Integrated lidar backscatter Quantifying the occurrence of supercooled

Integrated lidar backscatter: Quantifying the occurrence of supercooled water and specular reflection Robin Hogan and Anthony Illingworth • Enhanced algorithm for supercooled water detection (Hogan et al. 2003, QJ in press) • Specular reflection could be a problem for nadirpointing Earth. CARE lidar: how common is it?

Introduction • The integrated backscatter through a cloud of optical depth of is approximately (Platt 1973): – k is the extinction/backscatter ratio (18. 75 sr for droplets) – is the multiple scattering factor (~0. 7 for the CT 75 K) • For large optical depth it reduces to (2 k)-1 • If z 1 and z 2 encompass the 300 m around the strongest echo in a profile, we can identify thin liquid water layers with greater than, say, 0. 7

Results for lidar 5° from zenith • Chilbolton 2000 – Occurrence of supercooled layers with > 0. 7

Results for zenith pointing lidar • Chilbolton 1999 – Enhanced occurrence between -10 and -20°C

Supercooled water in models • A year of data from the Met Office and ECMWF – Easy to calculate occurrence of supercooled water with > 0. 7

Specular reflection • Specular reflection from planar crystals can occur within 1° from zenith or nadir – Enhanced backscatter with no accompanying increase in extinction: very low k – Integrated backscatter in ice can exceed the asymptote corresponding to optically thick liquid cloud (recall ~(2 k)-1) • To quantify, require lidar to be precisely at zenith: 20 days of data obtained so far at Chilbolton – Algorithm calculates integrated backscatter from 2 km up – Specular reflection deemed to occur if this integral is more than 1. 05 times the asymptote for liquid water – Excess above this value is attributed to pixels with highest – But allowance made for common scenario of liquid above ice

Results • Around 23% of ice cloudy profiles strongly affected – Specular reflection in 20% of cloudy pixels at 4 km – Big problem for interpreting backscatter measurements • Must analyse more data – Use model for temperature: specular reflection only for plates between -9 and -23°C?