1997)

8. A very large variety of ice characteristics from in situ measurements of cold clouds Ice Water Path (IWP) between 5 and 1000 g/m 2 Dimensions between 5 and 1000  m => impossible to observe the complete characteristic range with a single frequency domain => combination of instruments for complementary information (Heymsfield and Iaquinta, 2000)

9. Current Satellite Observations of Ice Cloud Properties Visible and Infrared cloud amount, height, optical thickness, ice water path, and droplet size BUT insensitive above IWP~50 g/m 2 and for particles with D> 80  m Characterization of ~50% of the ice clouds Passive Microwave Retrieval of IWP with existing frequencies below 190 GHz BUT insensitive below IWP~300g/m 2 and for particles with D<200  m Characterization of ~5% if the ice clouds => Millimeter and sub-millimeter wavelengths: A promising region to characterize the remaining ~45% of the ice clouds

10. Sensitivity to the Ice Water Path (standard atmosphere; 53° incidence) almost linear increase of  Tb with IWP below 300 GHz frequencies below 500 GHz not suitable for low IWP with small D

11.  Tb ratio sensitive to particle size, not to IWP for a given IWP,  Tb increases with particle size but levels off as particle sizes approach the wavelength Effect of particle size (standard atmosphere; 53° incidence)

12. Effect of particle shape and orientation Radiative transfer simulations: V-H > 0 => oblate spheroids V-H prolate spheroids A unique insight into cloud microphysics Scattering polarized signatures observed at 89 GHz: - if particles were spheres, zero polarization V-H > 0 V-H < 0

13. Suggested instrument - new measurements to be combined with other sensor observations for a full characterisation of ice cloud properties - channels suitable for detection of both thin cirrus and thick anvils - constant incident angle preferred with dual perpendicular polarizations - prospective study with EADS/ASTRIUM <65 kg 0.8mx0.8mx0.5m - similar instrument submitted to the ESA call for Earth Explorer Missions

14. Conclusions  Millimeter and sub-millimeter waves not yet used for operational meteorology  Technology is mature  Projects:-for geostationary satellite (but already some, 20 years ago…) -for polar orbitors  Soon catching up?? ??