NLC and the background atmosphere above ALOMAR

Abstract

Abstract. Noctilucent clouds (NLC) have been measured by the Rayleigh/Mie/Raman-lidar at the ALOMAR research facility in Northern Norway (69° N, 16° E). From 1997 to 2010 NLC were detected during more than 1850 h on 440 different days. Colocated MF-radar measurements and calculations with the Leibniz-Institute Middle Atmosphere (LIMA-) model are used to characterize the background atmosphere. Temperatures as well as horizontal winds at 83 km altitude show distinct differences during NLC compared to the absence of NLC. On seasonal mean it is colder and the winds are stronger westward when NLC are detected. The wind separation is a robust feature as it shows up in measurements as well as in model and it is consistent with the current understanding that lower temperatures support the existence of ice particles. For the whole 14-years data set there is no statistically significant relation between NLC occurrence and solar activity. On the other hand NLC occurrence and temperatures at 83 km show a significant anti-correlation, which suggests that the thermal state plays a major role for the existence of ice particles and dominates the pure Lyman-α influence on water vapor during certain years. We find the seasonal mean NLC altitudes to be correlated to both Lyman-α radiation and temperature. NLC above ALOMAR are strongly influenced by atmospheric tides. Diurnal and semidiurnal amplitudes and phases show partly pronounced year-to-year variations. In general, amplitudes as well as phases vary in a different manner. Amplitudes change by a factor of more than 3 and phases vary by up to 7 h. Such variability can impact NLC observations limited to fixed local times.