Presentation on theme: "Integrated lidar backscatter: Quantifying the occurrence of supercooled water and specular reflection Robin Hogan and Anthony Illingworth Enhanced algorithm."— Presentation transcript:
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 nadir- pointing EarthCARE 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 CT75K) For large optical depth it reduces to (2k) -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 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? –Problem solved with a high spectral resolution lidar?