12th Conference on Middle Atmosphere

Monday, 4 November 2002: 9:15 AM
Small-scale turbulence observations in the upper troposphere and lower stratosphere from MST radars
Gregory D. Nastrom, St. Cloud State University, St. Cloud, MN; and T. E. VanZandt, T. Tsuda, and J. Furumoto
MST radars offer a unique opportunity to infer the intensity of small-scale turbulence in the upper troposphere and lower stratosphere with very high resolution in time and height from the Doppler spectral widths. The intensity of turbulence in the inertial sub-range is a key variable in atmospheric science as it is directly linked to the eddy dissipation rate and the eddy diffusivity. A major problem is the need to correct the observed spectral widths for effects related to the interaction of the prevailing wind with the geometry of the sampling volume; indeed, in many cases the corrections are larger than the observed spectral width. Working with the hypothesis that the sampling volume is intersected by a small number of quasi-horizontal layers that are highly turbulent, rather than uniformly filled with turbulence, we derive an analytic expression for the effective radar beamwidth. This expression predicts that the effective beamwidth is a function of beam zenith angle and range. These predictions are tested using observations taken with the highly versatile MU radar in Japan during April, 2002. The turbulence intensities found using the effective beamwidths are then compared with those inferred using the new dual-beamwidth. Conclusions will be presented regarding the influence of intense layers on this technique, and on the general capability of MST radars for providing useful climatologies of small-scale turbulence.

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