MU radar observations of tropopause variations by using clear air echo characteristics

Abstract

We present in this paper the characteristics of clear air echoes revealed by the MU radar experiments in Shigaraki, Japan (34° 51′ N, 136° 06′ E). In particular, we study a relation between atmospheric stability, represented by Brunt Väisälä frequency squared, N2, and both the vertical echo power and the aspect sensitivity. On August 24–25, 1991, echo power was collected by steering the zenith angle of the antenna beam of the MU radar from the zenith to 28° with a step of 2°. Aspect sensitive echoes were detected in the lower stratosphere and some regions in the troposphere. We found that a ratio of vertical echo power to oblique echo power at 10° can represent the magnitude of aspect sensitivity. We compared profiles of both the vertical echo power and the aspect sensitivity with N2, inferred from a radiosonde sounding at the radar site. Cross correlation analysis indicates that a rapid increase of both vertical echo power and the aspect sensitivity near the tropopause usually coincides with a step-wise enhancement of N2. We also analyzed four other MU radar observations, continued 4–5 days each, and obtained a statistical results that the mean CCF values between profiles of N2 and vertical echo power and the aspect sensitivity are 0.89 and 0.86, respectively. Their mean lag distance was about 71 m and 134 m, with a standard deviation of 77 m and 176 m, respectively. That is, the increase in the vertical echo power and the aspect sensitivity generally occurred slightly higher altitude relative to a sharp increase in N2. However, we also found some exceptional cases when the lag distance was as large as several hundred meters or even negative.