Tidal structures within the LIMA model

Model results of LIMA (Leibniz Institute Middle Atmosphere Model) are used to reveal tides within the polar summer mesopause region. We specifically analyze temperature, water vapor, all wind components, and maximum backscatter coefficients that determine the brightness of noctilucent clouds as seen by lidars. Lidar measurements at ALOMAR (Arctic Lidar Observatory for Middle Atmosphere Research) in Northern Norway (69°N) and LIMA results show a high day-to-day variation of ice layer properties. Tidal variations are visible only after ice particle and atmospheric parameters from many days are overlayed and averaged. Lidar measurements of the maximum backscatter coefficient at ALOMAR from 2001 show a maximum at approximately 10 LT. The modeled maximum backscatter coefficient in summer 2001 also shows one distinctive maximum but considerably earlier (2 LT). By contrast, long-term observations at ALOMAR from 1997 to 2005 show a clear semidiurnal tidal signal indicating a high annual variability. The sun-synchronous diurnal tide is the prevalent mode in the maximum backscatter coefficient and in all other analyzed model parameters at latitudes poleward of 60°N, except for the vertical wind. In contrast, at lower latitudes eastward propagating non-migrating diurnal tidal components are prominent, especially wave-numbers 3 (DE3) and 4 (DE4). A spectral analysis of the tidal components as a function of latitude indicates that these non-migrating tides originate in the tropics.