11:55 AM - 12:10 PM
[AAS06-11] A study of Kelvin-Helmholtz billow in the Antarctic troposphere and lower stratosphere based on the PANSY radar observation using a frequency domain interferometry technique
Keywords:Kelvin-Helmholtz billows, polar meteorology, middle atmosphere
Atmospheric turbulence plays a potentially important role in the mixing and irreversible transport of heat, momentum, and minor constituents. Due to its small spatial and temporal scales, it is difficult to observe its structure directly and the entire time evolution from the onset to the disappearance. Thus, the role of turbulence is not well understood qualitatively. Kelvin-Helmholtz instabilities (KHIs) resulting from enhanced vertical shear in the atmosphere are among the significant sources of atmospheric turbulence; when KHI occurs, it generates Kelvin-Helmholtz waves (KH waves), which mix heat and materials up and down. Fukao et al. (2011) reported that small-scale phenomena with a duration as long as 30 minutes and depth smaller than 2.5 km like KH waves are often observed based on the MU radar observation (34.8°N, 136.1°E). The purpose of this study is to elucidate the dynamical structure and formation mechanism of KH waves in the Antarctic troposphere and lower stratosphere using the PANSY radar at Syowa Station (69.0°S, 39.6°E). We used the frequency-domain interferometry (FDI) technique with the high-resolution in both time and space.
The PANSY radar observation is usually performed with standard observation mode at a fixed central frequency providing line-of-sight wind velocities for five directions at temporal and vertical resolutions of 200 seconds and 150 m, respectively. On the other hand, the FDI observation mode uses five different frequencies of transmitted waves and provides echo power with a resolution higher than the transmitted pulse width. Note that for MST radar such as the MU radar and the PANSY radar, it is considered that the echo mainly comes from Bragg scattering by isotropic atmospheric turbulence. The FDI observation campaign by the PANSY radar performed from March 14-24 and August 2-12, 2019 (20 days in total) for the troposphere and lower stratosphere. The FDI mode observations (~22 min) and the standard mode observations (~ 7 min) were repeated. Radiosonde observations were also made every 4 hours. The FDI mode observations provide fine vertical profiles of vertical winds in addition to the echo power, while the standard observation mode provides vertical profiles of the three-dimensional winds, including vertical wind. Vertical profiles of horizontal winds, temperature, and humidity were also obtained from the radiosonde observations.
During the campaign, 73 structures that appear to be KH waves were captured. In this presentation, we focus on the KH waves detected at 1100UT-1730UT on 21 March around the height of 9.0 km (A) and 1100UT-1300UT on 22 March around the height of 7.0 km (B). In case (A), a wave-like structure dominant in the horizontal winds having a wave-period shorter than one day was observed in association with the KH waves. A hodograph analysis was conducted and found that this wave-like structure is likely due to an inertia-gravity wave (GW) having a northeast- or southwestward wave vector and upward group velocity. The slow ground-based phase velocity of about 2.3 m s-1 suggests that this inertia-gravity wave is an orographic one. These dynamical properties are consistent with the generation in the northeasterly wind of about 20 m s-1 near the surface during this period. In case (B), a strong vertical shear of meridional winds localized around the height of 7.0 km was observed. This structure corresponds to the strong wind region in the upper troposphere associated with a developed low-pressure system approaching Syowa Station. We will also discuss how this low-pressure system was developed and accompanied such a strong wind region in the upper troposphere, based on the numerical simulations using the Nonhydrostatic ICosahedral Atmospheric Model (NICAM).
The PANSY radar observation is usually performed with standard observation mode at a fixed central frequency providing line-of-sight wind velocities for five directions at temporal and vertical resolutions of 200 seconds and 150 m, respectively. On the other hand, the FDI observation mode uses five different frequencies of transmitted waves and provides echo power with a resolution higher than the transmitted pulse width. Note that for MST radar such as the MU radar and the PANSY radar, it is considered that the echo mainly comes from Bragg scattering by isotropic atmospheric turbulence. The FDI observation campaign by the PANSY radar performed from March 14-24 and August 2-12, 2019 (20 days in total) for the troposphere and lower stratosphere. The FDI mode observations (~22 min) and the standard mode observations (~ 7 min) were repeated. Radiosonde observations were also made every 4 hours. The FDI mode observations provide fine vertical profiles of vertical winds in addition to the echo power, while the standard observation mode provides vertical profiles of the three-dimensional winds, including vertical wind. Vertical profiles of horizontal winds, temperature, and humidity were also obtained from the radiosonde observations.
During the campaign, 73 structures that appear to be KH waves were captured. In this presentation, we focus on the KH waves detected at 1100UT-1730UT on 21 March around the height of 9.0 km (A) and 1100UT-1300UT on 22 March around the height of 7.0 km (B). In case (A), a wave-like structure dominant in the horizontal winds having a wave-period shorter than one day was observed in association with the KH waves. A hodograph analysis was conducted and found that this wave-like structure is likely due to an inertia-gravity wave (GW) having a northeast- or southwestward wave vector and upward group velocity. The slow ground-based phase velocity of about 2.3 m s-1 suggests that this inertia-gravity wave is an orographic one. These dynamical properties are consistent with the generation in the northeasterly wind of about 20 m s-1 near the surface during this period. In case (B), a strong vertical shear of meridional winds localized around the height of 7.0 km was observed. This structure corresponds to the strong wind region in the upper troposphere associated with a developed low-pressure system approaching Syowa Station. We will also discuss how this low-pressure system was developed and accompanied such a strong wind region in the upper troposphere, based on the numerical simulations using the Nonhydrostatic ICosahedral Atmospheric Model (NICAM).