Lecturer: Peter Dalin (IRF)
Date: 2009-10-15 10:30
Place: Aniara

A case study of the evolution of a Kelvin-Helmholtz wave and turbulence in noctilucent clouds

Peter Dalin
Swedish Institute of Space Physics

Bright and extensive noctilucent clouds (NLC) were observed in Aarhus (Denmark) on 3/4 July of 2008 with an automatic digital camera taking images every minute. This event was unique in the sense that bright NLC were seen at high elevation angles (more than 30 degrees) that allowed observing the evolution of a Kelvin-Helmholtz (KH) wave, resulted in well-developed turbulence. In particular, coherent vortex structures of a horseshoe-shaped form were observed for the first time in noctilucent clouds. The turbulent diffusion coefficient around the mesopause is estimated in the range of 162-667 m2/s that is rather close and partly inside the range of 2-200 m2/s for the summer mesosphere inferred from previous experimental and theoretical studies. At the same time, it has been demonstrated that it is of importance to take into account non-turbulent process such as the gravity wave motion that is always present in NLC layers. Unless non-turbulent process is taken into account, this certainly leads to overestimating of the value of the turbulent diffusion coefficient. More accurate characteristics of turbulence in NLC can be obtained by analyzing a sequence of high-resolution images with a high frame-rate high-resolution digital camera.

Created 2009-09-21 10:42:47 by Mats Holmström
Last changed 2009-09-23 12:55:51 by Mats Holmström