High-resolution turbulence observations in the stratosphere with LITOS

Abstract:

Although the stratosphere is mostly stably stratified, breaking gravity waves and instabilities produce turbulence and energy dissipation. This modifies the energy distribution from the troposphere to the mesosphere and is an important parameter for the vertical mixing of trace species. In order to precisely infer energy dissipation rates, the viscous subrange has to be resolved, which in the stratosphere lies at scales of centimeters and below. Our balloon-borne system LITOS (Leibniz-Institute Turbulence Observations in the Stratosphere) observes small-scale wind fluctuations with a vertical resolution of less than 1 mm. The dissipation rate is obtained by fitting a turbulence model to the measured spectrum of fluctuations.

Between 2008 and 2011 three flights were performed from Kiruna/Sweden (68°N, 21°E) during BEXUS campaigns as part of a large (~120 kg) payload. Recently, a new small version of LITOS (overall ~4 kg) was flown several times from Kühlungsborn/Germany (54°N, 12°E), thereof one during nighttime. Various turbulent layers with a vertical thickness in the order of a few 10 m have been observed. Stratospheric energy dissipation rates greatly vary within only a few 10 m, roughly between 10⁻⁸ and 10 W/kg, with a mean value of roughly 10⁻³ W/kg. Huge differences have been found in the altitudinal structure and strength of stratospheric turbulence. Results and differences between flights will be discussed in the geophysical context. Turbulence data will be compared with results from simultaneous radiosonde data (5–10 m vertical resolution).