radar/lidar observations of boundary layer clouds

Ewan O’Connor, Robin Hogan, Anthony Illingworth, Nicolas Gaussiat

Radar/lidar observations of boundary layer clouds

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

Vertically pointing radar and lidar

Radar: Z~D6

Sensitive to larger particles (drizzle, rain)

Lidar: ~D2

Sensitive to small particles

(droplets, aerosol)

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 %

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 - initialLWP - lidar corrected

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

http://www.met.rdg.ac.uk/radar/cloudnet/quicklooks/

Compare measured lwp to adiabatic lwp

• obtain ‘dilution coefficient’

Dilution coefficient versus depth of cloud

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 m

• 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

Drizzle below cloudDoppler radar and lidar - 4 observables (O’Connor et al. 2005)

• Radar/lidar ratio provides information on particle size

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

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

Profiles of lwc – no drizzleExamine radar/lidar profiles - retrieve LWC, N, D

Profiles of lwc – no drizzle

260 cm-3 90 cm-3 80 cm-3

Consistency shown between LWP estimates.


Cloud droplet sizes <12μm• no drizzle present

Cloud droplet sizes 18 μm• drizzle present

Agrees with Tripoli & Cotton (1980) critical size threshold

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

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

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

Drizzle-free stratocumulusZ = 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 Adiabatic profile: Z should vary as z2

Assume dLWC /dz is a constant, a

LWC(z) = az

Z(z) (az)2 / N

Aircraft data - ACE 2 Brenguier et al. (2000)

1005 UTC

1545 UTC

Reflectivity profiles

Refined techniqueAllow dilution from adiabatic profile of LWC

Z(z) k (az)2 / Nad

LWC(z) = k LWCad(z)

N = k Nad

D(z) = Dad(z)

Nad

Plots of N

High N, small D low Z

Nad = 264 cm-3

Plots of N

Nad = 91 cm-3

Plots of N

Nad = 82 cm-3

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

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