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) - PowerPoint PPT Presentation
Integrated lidar backscatter:Quantifying the occurrence of supercooled water andspecular 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?
IntroductionThe 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)-1If z1 and z2 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 zenithChilbolton 2000Occurrence of supercooled layers with > 0.7
Results for zenith pointing lidarChilbolton 1999Enhanced occurrence between -10 and -20C
Supercooled water in modelsA year of data from the Met Office and ECMWFEasy to calculate occurrence of supercooled water with > 0.7
Specular reflectionSpecular reflection from planar crystals can occur within 1 from zenith or nadirEnhanced 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 ~(2k)-1)To quantify, require lidar to be precisely at zenith: 20 days of data obtained so far at ChilboltonAlgorithm calculates integrated backscatter from 2 km upSpecular reflection deemed to occur if this integral is more than 1.05 times the asymptote for liquid waterExcess above this value is attributed to pixels with highest But allowance made for common scenario of liquid above ice
ResultsAround 23% of ice cloudy profiles strongly affectedSpecular reflection in 20% of cloudy pixels at 4 kmBig problem for interpreting backscatter measurementsMust analyse more dataUse model for temperature: specular reflection only for plates between -9 and -23C?