Radar/lidar observations of boundary layer clouds

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

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: b~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 - initial LWP - 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 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

Drizzle below cloud Doppler 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 drizzle Examine radar/lidar profiles - retrieve LWC, N, D

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

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

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

Data

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

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

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

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