integrated lidar backscatter: quantifying the occurrence of supercooled water and specular...

Integrated lidar backscatter:Quantifying the occurrence

of supercooled water andspecular reflection

Robin Hogan and Anthony Illingworth

• Enhanced algorithm for supercooled water detection (Hogan et al. 2003, QJ in press)

• Specular reflection could be a problem for nadir-pointing EarthCARE lidar: how common is it?

Introduction

• The integrated backscatter through a cloud of optical depth of is approximately (Platt 1973):

– k is the extinction/backscatter ratio (18.75 sr for droplets) is the multiple scattering factor (~0.7 for the CT75K)

• 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

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z

Results for lidar 5° from zenith

• Chilbolton 2000– Occurrence of supercooled layers with > 0.7

Results for zenith pointing lidar

• Chilbolton 1999– Enhanced occurrence between -10 and -20°C

Supercooled water in models

• A year of data from the Met Office and ECMWF– Easy to calculate occurrence of supercooled water with > 0.7

Specular reflection

• Specular reflection from planar crystals can occur within 1° from zenith or nadir– Enhanced backscatter with no accompanying increase in

extinction: very low k – Integrated backscatter in ice can exceed the asymptote

corresponding to optically thick liquid cloud (recall ~(2k)-1)

• To quantify, require lidar to be precisely at zenith: 20 days of data obtained 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 – But allowance made for common scenario of liquid above ice

Results

• Around 23% of ice cloudy profiles affected– Specular reflection in 20% of

cloudy pixels at 4 km– Big problem for interpreting

backscatter measurements

• Must analyse more data– Use model for temperature:

specular reflection only for plates between -9 and -23°C?

– Problem solved with a high spectral resolution lidar?