Radar/lidar observations of boundary layer clouds
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Transcript of Radar/lidar observations of boundary layer clouds
Ewan O’Connor, Robin Hogan, Anthony Illingworth, Nicolas Gaussiat
Radar/lidar observations of boundary layer clouds
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
Vertically pointing radar and lidar
Radar: Z~D6
Sensitive to larger particles (drizzle, rain)
Lidar: ~D2
Sensitive to small particles
(droplets, aerosol)
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 %
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
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/
Compare measured lwp to adiabatic lwp
• obtain ‘dilution coefficient’
Dilution coefficient versus depth of cloud
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
Drizzle below cloudDoppler radar and lidar - 4 observables (O’Connor et al. 2005)
• Radar/lidar ratio provides information on particle size
Drizzle below cloud– Retrieve three components of drizzle DSD (N, D, μ).– Can then calculate LWC, LWF and vertical air velocity, w.
Drizzle below cloud– Typical cell size is about 2-3 km– Updrafts correlate well with liquid water flux
Profiles of lwc – no drizzleExamine radar/lidar profiles - retrieve LWC, N, D
Profiles of lwc – no drizzle
260 cm-3 90 cm-3 80 cm-3
Consistency shown between LWP estimates.
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
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
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
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
Data
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
Aircraft data - ACE 2 Brenguier et al. (2000)
1005 UTC
1545 UTC
Reflectivity profiles
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
Plots of N
High N, small D low Z
Nad = 264 cm-3
Plots of N
Nad = 91 cm-3
Plots of N
Nad = 82 cm-3
Presence of drizzle can lead to an overestimate of N an overestimate of LWC (and LWP)
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
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
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/
Compare measured lwp to adiabatic lwp
• obtain ‘dilution coefficient’
Dilution coefficient versus depth of cloud
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
Drizzle below cloudDoppler radar and lidar - 4 observables (O’Connor et al. 2005)
• Radar/lidar ratio provides information on particle size
Drizzle below cloud– Retrieve three components of drizzle DSD (N, D, μ).– Can then calculate LWC, LWF and vertical air velocity, w.
Drizzle below cloud– Typical cell size is about 2-3 km– Updrafts correlate well with liquid water flux
Profiles of lwc – no drizzleExamine radar/lidar profiles - retrieve LWC, N, D
Profiles of lwc – no drizzle
260 cm-3 90 cm-3 80 cm-3
Consistency shown between LWP estimates.
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