Mean diurnal variations of noctilucent clouds during 7 years of lidar observations at ALOMAR

From 1997 to 2003, noctilucent clouds (NLC) were observed by lidar above the ALOMAR observatory in Northern Norway (69 N) during a total of 1880 measurement hours. This data set contains NLC signatures for 640 h, covering all local times, even during the highest solar background conditions. After data limitation imposing a threshold value of 4×10−10 m−1 sr−1 for the volume backscatter coefficient of the NLC particles, a measure for the cloud brightness, local time dependencies of the NLC occurrence frequency, altitude, and brightness were determined. On average, over the 7 years NLC occurred during the whole day and preferably in the early morning hours, with a maximum occurrence frequency of∼40% between 4 and 7 LT. Splitting the data into weak and strong clouds yields almost identical amplitudes of diurnal and semidiurnal variations for the occurrence of weak clouds, whereas the strong clouds are dominated by the diurnal variation. NLC occurrence, altitude, as well as brightness, show a remarkable persistence concerning diurnal and semidiurnal variations from 1997 to 2003, suggesting that NLC above ALOMAR are significantly controlled by atmospheric tides. The observed mean anti-phase behavior between cloud altitude and brightness is attributed to a phase shift between the semidiurnal components by ∼ 6 h. Investigation of data for each individual year regarding the prevailing oscillation periods of the NLC parameters showed different phase relationships, leading to a complex variability in the cloud parameters.