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Lidar observations of mixed-phase clouds

Lidar observations of mixed-phase clouds. Robin Hogan, Anthony Illingworth, Ewan O’Connor & Mukunda Dev Behera University of Reading UK Overview Enhanced algorithm for supercooled liquid water detection (Hogan et al. 2003, QJ in press)

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Lidar observations of mixed-phase clouds

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  1. Lidar observations ofmixed-phase clouds Robin Hogan, Anthony Illingworth, Ewan O’Connor & Mukunda Dev Behera University of Reading UK Overview • Enhanced algorithm for supercooled liquid water detection (Hogan et al. 2003, QJ in press) • Specular reflection from horizontally aligned plate crystals: A blessing or a curse? • Global distribution of stratiform supercooled water clouds from spaceborne lidar

  2. Small supercooled liquid droplets Large falling ice particles Lidar and mixed-phase clouds • Typical concs: ice 20 l-1, liquid droplets 20 000 l-1 • The same mass of water is ~10 times more optically thick as liquid than as ice, so lidar return also 10 times greater • By contrast, D6 dependence of radar makes the same mass of water ~1000 times more reflective as ice than as liquid! • Radiation calculations on 2 case studies suggest • When supercooled liquid present it is usually more radiatively important than the ice, even though tends to form thin layers • Crudely represented in current models

  3. Integrated lidar backscatter • The integrated backscatter through a cloud of optical depth of is approximately (Platt 1973): • k = extinction/backscatter ratio (18.75 sr for droplets) •  = multiple scattering factor (~0.7 for Chilbolton lidar ) • For large optical depth it reduces to  = (2k)-1 • If z1 and z2 encompass the 300 m around the strongest echo in a profile, we can identify thin liquid water layers with  greater than, say, 0.7

  4. Lidar echo • Example of supercooled water detection at Chilbolton Integrated lidar echo Microwave radiometer LWP

  5. Results for lidar 5° from zenith • Analysis of continuous Chilbolton CT75K lidar data from 2000 when looking off-zenith Frequency that cloud was observed Fraction of clouds containing supercooled water with >0.7

  6. Results for zenith pointing lidar • Analysis of Chilbolton lidar data from 1999 when pointing at zenith Enhanced occurrence between -10 and -20 °C: specular reflection from plates?

  7. Supercooled water in models • A year of data from the Met Office and ECMWF • Easy to calculate occurrence of supercooled water with  > 0.7 Prognostic ice and liquid+vapour variables Prognostic cloud water: ice/liquid diagnosed from temperature

  8. Specular reflection • Specular reflection from planar crystals can occur within 1° of zenith or nadir • Enhanced backscatter with no accompanying increase in extinction (very low k): radiative properties difficult to infer • Integrated backscatter in ice can exceed the asymptote corresponding to optically thick liquid cloud (recall ~(2k)-1) • Is locating plate crystals useful? Currently nadir viewing is being considered for spaceborne lidars Calipso and EarthCARE • To quantify, require lidar to be precisely at zenith: 20 days of data analysed so far at Chilbolton • Algorithm calculates integrated backscatter from 2 km up • Specular reflection deemed to occur if this integral is more than 1.05 times the asymptote for liquid water • Excess above this value is attributed to pixels with highest 

  9. Probable plate crystals

  10. Specularreflection: Results Pristine crystals are columns or needles ~-23°C • Around 20% of ice cloud profiles are strongly affected by specular reflection: enhancement > a factor of 2 • PDF of the maximum backscatter suggests that a further 30% of profiles are affected by specular reflection to a lesser extent • Big problem for interpreting backscatter measurements from space in terms of the radiative properties of ice clouds • Recommend operate spaceborne lidar a few degrees from nadir Pristine crystals are plates ~-9°C Fraction of clouds with specularly reflecting crystals

  11. Supercooled liquid waterfrom the LITE lidar on the space shuttle in 1994 • LITE took 45 hours of data 9-20 September 1994 • We use 532 nm channel: appeared most sensitive • Frequent changes in gain: only 10.5 hrs of data for which saturation level high enough to detect supercooled layers unambiguously • Even then liquid water often saturated receiver, and multiple scattering more uncertain from space, so integral method not reliable • 280 000 km of ground covered: equivalent to 160 days of surface observation!

  12. Occurrence versus height & latitude

  13. … versus temperature & latitude

  14. Comparison with Chilbolton Cloud fraction: much better coverage from space Lidar does not need to penetrate the cloudy boundary layer Liquid detection: very similar, esp. below -10°C

  15. Conclusions • Have shown that spaceborne lidar can identify supercooled liquid water clouds across the globe • Problems with LITE: saturation & severe multiple scattering • We will use long-term spaceborne lidar data: IceSat: launched 12 Jan 2003: High polar orbit, 2 wavelengths Calipso: launch in Dec 2004: Includes depolarisation channel

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