First measurements of neutral wind and turbulence in the mesosphere and lower thermosphere over Taiwan with a chemical release experiment

Abstract

Neutral winds and turbulence structure in the mesosphere and lower thermosphere were measured with a sounding rocket chemical release experiment carried out in Taiwan. Trimethyl aluminum (TMA) was used as a tracer of the drift of the background atmosphere. The results show that the measured neutral wind had a maximum speed of 154 m/s at a height around 105 km. The wind vectors were found to rotate clockwise with height in the altitude range from about 98 to 121 km. On the basis of the hodograph analysis of the measured neutral wind vectors, an upward propagating inertio‐gravity wave with intrinsic period of 11.2 hours and vertical wavelength of 19.5 km was believed to be primarily responsible for the height variations of the neutral wind and the Richardson number. Large vertical wind shears were observed near 100 km and 106 km. Turbulent structures were observed in much of the trail below 110 km, but there were enhanced structures in the altitude range between the two large shears, i.e., at and near the altitude of the maximum wind speed. Comparing the positions of the turbulent features with expected atmospheric stability zones induced by an upward propagating gravity wave indicates that the turbulence structures were primarily located within the wave‐induced convectively unstable zone. The structures are therefore interpreted as counterrotating vortices within the convective instability zone of a breaking gravity wave. Moreover, the relation between the horizontal separations λ (about 1.8 km) of the turbulent structures and the vertical extent h (about 0.7 and 1.5 km) of the wind shear zones with Richardson numbers less than 0.25 did not conform to the predicted λ/h ratio of a factor of approximately 8 predicted by simple linear Kelvin‐Helmholtz instability theory for the primary billow structures associated with the instability. These results suggest that the observed structures were not the primary billows but were more likely associated with a secondary instability, such as the counterrotating vortices that develop later in the evolution of the instability. In general, the observations reported here support the interpretation that the turbulence evident in the trail was very likely generated in the convectively unstable zone induced by the inertio‐gravity wave propagating through the region with large temperature gradients −32 K/km produced by wave breaking processes.