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)