Turbulence generation by Kelvin-Helmholtz instability in the tropical tropopause layer observed with a 47 MHz range imaging radar

摘要

Detailed structure of Kelvin-Helmholtz (KH) instability in the tropical tropopause layer is presented using a 47 MHz clear-air Doppler radar, referred to as the Equatorial Atmosphere Radar (EAR), installed at West Sumatra, Indonesia (0.20°S, 100.32°E), During 12-20 December 2008, radiosondes revealed that the altitude of temperature minimum was 16.1-16.4 km from 12-14 December and then descended with time and reached 13.4 km on 20 December. The altitude of easterly wind maximum corresponded well with that of the temperature minimum, and enhanced eastward vertical wind shear was produced above it because of a sharp decrease of easterly wind with altitude. During this period, to attain high range resolution of a few tens of meters, the EAR was operated in range imaging mode using frequency diversity. EAR observations revealed that enhanced radar echo above the temperature minimum had a thin, layered structure less than or about 200 m in depth during 12-14 December and then frequently showed upward wisp-like structures resulting from KH instability. In the intensive KH instability cases on 16 and 18 December, KH billows had a depth of up to –1 km, and upward and downward velocities reaching greater than 2 m s~(-1) were observed within the billows: Considerations using the linear wave theory suggest that a breaking of equatorial Kelvin wave might have been responsible for the onset of well-developed KH instability.