Gravity waves (GWs) have temporally and spatially small scales. Observations of GWs have been limited especially in the polar region due to its harsh environment. The purpose of this study is to elucidate the statistical characteristics of GWs in the Antarctic troposphere and lower stratosphere based on the continuous data over a year from 1 October 2015 to 30 September 2016 using the full system of the Program of the Antarctic Syowa MST/IS Radar (PANSY) radar at Syowa Station (69.0°S, 39.6°E). Such continuous observations over a long duration are unprecedented for high‐power Mesosphere‐Stratosphere‐Troposphere (MST) radars at any latitude. The frequency power spectra of horizontal wind fluctuations reveal a clear isolated maximum around the inertial frequency (f) in the lower stratosphere. A statistical analysis is performed, focusing on the GWs with near‐inertial frequencies (NIGWs) that are dominant in the lower stratosphere. According to the results of hodograph analyses, there are a considerable proportion of NIGWs propagating energy downward in the upper troposphere during all seasons, as well as in the winter stratosphere above a height of 15 km, whereas NIGWs with upward group velocities are dominant in the lower troposphere and the lowermost stratosphere. These results suggest that there are NIGW sources on the ground and around the tropopause during all seasons and in the stratosphere and/or above in winter. Plausible candidates for these sources are topography, the tropospheric jet, and the polar night jet. The statistical characteristics of NIGWs, such as horizontal phase velocity, provide powerful support for our inferences of wave sources.

The momentum fluxes associated with gravity waves (GWs) significantly vary both in time and space. It is important to qualify the intermittency of GWs because the intermittency largely affects the vertical profile of momentum flux convergences in the middle atmosphere. In this study, 1 year of continuous observation with high time/height resolution and accuracy was provided by the PANSY radar, a Mesosphere‐Stratosphere‐Troposphere (MST) radar at Syowa Station (69.01°S, 39.59°E) in the Antarctic. The PANSY radar is used to study the intermittency of GWs in the troposphere and lower stratosphere. In all seasons, the intermittency is large in the troposphere with Gini coefficients of 0.4–0.6 for absolute momentum fluxes ( ρo||u′w′true¯) and 0.4–0.7 for vertical wind variances multiplied by the density ( ρotruew′2¯). However, the intermittency is small in the lower stratosphere with Gini coefficients of 0.3–0.5 for ρo||u′w′true¯ and 0.2–0.4 for ρotruew′2¯. The seasonal characteristics in the lower stratosphere are different between ρo||u′w′true¯ and ρotruew′2¯: small Gini coefficients are observed in May–July for ρo||u′w′true¯ and in December–February for ρotruew′2¯. Simple model experiments with ideal distributions of GWs as a function of momentum flux magnitude and ground‐based phase velocity are performed. These results indicate that the vertical profiles of the Gini coefficient obtained by the PANSY radar are largely affected by orographic GWs with large amplitudes. The results also suggest that the generation and the partial reflection of GWs play an important role in determining the vertical profile of GW intermittency.

Using wind data over three years from

Kelvin-Helmholtz (K-H) instability is shear instability that occurs in a stably stratified fluid with strong vertical shear of the horizontal winds. It is well known that a necessary condition for K-H instability is that the Richardson number ( 𝐴𝐴 Ri ) must be <0. 25 (e.g., Miles & Howard, 1964;Scorer, 1969). The Richardson number can be estimated as follows: 𝐴𝐴 Ri = 𝑁𝑁 2 ∕|d𝑼𝑼 ∕d𝑧𝑧| 2 , where 𝐴𝐴 𝐴𝐴 2 is the square of the buoyancy (Brunt-Vӓisӓlӓ) frequency and 𝐴𝐴 |d𝑼𝑼 ∕d𝑧𝑧| is the magnitude of the vertical shear vector of the horizontal winds. The square of the buoyancy frequency is calculated as 𝐴𝐴 𝐴𝐴 2 = 𝑔𝑔𝑔𝑔 −1 d𝑔𝑔∕d𝑧𝑧 , where 𝐴𝐴 𝐴𝐴 is the gravitational acceleration and 𝐴𝐴 𝐴𝐴 is potential temperature. As seen in the 𝐴𝐴 Ri formula, a sufficiently large vertical shear 𝐴𝐴 |d𝑼𝑼 ∕d𝑧𝑧| can initiate instability even if the atmosphere is stably stratified. K-H instability is considered one of the main sources of clear air turbulence that affects the safety and comfort of air travel (e.g., Ralph et al., 1997).