Spaced antenna VHF radar observations of tropospheric velocities and irregularities

Abstract

The application of spaced antenna techniques to VHF radar studies of the motions and structure of irregularities in the clear atmosphere is discussed. Observations made with the 53.5 MHz SOUSY‐VHF radar working at vertical incidence and using spaced antennas for reception confirm previous suggestions that echoes from the troposphere have contributions from specular (i.e., partial) reflection, also called ‘Fresnel reflection,’ as well as random scatter. Measurements of the angular width and spatial coherence show the reflecting regions to be thin and quasi‐horizontal in structure. These reflecting regions also are called sheets. Reflected to scattered power ratios are in the range between 0.2 and 10. The angular spectrum of reflecting/scattering structures is investigated by means of a radar interferometer technique. The angular width and the apparent tilt of the sheets is measured to be a few degrees. It has been possible to measure horizontal velocities by the drift method even on those returns where the angular widths are so narrow that the use of conventional Doppler techniques would be impracticable. The drift velocities agree well with balloon measured winds. It is found that stable, reflecting sheets occur primarily in regions of high dynamic stability. In regions of lower dynamic stability, scattering from turbulent irregularities is observed. In this case the mean dissipation rate, diffusion coefficient, and outer scale of the inertial subrange of turbulence are estimated from measured values of correlation time and Richardson number. The estimates are in accord with other turbulence measurements.