Monitoring of noctilucent clouds by lidar

.Noctilucent clouds are optically thin clouds that appear in the summer polar regions at an altitude of about 83 km. These clouds are made up of small ice particles that grow (r < 100 nm) under the cold temperature conditions (T < 150 km) of the summer mesopause region. While noctilucent clouds (NLCs) have been known to exist for over 100 years, the processes that govern their formation and evolution remain elusive. This is due to the lack of measurements in and around the mesopause region. NLCs have been linked to global change processes as their supply of water vapor is provided by methane oxidation in the middle atmosphere and increasing carbon dioxide concentrations near the mesopause may augment the cold temperatures. Therefore, long term monitoring of NLC characteristics and their behavior may indicate important changes taking place in the mesosphere environment related to the rise in greenhouse gases. Lidars operating at high latitudes in the summer months have begun to contribute significantly to the study of NLCs by providing accurate measurements of the cloud's height, thickness, backscatter strength, surrounding temperature environment, and temporal evolution. This paper will review the contributions of lidars to NLC studies and discuss the aspects of lidars monitoring long-term changes in NLC characteristics