Long-term variations of noctilucent clouds at ALOMAR

Noctilucent clouds (NLC) are measured by the Rayleigh/Mie/Raman-lidar at the ALOMAR research facility in Northern Norway (69°N, 16°E) since 1994. The data set contains about 2860 h of NLC detections and is investigated for the first time regarding trends. NLC properties depend on cloud brightness which is taken into account by the use of several cloud classes, related to brightness ranges. For NLC brighter than the long-term detection limit and strong NLC, respectively, the trend terms show increasing occurrence frequency (+9%/dec and+5%/dec) and brightness (+1.7×10−10 m−1 sr−1/dec and +1.5×−10 m−1 sr−1/dec) from 1998 to 2015. In the same period the altitude of faint and long-term limit clouds decreases (−66 m/dec and −108 m/dec). Over the entire time period of 22 years strong clouds show an increasing altitude by +76 m/dec. NLC properties are affected differently by solar and atmospheric parameters. In general, Lyman-α and stratospheric ozone impact all three NLC parameters, temperature at 83 km impacts mainly the NLC altitude. Time series of RMR lidar and SBUV satellite instruments match best for NLC occurrence frequency and brightness when restricting SBUV to the morning data at longitudes around ALOMAR (64-74°N, 8-24°E/0-9 LT). This suggests longitudinal dependent trends, which is confirmed by trend investigations of longitudinal subsets of the SBUV data set.