1. Radar/lidar observations of boundary layer clouds
Ewan O’Connor, Robin Hogan, Anthony Illingworth, Nicolas Gaussiat

2. 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

3. Vertically pointing radar and lidar
Radar: Z~D6
Sensitive to larger particles (drizzle, rain)
Lidar: b~D2
Sensitive to small particles (droplets, aerosol)

4. 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 %

5. 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 - initial
LWP - lidar corrected

6. 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

7. Compare measured lwp to adiabatic lwp
obtain ‘dilution coefficient’
Dilution coefficient versus
depth of cloud

8. 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 mm
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

10. Drizzle below cloud
Doppler radar and lidar - 4 observables (O’Connor et al. 2005)
Radar/lidar ratio provides information on particle size

11. Drizzle below cloud
Retrieve three components of drizzle DSD (N, D, μ).
Can then calculate LWC, LWF and vertical air velocity, w.

12. Drizzle below cloud Typical cell size is about 2-3 km
Updrafts correlate well with liquid water flux

13. Profiles of lwc – no drizzle
Examine radar/lidar profiles - retrieve LWC, N, D

14. Profiles of lwc – no drizzle
260 cm-3
90 cm-3
80 cm-3
Consistency shown between LWP estimates.

15. 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

16. 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

17. 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

18. 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

19. Data

20. Drizzle-free stratocumulus
Z = 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
Assume dLWC /dz is a constant, a
 LWC(z) = az
Z(z)  (az)2 / N
Adiabatic profile: Z should vary as z2

21. Aircraft data - ACE 2
Brenguier et al. (2000)
1005 UTC
1545 UTC
Reflectivity profiles

22. Refined technique Nad Allow dilution from adiabatic profile of LWC
LWC(z) = k LWCad(z)
N = k Nad
D(z) = Dad(z)
Z(z)  k (az)2 / Nad
Nad

23. Plots of N
High N, small D  low Z
Nad = 264 cm-3

24. Plots of N
Nad = 91 cm-3

25. Plots of N
Nad = 82 cm-3

26. Presence of drizzle can lead to an overestimate of N
 an overestimate of LWC (and LWP)

28. 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

29. 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 - initial
LWP - lidar corrected

30. 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

31. Compare measured lwp to adiabatic lwp
obtain ‘dilution coefficient’
Dilution coefficient versus
depth of cloud

32. 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 mm
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

34. Drizzle below cloud
Doppler radar and lidar - 4 observables (O’Connor et al. 2005)
Radar/lidar ratio provides information on particle size

35. Drizzle below cloud
Retrieve three components of drizzle DSD (N, D, μ).
Can then calculate LWC, LWF and vertical air velocity, w.

36. Drizzle below cloud Typical cell size is about 2-3 km
Updrafts correlate well with liquid water flux

37. Profiles of lwc – no drizzle
Examine radar/lidar profiles - retrieve LWC, N, D

38. Profiles of lwc – no drizzle
260 cm-3
90 cm-3
80 cm-3
Consistency shown between LWP estimates.

39. 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