VHF radar echoes in the vicinity of tropopause during the passage of tropical cyclone: first observations from the Gadanki MST radar

Abstract

First observations on the characteristics of VHF radar echoes in the vicinity of tropopause (VOT) during depression or passage of tropical cyclone made using the Gadanki MST radar are presented. The most significant and new observation is that signal-to-noise ratio (SNR) in zenith and off-zenith beams are nearly equal in the VOT when a cyclone is located close to the radar site. During depression, however, a slight decrease in echo strength with zenith angle was found. Observations made during such disturbed conditions are in contrast to the usual aspect-sensitive characteristics of the radar echoes. Spectral widths during disturbed atmospheric conditions in general are found to be much larger than that of normal condition. Further, spectral widths observed in the vertical beam during cyclone are considerably larger than that during normal condition. These features, however, have not been noticed at other height regions during such atmospheric conditions. All these observations clearly indicate preferential enhancement in turbulence activity in the VOT. These observations constitute the first experimental evidence of such a zenith angle dependence of VHF radar echo characteristics in the VOT during depression and cyclone. These results are presented and discussed in the light of present understanding of turbulence activity in the VOT.