Earth Planets Space, Vol. 61 (No. 4), pp. 545-549, 2009
Professor Emeritus, Kyoto University, Uji, Kyoto, Japan
(Received July 27, 2007; Revised August 19, 2008; Accepted December 12, 2008; Online published May 14, 2009)
Doppler radar observations have made a large contribution towards improving our understanding of middle- and upper-atmosphere dynamics. This radar echo is mainly due to radiowave scattering by atmospheric turbulence, which occurs almost universally throughout the atmosphere as a result of gravity wave (GW) breaking. In the strictness sense, the radar pulse is back-scattered by radio refractive index (RRI) perturbations caused by turbulence whose size along the radar beam is half of the radar wavelength. Understanding of the validity of the MST (mesosphere, stratosphere and troposphere)-radar observation requires a precise knowledge of how RRI perturbations are formed and behave. A basic property of the RRI of the atmosphere is that it has a static vertical gradient in terms of distribution and the turbulence velocity is divergence-free. The RRI depends on three atmospheric propertieshumidity, air density and electron density. These three elements each have a static vertical gradient along which turbulence transports each element, hereby perturbing the density distribution of each element; that is to say, it produces the RRI irregularity for scattering the radar pulse. Since the turbulence motion and winds are divergence-free, PRI perturbation co-moves with the total air flow, i.e. turbulence motion plus winds. This can be true even in the mesosphere where the perturbation is controlled electromagnetically. Co-movement of turbulence with local winds has been shown from a comparison of observations with radars and radiosondes. In addition to tracking turbulence as wind-tracers, MST-radar observations provide important data in the study of atmosphere turbulence dynamics.
Key words: Radio refractive index perturbations, turbulence, Doppler radar observations, atmospheric turbulence dynamics.