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

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

Small supercooled liquid droplets

Large falling ice particles

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 = (2k)-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

21

212

1

ek

dzz

z

• Example of supercooled water detection at Chilbolton

Lidar echo

Integrated lidar echo

Microwave radiometer LWP

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

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?

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

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 ~(2k)-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

Probable plate crystals

Specularreflection:

Results

• 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

~-23°C

Pristine crystals are columns or needles

Pristine crystals are plates

Fraction of clouds with specularly reflecting crystals

~-9°C

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!

Occurrence versus height & latitude

… versus temperature & latitude

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

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