VHF radar measurements of small-scale and meso-scale dynamical processes in the middle atmosphere

Abstract
The principle of VHF radar observations of the atmosphere is briefly summarized. Gravity-wave and turbulence sources, as seen by VHF radar, are described, followed by an outline of observations of upward-propagating gravity waves and the corresponding transport of energy and momentum. Frequency and wavenumber spectra are discussed in terms of a universal spectrum of waves and quasi-two-dimensional turbulence. The divergence of vertical flux of horizontal momentum, which indicates the interaction of waves with the mean wind, is considered. Saturation of gravity waves often occurs in the middle atmosphere where it can cause turbulence. This controls the vertical transport of passive tracers by means of turbulent diffusion. These phenomena are investigated by VHF radars, particularly in the mesosphere. The phenomenology of ‘turbulence echoes' from the mesosphere, influenced considerably by the electron density profile, is examined. The generation mechanisms of turbulence and its coexistence with stable stratifications are explored. The presented model offers an explanation of the different features of the observed radar echoes and provides a better understanding of the interaction of meso-scale and small-scale phenomena of waves and turbulence in the middle atmosphere. Finally, a summary of some remaining questions that could be solved by further collaborative efforts, including the application of radars, is given.