Kelvin‐Helmholtz Billows in the Troposphere and Lower Stratosphere Detected by the PANSY Radar at Syowa Station in the Antarctic

Abstract

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).

Numerical Simulation of Orographic Gravity Waves Observed Over Syowa Station: Wave Propagation and Breaking in the Troposphere and Lower Stratosphere

“…Recent studies based on observations from a Mesosphere-Stratosphere-Troposphere (MST) radar at Syowa Station provide new insights into GWs in the Antarctic region, including their momentum fluxes (Sato et al, 2014(Sato et al, , 2017Shibuya et al, 2017), wave parameters (Minamihara et al, 2018), and their intermittency (Minamihara et al, 2020). The estimated turbulent energy dissipation rate is also used to examine the relation between GWs and turbulence (Kohma et al, 2019(Kohma et al, , 2021Minamihara et al, 2023). Although there have been several studies based on numerical simulations of GWs observed over Syowa Station (e.g., Minamihara et al, 2023;Shibuya et al, 2017), they are based on calculations with horizontal grid spacings of 2.8 km or larger.…”

“…The estimated turbulent energy dissipation rate is also used to examine the relation between GWs and turbulence (Kohma et al, 2019(Kohma et al, , 2021Minamihara et al, 2023). Although there have been several studies based on numerical simulations of GWs observed over Syowa Station (e.g., Minamihara et al, 2023;Shibuya et al, 2017), they are based on calculations with horizontal grid spacings of 2.8 km or larger.…”