Integrated lidar backscatter: Quantifying the occurrence of supercooled water and specular reflection

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Integrated lidar backscatterQuantifying 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?
  

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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 (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

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Results for lidar 5 from zenith

• Chilbolton 2000
• Occurrence of supercooled layers with ? gt 0.7

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Results for zenith pointing lidar

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

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Supercooled water in models

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

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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 ?(2?k)-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

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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 -23C?
• Problem solved with a high spectral resolution
  lidar?