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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
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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
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
Observations Lidar backscatter Radar reflectivity
Observations Doppler velocity Doppler spectral width
Observations Radar Reflectivity Lidar backscatter
Derived Parameters Median Diameter Shape parameter
Derived Parameters Liquid Water Content Liquid Water Flux
Derived Parameters Air velocity Droplet fall velocity
Observations Lidar backscatter Radar reflectivity
Observations Doppler velocity Doppler spectral width
Derived Parameters Median Diameter Shape parameter
Derived Parameters Liquid Water Content Liquid Water Flux
Derived Parameters Air velocity Droplet fall velocity
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
w from cloud mode w from cloud mode Technique 3: Doppler spectra • Identify cloud mode and drizzle mode - determine w • Infer Z of drizzle mode and 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
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)