Investigations from lidar measurements at 54°N, 69°N and 78°N
One of the research topics of the IAP is the examination of noctilucent clouds (NLC) with different lidars at different locations. In north germany and in summer time NLC can sometimes be observed bye naked eye or camera. The figure on the right shows a noctilucent cloud on July, 24, 2005 at 84 km altitude above the Baltic see. The direct observation is possible because the NLC is still in sunlight even though the sun is below horizon. The background on the other hand is dark. Such observations are not possible at day. The relative dark clouds are outshined by the sun. The observation of noctilucent clouds at more northern latitudes in the polar region is only possible in exceptional cases. This observation gap is closed at the IAP by lidar measurements.
Noctilucent clouds can only exist at very cold temperatures (<~130° C). Such low temperatures are extreme events above Kühlungsborn and occur only during the summer months June/July (see temperature structure above Kühlungsborn). A comparison of mean temperatures with the occurrence rate of NLC (figure 1) shows that the temperature above Kühlungsborn is on average to warm for the formation of ice clouds. NLC are accordant seldom (<10% of the time). In exceptional cases NLC have been observed further south as observed by our digital camera network.
The occurrence rate further north increases rapidly because the temperature in summer time in the mesopause region decreases towards the pol (see temperature structure from 69°N and 78°N). Beside lidar measurements with the K lidar and RMR lidar at Kühlungsborn, 54° N, NLC are observed since many years by the ALOMAR RMR lidar at 69° N. At polar latitudes observations must be performed during daylight conditions resulting in corresponding technical complexity. Measurements similar to figure 2 about several years at ALOMAR show that NLC at this latitude occur considerably frequently (~38 % of the time) and that the occurrence rate depends on the time of day. Particle properties are derived by three wavelengths measurements. The Observations at ALOMAR show that NLC at polar region is a typical phenomena with long term changes.
Further north measurements have been performed between 2001 and 2003 with the mobile K lidar. The measurements at Spitsbergen, 78° N show, that noctilucent clouds occur 75% of the time during summer time. The simultaneous measured temperatures are colder compared to the other stations. The occurrence rate of NLC increases further towards the pol. The mean altitude of NLC is at the same time relative constant with only a few hundred meter difference between all lidar stations.
Figure 1: Comparison of night mean temperatures with the occurrence of NLC and MSE above Kühlungsborn, 54°N. Figure adopted from Gerding et al., JGR, 2007.
Temporal evolution of a NLC at ALOMAR, 69° N.