Analysis of gravity waves structures visible in noctilucent cloud images

The noctilucent clouds (NLC) are high-altitude bright cloud formations visible under certain conditions from high-latitude places during the summer months. Even if the exact nature of these clouds still remains a mystery, they are an efficient tracer of the dynamic processes at their level, particularly the gravity waves propagating from the stratosphere through the mesopause layer. In this paper, we describe a technique developed to analyze the structures visible in the NLC images taken every summer night since 2004 from Stockholm, Sweden (59.4°N). The parameters of 30 short-period gravity wave events have been measured and compared with older datasets obtained mostly from low and mid-latitude sites, using airglow imaging techniques. The horizontal wavelengths are in good agreement with previous results while the observed horizontal phase speeds exhibit smaller values than for other sites. The directionality of the waves presents strong poleward preference, traditionally observed during the summer season. This anisotropy and the difference in the phase speed distribution cannot be explained by the filtering due to the background wind field but more probably by the position of the gravity waves sources, located to the south of the observation site.