An investigation of tilted aspect-sensitive scatterers in the lower atmosphere using the MU and Aberystwyth VHF radars

This study investigates subtle variations of the zenith and azimuth depende nce of VHF-radar echo power in the troposphere and lower stratosphere. Usin g the middle and upper atmosphere (MU) and Aberystwyth radars, we reanalyze data from two areas of the literature on tilted aspect-sensitive scatterer s, linking results from the spatial interferometry (SI) and Doppler beam-sw inging (DBS) techniques. Whereas wind profilers commonly use three or five radar beams, we examine a MU radar data set with 64 beam positions, so that maps of echo power distribution can be plotted as far as 5 degrees from ze nith. The power distribution pattern is often skewed, with the azimuth of m aximum power being closely related to the wind shear caused by, for example , inertia-gravity waves in the lower stratosphere. The results imply that i nertia-gravity wave motions are closely coupled to the smaller-scale wind f ield, causing patches of Kelvin-Helmholtz instability and/or steepening of other shorter-period gravity waves. These effects can alter the distributio n of the tilts of aspect-sensitive scatterers and explain the skewed echo p ower patterns. The deviations of vertical-beam incidence angle measured by SI are found to be inappropriate for off-vertical beams, and it also appear s impossible for basic DBS systems to be used for measuring vertical-beam i ncidence angles. Further tests of mountain wave data are consistent with th e tilted layer model and help to confirm that the azimuth of gravity waves may be calculated using radar echo-power imbalances.