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Remote sensing of Stratocumulus using radar/lidar synergy
Ewan O’Connor, Anthony Illingworth & Robin Hogan
University of Reading
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
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
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
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
Observations
Lidarbackscatter
Radarreflectivity
Observations
Dopplerspectral width
Dopplervelocity
Observations
Lidarbackscatter
RadarReflectivity
Derived Parameters
MedianDiameter
Shapeparameter
Derived Parameters
LiquidWaterFlux
LiquidWaterContent
Derived Parameters
Droplet fallvelocity
Airvelocity
Cellular Structure
Observations
Lidarbackscatter
Radarreflectivity
Observations
Dopplerspectral width
Dopplervelocity
Derived Parameters
MedianDiameter
Shapeparameter
Derived Parameters
LiquidWaterFlux
LiquidWaterContent
Derived Parameters
Droplet fallvelocity
Airvelocity
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
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
Doppler spectra• Drizzle droplets have significant terminal velocities (>1 m
s-1)
• Much higher reflectivity since Z = ND6
Doppler spectra • Can use spectral and drizzle techniques to obtain w in
cloud and below cloud in drizzle
Doppler spectra • Can use spectral and drizzle techniques to obtain w in
cloud and below cloud in drizzle
Doppler spectra • Can use spectral and drizzle techniques to obtain w in
cloud and below cloud in drizzle
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
Chilbolton observations• Sc present 26% of the time
• 50% of Sc seen by radar contains drizzle droplets
Observations
Observations
Derived Parameters
Derived Parameters
Derived Parameters
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)
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)