Rapid variations in echo power maps of VHF radar backscatter from the lower atmosphere

For the first time, echo power maps of aspect-sensitive VHF backscatter are shown, with good time and spatial resolutions, for angles 0°-7° from zenith. Sequences of power maps show large changes in appearance over timescales of a few minutes and height intervals of a few hundred metres. Often, individual power maps are consistent with tilted and distorted specular-type scattering layers, rather than anisotropic turbulence, and the direction of maximum echo power is sometimes several degrees off-vertical. Nevertheless, after time-averaging the variable echo-power patterns, the average pattern can become almost circular and centred on zenith, as has been assumed in the past. Echo power maps measured in strong windshear beneath the jet stream show a skewing of the echo power distribution. However, some power maps in the lower stratosphere, despite stronger wind shear, appear more constrained and their maximum echo power remains closer to zenith.