A Case Study of Two-Dimensional Stratified Turbulence

Abstract

Data from a period with intensive measurements in the lowest 1000 m of a marine atmosphere over the Baltic Sea with strongly stable stratification and a turbulent boundary layer less than 50 m deep have been analyzed with respect to the mesoscale flow regime. Analysis of the wind data shows that horizontal wind fluctuations take place in vertically decoupled layers of the order of a few hundred meters deep. In spite of sometimes large shear, turbulence levels are shown to be very low, implying a ?dissipation travel distance? of the order 1000 km. Airborne measurements have been used to produce wavenumber spectra of the zonal and meridional components of the wind and the temperature over the spectral range 10?4 ? ? ? 1 radians m?1 and frequency spectra for the vertical wind component. Extended series of continuous wind measurements on a tower were used to derive frequency spectra of the wind, which were Taylor-transformed to wavenumber spectra, showing good agreement with the spectra derived from the airborne measurements. The horizontal wavenumber spectra show no significant height variation for the 30?500-m layer studied. For the approximate wavenumber range 10?5 ? ? ? 10?3 radians m?1, these spectra fall off as ??5/3, with a spectral level only about a factor of 2 lower than the corresponding Global Atmospheric Sampling Program spectra from the upper troposphere and the lower stratosphere. Vertical velocity spectra are very close in shape and spectral amplitude to corresponding spectra derived from Doppler radar measurements at several places around the world during ?quiet-time conditions.? However, an important difference of the present spectra is that they show no sign of scaling with the local Brunt?Väisälä frequency. The temperature spectra are closely similar in shape to the horizontal velocity spectra, and their amplitude scale with the square of the local Brunt?Väisälä frequency. Detailed analysis of the spectra for this wavenumber range suggests that the spectra of horizontal motion reflect quasi-two-dimensional stratified turbulence, but that the vertical velocity spectra reflect wave activity. In the wavenumber range 10?3 ? ? ? 10?2 radians m?1 the horizontal wind spectra fall off approximately as ??9/4, which suggests that internal waves may make up this spectral region, an assertion that is also supported by application of a polarization relation valid for waves. Temperature?vertical velocity cross spectra show no signs of linear, monochromatic waves. Instead it is possible that the waves in this region are breaking, a process that has previously been suggested as a possible source for the quasi-two-dimensional turbulence.