SIMULTANEOUS OBSERVATIONS OF NOCTILUCENT CLOUDS AND MESOPAUSE TEMPERATURES BY LIDAR

During the night of August 5-6, 1989, geometric and optical properties of a noctilucent cloud (NLC) as well as the thermal structure of the atmosphere above the cloud were measured by ground-based lidar. The data were collected at Andenes, Norway (69 degrees N, 16 degrees E). The cloud was observed from 2220 UT until 0010 UT, during which time its mean altitude was 82.6 km. Temperatures were measured from 2129 UT until 0017 UT in the altitude range of 86 to 94 km. A persistent temperature minimum was located in the 87- to 89-km altitude region with temperatures dropping from close to 140 K at the beginning of the NLC observation to around 130 K after midnight. In a 2-km-wide layer below this minimum the temperature was persistently too high to provide supersaturation of water assuming realistic water concentrations. Comparison with other temperature measurements in the first half of August 1989 shows that the thermal conditions at the mesopause on August 5-6, 1989, were not especially favorable for the formation of NLCs, while on other occasions with lower mesopause temperatures no NLCs were observed. We conclude that the occurrence of NLCs only limitedly depends on the local thermal conditions at the display height and above. Instead, the conditions along the whole NLC particle trajectory have to be taken into account. This implies that horizontal and vertical advection are important factors controlling the occurrence of NLC displays in late summer at this latitude,