Complete maps of the aspect sensitivity of VHF atmospheric radar echoes

R M Worthington,R D Palmer,S Fukao

doi:10.1007/s005850050836

Abstract

Using the MU radar at Shigaraki, Japan (34.85∞N, 136.10∞E), we measure the power distribution pattern of VHF radar echoes from the mid-troposphere. The large number of radar beam-pointing directions (320) allows the mapping of echo power from 0∞ to 40∞ from zenith, and also the dependence on azimuth, which has not been achieved before at VHF wavelengths. The results show how vertical shear of the horizontal wind is associated with a definite skewing of the VHF echo power distribution, for beam angles as far as 30∞ or more from zenith, so that aspect sensitivity cannot be assumed negligible at any beam-pointing angle that most existing VHF radars are able to use. Consequently, the use of VHF echo power to calculate intensity of atmospheric turbulence, which assumes only isotropic backscatter at large beam zenith angles, will sometimes not be valid.

Highlights

VHF radar wind-pro®lers are widely used worldwide, but no high-resolution measurements have been made showing the dependence of their backscattered power on the radar beam-pointing direction
An explanation for the results shown here, and those in earlier papers, may be that small-scale Kelvin-Helmholtz Instabilities (KHI) or steepening in regions of windshear are tilting the aspect-sensitive scatterers from horizontal (Worthington and Thomas, 1997, hereafter WT97)
The wind shears may be the large-scale shears above and below the jet stream maximum, or those caused by inertia-gravity waves, for example

Summary

Introduction

VHF radar wind-pro®lers are widely used worldwide, but no high-resolution measurements have been made showing the dependence of their backscattered power on the radar beam-pointing direction (i.e. as a function of both zenith and azimuth angles, from vertical to large zenith angles). WT97 found that even fairly weak windshears (as measured by radar over height intervals of 300 m) cause the echo power distribution to be skewed, and for zenith angles as large as 12° (Fig. 6 of WT97), which is not consistent with the common assumption of isotropic echoes for VHF radar beams pointing >10° from zenith (e.g. Hocking and Mu, 1997).

Results

Conclusion