NOCTILUCENT CLOUDS AND THE THERMAL STRUCTURE NEAR THE ARCTIC MESOPAUSE IN SUMMER

In the summers of 1993 and 1994 a series of meteorological rockets and sounding rockets were launched from the Andoya Rocket Range (69 degre es N) during the SCALE and ECHO campaigns in order to investigate the state of the mesosphere and lower thermosphere (SCALE = ''SCAttering L ayer Experiment''; ECHO indicates that radar and lidar echoes are inve stigated), At the same location a Rayleigh lidar was operational durin g these campaigns and searched for enhanced backscatter signals fi om the upper mesosphere indicative of the presence of noctilucent clouds (NLC). In five cases the lidar detected a NLC and the atmospheric temp erature profile was obtained simultaneously by in situ techniques, A l iterature survey shows that there are only three more cases of unambig uous and simultaneous observations of NLC temperature and altitude, Th e temperature profiles obtained during SCALE and ECHO are as expected for the high-latitude summer season: The mean mesopause temperature is 135 K at an altitude of 87.5 kin, The variability of the temperatures is smaller above similar to 84 km altitude than below, The mean tempe rature below the mesopause shows a remarkable repeatability ever since the first measurements 30 years ago; at the lower edge of the NLC hei ghts (82 km) it is again and again observed. to be in the range 150 +/ - 2 K and the variability within each data set is only a few Kelvins, Such an ''equithermal submesopause'' in summer puts a serious constrai nt on any model prediction of secular changes of temperatures in the u pper mesosphere, The mean altitude of the NLCs as determined from our lidar measurements is 83.1 km which is surprisingly close to the very first height determinations more than 100 years ago, It is conceivable that this repeatability in altitude reflects a similar repeatability of the thermal structure. There is no apparent correlation between the conditions at the mesopause and the occurrence of NLCs at lower altit udes, The physical reason behind this is presumably related to the fac t that the wind direction changes at similar to 87 km height, which im plies that the air masses observed above the rocket site near mesopaus e altitudes have been advected from different locations than those at NLC heights. The NLCs are always located below the mesopause, and the temperature in the NLC layer is observed to be lower than 154 K, Our r esults are compatible with the idea that NLCs consist of ice particles which start to nucleate around the mesopause, sediment to lower altit udes while growing, become observable by the lidar and/or by the naked eye, and finally evaporate once they approach the higher temperatures around 82 km.