Frequency spectra and vertical profiles of wind fluctuations in the summer Antarctic mesosphere revealed by MST radar observations

Abstract

Continuous observations of polar mesosphere summer echoes at heights from 81–93 km were performed using the first Mesosphere‐Stratosphere‐Troposphere/Incoherent Scatter radar in the Antarctic over the three summer periods of 2013/2014, 2014/2015, and 2015/2016. Power spectra of horizontal and vertical wind fluctuations, and momentum flux spectra in a wide‐frequency range from (8 min)−1 to (20 days) −1 were first estimated for the Antarctic summer mesosphere. The horizontal (vertical) wind power spectra obey a power law with an exponent of approximately −2 (−1) at frequencies higher than the inertial frequency of (13 h)−1 and have isolated peaks at about 1 day and a half day. In addition, an isolated peak of a quasi‐2 day period is observed in the horizontal wind spectra but is absent from the vertical wind spectra, which is consistent with the characteristics of a normal‐mode Rossby‐gravity wave. Zonal (meridional) momentum flux spectra are mainly positive (negative), and large fluxes are observed in a relatively low‐frequency range from (1 day)−1 to (1 h)−1. A case study was performed to investigate vertical profiles of momentum fluxes associated with gravity waves and time mean winds on and around 3 January 2015 when a minor stratospheric warming occurred in the Northern Hemisphere. A significant momentum flux convergence corresponding to an eastward acceleration of ~200 m s−1 d−1 was observed before the warming and became stronger after the warming when mean zonal wind weakened. The strong wave forcing roughly accorded with the Coriolis force of mean meridional winds.

Characteristics of Mesosphere Echoes over Antarctica Obtained Using PANSY and MF Radars

“…1b to better understand their height distributions. Summer echoes in December, January, and February are mostly confined to altitudes of 80−90 km and are rarely seen outside this region, which is a well-known feature of PMSEs (e.g., Sato et al 2014Sato et al , 2017. Non-summer echoes between mid-March and mid-November are seen mostly at altitudes of 55−80 km, which is in line with the reported features of PMWEs over Syowa (Nishiyama et al 2015).…”

“…PANSY echoes reproduced reported results by Sato et al (2014Sato et al ( , 2017 and Nishiyama et al (2015): summer echoes (PMSEs) were mostly confined to the height region of 80−90 km, and winter echoes (PMWEs) were mostly below 80 km during the daytime and for a few extended hours after sunset. This somewhat asymmetric structure about the local noon in winter was also depicted in the MAARSY observations in the Arctic (Latteck and Strelnikova 2015) although not explicitly mentioned.…”

Statistical Characteristics of Gravity Waves With Near‐Inertial Frequencies in the Antarctic Troposphere and Lower Stratosphere Observed by the PANSY Radar

“…The momentum flux frequency spectra ( ω Re [ U ( ω , z ) W * ( ω , z )] and ω Re [ V ( ω , z ) W * ( ω , z )]) are also estimated (see Sato et al, , for details of this method) and are shown in their flux‐content form in Figure as a function of frequency and height. In the troposphere, ω Re [ U ( ω , z ) W * ( ω , z )] and ω Re [ V ( ω , z ) W * ( ω , z )] are negative for ω < 2 π /12 hr.…”

Observing Mesospheric Turbulence With Specular Meteor Radars: A Novel Method for Estimating Second‐Order Statistics of Wind Velocity

“…From and G 4 h′ ( τ ,0), it is possible to see a strong peak around 3‐hr wave periods—this is consistent with the mean wind residuals shown in Figure and the oscillations that can be seen in the corresponding autocorrelation function. Sato et al () also report the presence of a 3‐hr wave. Our spectra only contains 1 day of measurements, so the fidelity is not as good.…”