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