Radar and lidar calibration by wbf21256

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									       Radar and lidar
          calibration




Ewan O’Connor, Robin
Hogan, Anthony Illingworth,
Nicolas Gaussiat, Dominique
Bouniol, Darcy Ladd, Henk
Klein Baltink
                                             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
   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
Radar calibration – in rain
              Radar calibration – in rain
• Compare observations with theory




 Problem: apparent bias in observations
                   Radar calibration – in rain

Why bias in observations?

  – Additional problem in rain -
    Radome gets wet

  – Causes severe attenuation
       9 – 13 dB
                  Radar calibration – in rain
• Solution
  – Keep radome dry in rain (use cover)
  – Observations now agree with theory
  – Estimate radome attenuation
                   Radar calibration – in rain

  Compare independent
methods – good agreement
                 Inter-calibration
                 Cabauw,The
                 Netherlands




Chilbolton, UK   SIRTA, Palaiseau
                 (Paris), France
                                    Inter-calibration
French radar 94 GHz RASTA is mobile
  – Solution: drive to each site and place radars next to one another
                  Inter-calibration – results
– Add 6dB to RASTA
– 13dB radome attenuation
  (c.f. 11dB for Galileo)
  Agreement between all 3
  radars ~1dB
Lidar calibration – molecular
Lidar calibration – molecular
            Lidar calibration – liquid layers

Optically thick liquid
layers appear to have
constant integrated
backscatter (B) -
except when
precipitating
            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
            Lidar calibration – liquid layers
Multiple scattering
depends on lidar design,
range, and droplet size
Use factor η to
describe effect
B = 1/2ηS
η variation with range
and lidar design can be
calculated (Eloranta
1998)

Calibration within 7%
Dual wavelength microwave radiometer
 – Brightness temperatures -> 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

								
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