Tidal variations of noctilucent clouds measured at 69°N latitude by groundbased lidar

During June/July 1997 we observed with a groundbased Lidar noctilucent clouds (NLCs) overhead the ALOMAR observatory in Northern Norway (69 degrees N, 16 degrees E). These daylight observations of NLC were made possible by use of a very small field-of-view for the lidar receiving telescope in combination with a narrowband double-etalon Fabry-Perot interferometer in front of the photon counting detectors. During 148 hours of lidar measurements, NLCs were observed for 69 hours. Thus, the mean occurrence frequency of NLC was 46 %. The observed NLCs exhibited marked variations of the layer altitude and backscatter ratio with local time (LT). The temporal behaviour of both the NLC altitude and NLC backscatter ratio are dominated by a semidiurnal variation. The NLC altitude attains maxima near 13 LT and 01 LT and minima near 07 LT and 19 LT. The backscatter ratio clearly maximizes at times of minimum NLC layer altitude. These observations point to a strong control of variations in NLC altitude and backscatter ratio by a stable semidiurnal tide. The mean centroid NLC altitude is 82.7 km.