Ewan O

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Radar/lidar observations of boundary layer clouds

• Ewan OConnor, Robin Hogan, Anthony Illingworth,
  Nicolas Gaussiat

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Overview

• Radar and lidar can measure boundary layer clouds
  at high resolution
• Cloud boundaries - radar and lidar
• LWP microwave radiometer
• LWC cloud boundaries and LWP
• Cloudnet compare forecast models and
  observations
• 4 European remote-sensing sites (currently), 7
  models (currently)
• Cloud fraction, liquid water content statistics
• Microphysical profiles
• LWC - dual-wavelength radar
• Drizzle properties - Doppler radar and lidar
• Drop concentration and size radar and lidar

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Statistics - liquid water clouds

• 2 year database
• Use lidar to detect liquid cloud base
• Low liquid water clouds present 23 of the time
  (above 400 m)
• Summer 25
• Winter 20
• Use radar to determine presence of drizzle
• 46 of clouds detected by lidar contain
  occasional large droplets
• Summer 42
• Winter 52

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LWC - Scaled adiabatic method

• Use lidar/radar to determine cloud boundaries
• Use model to estimate adiabatic gradient of lwc
• Scale adiabatic lwc profile to match lwp from
  radiometers

http//www.met.rdg.ac.uk/radar/cloudnet/quicklooks
/
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• Compare measured lwp to adiabatic lwp
• obtain dilution coefficient
• Dilution coefficient versus
• depth of cloud

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Stratocumulus liquid water content

• Problem of using radar to infer liquid water
  content
• Very different moments of a bimodal size
  distribution
• LWC dominated by 10 ?m cloud droplets
• Radar reflectivity often dominated by drizzle
  drops 200 mm
• An alternative is to use dual-frequency radar
• Radar attenuation proportional to LWC, increases
  with frequency
• Therefore rate of change with height of the
  difference in 35-GHz and 94-GHz yields LWC with
  no size assumptions necessary
• Each 1 dB difference corresponds to an LWP of
  120 g m-2
• Can be difficult to implement in practice
• Need very precise Z measurements
• Typically several minutes of averaging is
  required
• Need linear response throughout dynamic range of
  both radars

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Drizzle below cloud

• Doppler radar and lidar - 4 observables
  (OConnor et al. 2005)
• Radar/lidar ratio provides information on
  particle size

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Drizzle below cloud

• Retrieve three components of drizzle DSD (N, D,
  µ).
• Can then calculate LWC, LWF and vertical air
  velocity, w.

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Drizzle below cloud

• Typical cell size is about 2-3 km
• Updrafts correlate well with liquid water flux

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Profiles of lwc no drizzle

• Examine radar/lidar profiles - retrieve LWC, N, D

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Profiles of lwc no drizzle
260 cm-3
90 cm-3
80 cm-3

• Consistency shown between LWP estimates.

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Profiles of lwc no drizzle

• Cloud droplet sizes lt12µm
• no drizzle present
• Cloud droplet sizes 18 µm
• drizzle present
• Agrees with Tripoli Cotton (1980) critical size
  threshold