RAYLEIGH LIDAR DETECTION OF AEROSOL ECHOES FROM NOCTILUCENT CLOUD ALTITUDES AT THE ARCTIC-CIRCLE

During 3 out of 16 observations runs in July and August 1993 the Rayle igh Lidar at the Andoya Rocket Range (69-degrees-N, 16-degrees-E) in N orthern Norway detected aerosol echoes from noctilucent cloud altitude s on July 28, August 7, and August 9. The geometric elevation of the c enter of the Sun was from +1.3-degrees to -4.5-degrees during aerosol detection. These three events differed significantly in peak signal st rength, altitude, cloud layer shape, altitude integrated signal, and t emporal evolution. Aerosol echoes were seen from the altitude range 81 to 87 km. The strongest aerosol event showed a peak backscatter ratio of 240 at 83.2 km altitude equivalent to the molecular (Rayleigh) sca ttering signal from 41.5 km. The weakest event had a peak backscatter ratio of 7 at 84.8 km with a Rayleigh equivalent altitude of 73.3 km. The zenith optical thickness of the aerosol layers varied by approxima tely two orders of magnitude. Detection times ranged from longer than 5 hours to as short as 15 minutes. The temporal evolution during the e vents suggests that single clouds were drifting through the laser beam which has a diameter of approximately 4 m at 85 km altitude. All even ts occurred before local midnight and the gross temporal evolution is compatible with tidal models for the diurnal variation of the visibili ty in PMCs and NLCs although there is considerably more structure in t he lidar data than predictable by such a model. The estimated zenith o ptical thickness is within the bounds of microphysical NLC models.