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

Abstract

This study investigates subtle variations of the zenith and azimuth dependence of VHF‐radar echo power in the troposphere and lower stratosphere. Using the middle and upper atmosphere (MU) and Aberystwyth radars, we reanalyze data from two areas of the literature on tilted aspect‐sensitive scatterers, linking results from the spatial interferometry (SI) and Doppler beam‐swinging (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° from zenith. The power distribution pattern is often skewed, with the azimuth of maximum power being closely related to the wind shear caused by, for example, inertia‐gravity waves in the lower stratosphere. The results imply that inertia‐gravity wave motions are closely coupled to the smaller‐scale wind field, causing patches of Kelvin‐Helmholtz instability and/or steepening of other shorter‐period gravity waves. These effects can alter the distribution of the tilts of aspect‐sensitive scatterers and explain the skewed echo power patterns. The deviations of vertical‐beam incidence angle measured by SI are found to be inappropriate for off‐vertical beams, and it also appears impossible for basic DBS systems to be used for measuring vertical‐beam incidence angles. Further tests of mountain wave data are consistent with the tilted layer model and help to confirm that the azimuth of gravity waves may be calculated using radar echo‐power imbalances.