VHF radar observations of turbulent structures in the polar mesopause region

P Czechowsky,R Rüster

doi:10.1007/s00585-997-1028-8

Abstract

Abstract. The mobile SOUSY VHF Radar was operated in the summer of 1987 during the MAC/SINE campaign in northern Norway to study the polar mesosphere summer echoes (PMSE). Measurements of the spectral width indicate that two types of structures occur. In general mesospheric layers are bifurcated exhibiting a narrow spectral width and a well-defined aspect sensitivity. However, for about 10% of the observation time cells of enhanced turbulence characterized by extremely broad spectral widths appear predominantly in the upper sublayer above 86 km. Identification and separation of beam and shear broadening allows a determination of the turbulence-induced component of the spectral width. This case study reveals that during several events these cloud-like structures of enhanced turbulence move with an apparent velocity of several tens of meters per second which is almost identical with the phase trace velocity of simultaneously observed waves. Since, at that time, the Richardson number was less than a quarter, it was concluded that these turbulent cells were generated by a Kelvin-Helmholtz mechanism. The horizontal extent of these structures was calculated to be less than 40 km. A general relation between spectral width and echo power was not detected. The turbulent component of the spectral width was used to calculate typical values of the energy dissipation rate at times where narrow spectral width dominates and during periods of enhanced turbulence. In addition, the outer scale of the inertial subrange (buoyancy scale) was estimated. For the first time the occurrence and motion of this type of structures of enhanced spectral width is analyzed and discussed in detail.

Highlights

Radar measurements at Andenes, Norway (69N, 16E) were performed with the mobile SOUSY system to study the nature of echo structures, wave dynamics and Correspondence to: P
The mobile SOUSY VHF Radar was operated in the summer of 1987 during the MAC/SINE campaign in northern Norway to study the polar mesosphere summer echoes (PMSE)
During a very active period of about 3 h, starting at 0905 LT, the signal-tonoise ratio (S/N) exceeds 36 dB indicated by the dark red color

Summary

Introduction

Radar measurements at Andenes, Norway (69N, 16E) were performed with the mobile SOUSY system to study the nature of echo structures, wave dynamics and Correspondence to: P. Scattering was detected from layers at heights between about 80 to 92 km, which are quite often split into two sublayers, one above and one below 86 km In some cases these structures descend with an apparent velocity of about 1 km hÀ1 (see e.g., Ecklund and Balsley, 1981; RuÈ ster, 1984; Czechowsky et al, 1989; Cho and Kelly, 1993). In general these polar mesospheric summer echoes (PMSE) are dominated by narrow spectral widths and strong aspect sensitivities (Czechowsky et al, 1988). Power bursts which were observed in connection with KelvinHelmholtz instabilities (KH1) as reported by Klostermeyer and RuÈ ster (1984) and by RuÈ ster and Klostermeyer (1987) were detected only twice in connection with these cells

Results

Discussion

Conclusion