Complementary criteria for identifying regions of intense atmospheric turbulence using lower VHF radar

Use is made of data from three campaigns of combined multi-beam lower VHF radar and radiosonde observations of the troposphere and lower stratosphere at Aberystwyth (52.4°N, 4.1°W). These provide altitude profiles of the usual radar signal parameters in addition to those of the Brunt-Väisälä frequency, ω_B, and the gradient Richardson number, Ri. It is shown that the signal strength observed by a radar beam directed 12° off-vertical, which is expected to represent backscatter alone, is not necessarily enhanced within regions identified by Ri as containing intense turbulence. Furthermore, perturbations of the signal strength altitude profiles are associated with those of the ω²_B profile. The possibility that Fresnel scatter contributes to radar returns at such a large zenith angle cannot be discounted. It is shown that regions of intense turbulence can, however, be identified by a combination of an absence of aspect sensitivity of radar returns, an enhancement of vertical beam spectral width, which has been corrected for the effect of beam broadening, and values of Ri close to or less than 1/4. This method has highlighted the presence of layers of intense turbulence which follow descending phase fronts of long-period gravity waves in the lower stratosphere.