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Ewan O’Connor, Robin Hogan, Anthony Illingworth, Nicolas Gaussiat
Radar/lidar observations of boundary layer clouds
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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– 3 remote-sensing sites (currently), 6 models (currently)– Cloud fraction, liquid water content statistics
• Microphysical profiles:– Water vapour mixing ratio - Raman lidar– LWC - dual-wavelength radar – Drizzle properties - Doppler radar and lidar– Drop concentration and size – radar and lidar
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Vertically pointing radar and lidar
Radar: Z~D6
Sensitive to larger particles (drizzle, rain)
Lidar: ~D2
Sensitive to small particles
(droplets, aerosol)
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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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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 - initialLWP - lidar corrected
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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 m
• 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 cloudDoppler radar and lidar - 4 observables (O’Connor 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 drizzleExamine 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 <12μm• no drizzle present
Cloud droplet sizes 18 μm• drizzle present
Agrees with Tripoli & Cotton (1980) critical size threshold
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Conclusion • Relevant Sc properties can be measured using
remote sensing;– Ideally utilise radar, lidar and microwave radiometer
measurements together.– Cloudnet project provides yearly/monthly statistics for cloud
fraction and liquid water content including comparisons between observations and models.
– Soon - number concentration and size, drizzle properties.– Humidity structure, turbulence.
– Satellite measurements• A-Train (Cloudsat + Calipso + Aqua)• EarthCARE• IceSat
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Importance of Stratocumulus• Most common cloud type globally • Global coverage 26%
– Ocean 34%– Land 18%
• Average net radiative effect is about –65 W m-2
• Cooling effect on climate
Mean annual low cloud amount – ISCCP
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Cloud Parameters• Use radar and lidar to provide vertical profiles of:
– Cloud droplet size distribution (N, mean D, broad/narrow)
– Drizzle droplet size distribution (N, mean D, broad/narrow)
• Relate drizzle to cloud N• Is stratocumulus adiabatic? Entrainment rates
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Data
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Drizzle-free stratocumulusZ = ND6 & LWC ND3
Z LWC2/N
Assume adiabatic ascent and constant N LWC increases linearly with height
(z)
If we know T and p dLWC /dz Adiabatic profile: Z should vary as z2
Assume dLWC /dz is a constant, a
LWC(z) = az
Z(z) (az)2 / N
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Aircraft data - ACE 2 Brenguier et al. (2000)
1005 UTC
1545 UTC
Reflectivity profiles
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Refined techniqueAllow dilution from adiabatic profile of LWC
Z(z) k (az)2 / Nad
LWC(z) = k LWCad(z)
N = k Nad
D(z) = Dad(z)
Nad
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Plots of N
High N, small D low Z
Nad = 264 cm-3
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Plots of N
Nad = 91 cm-3
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Plots of N
Nad = 82 cm-3
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Presence of drizzle can lead to an overestimate of N an overestimate of LWC (and LWP)
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Conclusion• Consistency shown between LWP estimates from this
technique, and from microwave radiometers.• Additional techniques to investigate Sc are also available:
– Doppler radar/lidar – Drizzle properties (O’Connor et al. 2004)– Dual wavelength radar – LWC profile (Gaussiat et al.)– Doppler spectra
• Raman humidity measurements – WV structure, mixed layer depths
• Aircraft verification?• CloudNet – 3 years, 3 sites, provide climatology of Sc properties
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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 - initialLWP - lidar corrected
![Page 30: Radar/lidar observations of boundary layer clouds](https://reader036.vdocuments.mx/reader036/viewer/2022062305/56814e5e550346895dbbfd70/html5/thumbnails/30.jpg)
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/
![Page 31: Radar/lidar observations of boundary layer clouds](https://reader036.vdocuments.mx/reader036/viewer/2022062305/56814e5e550346895dbbfd70/html5/thumbnails/31.jpg)
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 m
• 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
![Page 33: Radar/lidar observations of boundary layer clouds](https://reader036.vdocuments.mx/reader036/viewer/2022062305/56814e5e550346895dbbfd70/html5/thumbnails/33.jpg)
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Drizzle below cloudDoppler radar and lidar - 4 observables (O’Connor 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 drizzleExamine 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 <12μm• no drizzle present
Cloud droplet sizes 18 μm• drizzle present
Agrees with Tripoli & Cotton (1980) critical size threshold