Tidal structures in noctilucent clouds (NLC)
Atmospheric tides profoundly affect the upper mesosphere because they introduce large atmospheric variations, transport energy, and momentum from lower atmospheric regions. The classical theory distinguishes between migrating and nonmigrating tides. Migrating or sun-synchronous tides propagate westward with the apparent motion of the sun and are excited due to solar radiation absorption by tropospheric water vapor and stratospheric ozone. Nonmigrating tides represent the remaining set of atmospheric tides which are not sun-synchronous. They may be forced by diverse mechanisms, such as longitudinal variations in the heating rates, induced by variations in trace gas concentrations, land-sea differences or topography. Tidal oscillations have periods that are some integer fraction of a solar day. An outstanding phenomenon of the upper mesosphere region are noctilucent clouds (NLC) which are the highest clouds of the Earth's atmosphere. Observations at ALOMAR 69°N show significant tidal signatures in NLC measurements. These observations are compared with model results of LIMA and LIMA-ICE. The modeled brightness as a function of local time at ALOMAR shows one maximum at 02 LT for the year 2001. The lidar data exhibits a strong peak at 10 LT. The maximum and minimum brightness modeled by LIMA and observed at ALOMAR nicely agree within 10 %. The phase shift of 8 hours may be explained by the fact that in a model the vertical propagation of tidal signals from lower parts of the atmosphere is somewhat different from nature. On a global scale the maximum of the modeled mean seasonal brightness propagate with a period of about 24 hours to the west indicating a diurnal sun-synchronous tide. Nevertheless, by comparing brightness at different times the magnitude of the maximum does not propagate homogenous but indicates a contribution of other components which are not sun-synchronous such as nonmigrating tides, waves or the nonlinearity of the microphysics.
Atmospheric tides are present in the LIMA model. Regarding ice layers, various atmospheric parameters influence the growth of ice particles and thereby the brightness of NLC. Concerning the comparison between model and observation there is a good agreement of the amplitude of βmax, but the model exhibits a phase shift of 8 hours. At ALOMAR a high correlation between low temperatures and bright NLC exist. This indicates that temperature is the main driver for the diurnal variation of the seasonal mean brightness of NLC. In the future we will extend the model analysis to several years and will study in detail the summer-to-summer variations of various atmospheric parameters with respect to the formation of ice layers in the polar summer mesopause region.
Diurnal variation of NLC brightness (βmax) above ALOMAR. Calculations of LIMA-ICE (red) and lidar measurements (black) are shown.