Ewan O

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Radar and lidar calibration

• Ewan OConnor, Robin Hogan, Anthony Illingworth,
  Nicolas Gaussiat, Dominique Bouniol, Darcy Ladd,
  Henk Klein Baltink

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Overview

• Radar calibration
• Typical methods
• Comparison with other wavelengths
• Calibration in rain
• Inter-calibration
• Consistency between cloud radars
• Lidar calibration
• Molecular calibration
• Calibration using liquid water layers

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Radar calibration by comparison

• Use calibration target of known backscatter
  cross-section
• Link budget calculation
• Compare to calibrated radar
• At Chilbolton use 3GHz polarimetric scanning
  radar
• 3GHz calibrated using Z, ZDR, KDP, redundancy in
  heavy rain (Goddard et al., 1994)
• Correct for gaseous attenuation at 35/94GHz
• Check for Mie scattering, attenuation at 35/94GHz

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Radar calibration in rain
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Radar calibration in rain

• Compare observations with theory

Problem apparent bias in observations
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Radar calibration in rain

• Why bias in observations?
• Additional problem in rain - Radome gets wet
• Causes severe attenuation
• 9 13 dB

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Radar calibration in rain

• Solution
• Keep radome dry in rain (use cover)
• Observations now agree with theory
• Estimate radome attenuation

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Radar calibration in rain

• Compare independent methods good agreement

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Inter-calibration
Cabauw,The Netherlands
SIRTA, Palaiseau (Paris), France
Chilbolton, UK
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Inter-calibration

• French radar 94 GHz RASTA is mobile
• Solution drive to each site and place radars
  next to one another

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Inter-calibration results

• Add 6dB to RASTA
• 13dB radome attenuation (c.f. 11dB for Galileo)
• Agreement between all 3 radars 1dB

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Lidar calibration molecular
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Lidar calibration molecular
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Lidar calibration liquid layers

• Optically thick liquid layers appear to have
  constant integrated backscatter (B) - except when
  precipitating

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Lidar calibration liquid layers

• Calculate theoretical lidar ratio, S
• At 905 nm S is constant for the range of liquid
  cloud droplet sizes
• Theory says
• B 1/2S
• Calibration method scale observed B (from liquid
  water clouds) until equal to theoretical 1/2S
• However, additional complication due to multiple
  scattering

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Lidar calibration liquid layers

• Multiple scattering depends on lidar design,
  range, and droplet size
• Use factor ? to describe effect
• B 1/2?S
• ? variation with range and lidar design can be
  calculated (Eloranta 1998)
• Calibration within 7

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Dual wavelength microwave radiometer

• Brightness temperatures -gt Liquid water path
• Improved technique Nicolas Gaussiat
• Use lidar to determine whether clear sky or not
• Adjust coefficients to account for instrument
  drift
• Removes offset for low LWP

LWP - initial
LWP - lidar corrected