Morphology and sources of turbulence in the mesosphere during DYANA

Abstract

During the DYANA campaign in early 1990 turbulent parameters were measured at various places by means of in situ and ground supported techniques. Rocket borne instruments detected small-scale fluctuations of neutral (TOTAL instrument) and ion (PIP instrument) number densities in the mesosphere and lower thermosphere. A total of six flights was successfully performed in Andøya (69°N) and two in Biscarosse (44°N). Altitude profiles of turbulent parameters, such as turbulent energy dissipation rates ε. and turbulent diffusion coefficients K were derived from the fluctuations. Thanks to improvements in the instrumental capabilities, the reliability of the absolute values is unprecedented. The mean turbulent energy dissipation rates measured both by TOTAL and PIP in Andøya show very low values ( < l mW/kg) in the lower mesosphere, increasing to ~ 10 W/kg in the upper mesosphere. The corresponding heating rates are approximately 0.05 and 1 K/day, respectively. The two flights performed in Biscarosse showed similar low values. Nearly simultaneously with the sounding rocket flights, temperatures and winds were measured by meteorological rockets and by lidars. This allows study of the relationship between the occurrence of turbulence and the atmospheric stability, parameterized by the Richardson number. In 14 out of 18 cases strong turbulent layers were accompanied by low Richardson numbers. Both wind shear and convectively generated instabilities were observed. Given a particular altitude in the mesosphere, the TOTAL and PIP instrument detected turbulence in approximately 40–60% of all flights. In addition to the in situ measurements, energy dissipation rates were measured around 75 km by the chaff dispersion technique at Heiss Island (81°N) and at Volgograd (48°N). Much higher ε values were observed (typically 100 mW/kg) using this procedure than those values obtained by in situ measurements. However, due to non-turbulent dispersion processes, such as small-scale gravity waves and wind shears, the absolute ε values from this technique are considered upper limits.