Measurement of anisotropic permittivity structure of upper troposphere with clear‐air radar

Abstract

When radar echoes are obtained from the upper troposphere, tropopause and lower stratosphere, one characteristic reported by several groups is a variation in echo strength with pointing angle. These observations are interpreted in terms of an anisotropic structure to the permittivity of the air in that region. The present paper attempts to make some quantitative estimates of this anisotropy. The measurements analyzed were obtained with the SOUSY 50 MHz radar installed in the 1000‐foot‐diameter antenna at Arecibo. Observations were made at eight zenith angles, from 0° to 11.7°, and at two azimuths, north and east. Echo strength as a function of zenith angle was obtained for these two azimuths and for many heights, with a range resolution of 150 m. These data were then fit to an anisotropic scattering model, and the possible anisotropy parameters were found. Horizontally, there was a range of uncertainty between the degree of anisotropy and its orientation. This uncertainty was inherent in the original measurements, since only two azimuths were used. However, despite this latitude, the vertical anisotropy was quite closely determined. For most heights, the ratio of vertical to horizontal correlation distance of the permittivity field was 0.2 or less.