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Remote sensing of Stratocumulus using radar/lidar synergy
Ewan O’Connor, Anthony Illingworth & Robin Hogan
University of Reading
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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
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Role of drizzle• Ubiquitous in clouds deeper than 300m
• Determines cloud lifetime and evolution
• Alters droplet spectra
• Implications for the processing of aerosol particles
• Feedback on BL dynamics through evaporative cooling
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Algorithm• Assume gamma distribution of the form
• Radar reflectivity, Z
• Lidar backscatter extinction coefficient ( )
• Ratio of Z to gives first guess of D0
00
67.3exp
D
DDNDn
0
6dDDDnZ
0
2
2dDDDn
40D
Z
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Algorithm• Doppler spectral width, v and improved D0
• D0 and v VT, Z-weighted terminal fall velocity
• Air velocity, w (+ve upwards)
• LWC and LWF
40467.3
1
3
72D
Z
0
6
0
6
dDDDn
dDDVDDnVT
TVwV
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Observations
Lidarbackscatter
Radarreflectivity
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Observations
Dopplerspectral width
Dopplervelocity
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Observations
Lidarbackscatter
RadarReflectivity
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Derived Parameters
MedianDiameter
Shapeparameter
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Derived Parameters
LiquidWaterFlux
LiquidWaterContent
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Derived Parameters
Droplet fallvelocity
Airvelocity
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Cellular Structure
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Observations
Lidarbackscatter
Radarreflectivity
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Observations
Dopplerspectral width
Dopplervelocity
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Derived Parameters
MedianDiameter
Shapeparameter
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Derived Parameters
LiquidWaterFlux
LiquidWaterContent
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Derived Parameters
Droplet fallvelocity
Airvelocity
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Technique 3: Doppler spectra• Can use Doppler spectra to infer vertical air velocity, w,
since small cloud droplets act as tracers (4 cm s-1)
• Shows cellular nature of updrafts and downdrafts
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Technique 3: Doppler spectra• Identify cloud mode and drizzle mode - determine w
• Infer Z of drizzle mode and cloud mode
w from cloud mode w from cloud mode
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Doppler spectra• Drizzle droplets have significant terminal velocities (>1 m
s-1)
• Much higher reflectivity since Z = ND6
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Doppler spectra • Can use spectral and drizzle techniques to obtain w in
cloud and below cloud in drizzle
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Doppler spectra • Can use spectral and drizzle techniques to obtain w in
cloud and below cloud in drizzle
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Doppler spectra • Can use spectral and drizzle techniques to obtain w in
cloud and below cloud in drizzle
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Conclusion• Can infer droplet number concentration in Sc
• Drizzle drop spectra and liquid water content/fluxes
• Dynamic motions/overturning in Sc
• Consistency shown between w derived in drizzle and obtained from Doppler spectra
• CloudNet – 3 years, 3 sites with radar and lidar
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Chilbolton observations• Sc present 26% of the time
• 50% of Sc seen by radar contains drizzle droplets
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Observations
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Observations
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Derived Parameters
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Derived Parameters
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Derived Parameters
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Drizzle flux versus radar reflectivity calculated from ASTEX spectra
calculated from FSSP and 2DC size spectra measured by the Met Office C-130 during the Atlantic Stratocumulus Transition Experiment (ASTEX)
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Spaceborne radar
• Global values of liquid water flux from a Z/LWF relationship suitable for 94GHz radar
• LWF (g m-2 s-1) = 0.0093 Z 0.69 (mm-6 m-3)