Estimate of the global distribution of stratiform supercooled liquid water clouds using the LITE lidar

Supercooled liquid water clouds can occur in the form of thin layers that have a much larger radiative impact than ice clouds of the same water content because of their smaller particle size, yet they are poorly represented in climate models. Such clouds may be easily distinguished from ice by their high lidar backscatter coefficient and sharp backscatter gradient at cloud top. In this paper, data from the Lidar In-space Technology Experiment ( LITE), which flew on the space shuttle in 1994, are used to estimate the fraction of clouds that contain supercooled liquid water over the latitude range +/- 60degrees. Around 20% of clouds between - 10 degreesC and - 15 degreesC were found to contain liquid water, falling with temperature to essentially zero below - 35 degreesC. Even from this limited dataset some clear latitudinal clear trends were evident, with a distinctly more frequent occurrence of supercooled water in clouds associated with mid-latitude weather systems in the southern hemisphere, as well as in tropical clouds warmer than around - 15 degreesC. The results between 40 and 60 degreesN agree well with the distribution previously found at Chilbolton in Southern England ( 51 degreesN), implying that the forthcoming long-term lidar observations from space will be able to infer the global distribution of mixed-phase clouds with much greater accuracy and vertical resolution than has been possible until now.