Azimuth angle variations of specular reflection echoes in the lower atmosphere observed with the MU radar

Abstract

Abstract We studied azimuth angle variations of clear-air echoes in the troposphere and lower stratosphere, observed by steering the antenna beam of the MU radar into 12 positions, the azimuth angle being changed every 30 ° at a zenith angle of 6 °. Azimuth angle variations of the echo intensity were recognized in a large height range in the lower stratosphere, although they were also sometimes found in the troposphere, which was approximated by a fairly smooth sinusoidal curve, with one or two cycles in 360 ° azimuth angle. Moreover, their structure showed continuous progression with time and altitude. The range of variations was generally greater in the lower stratosphere, the ratio of the maximum to the minimum sometimes exceeding 15 dB. The large azimuth angle variations were associated with the aspect sensitivity of the echo power, suggesting that they were caused by the characteristics of specular reflection rather than the effects of localized turbulence scattering, and suggesting that the reflection surface was corrugated, probably the result of the effects of gravity waves. A numerical model qualitatively explained the fundamental behavior of the azimuth angle dependence of the echo power, it being assumed that the vertical displacement of the reflection surface showed sinusoidal variation caused by the dynamical effects of a monochromatic gravity wave.