Gravity Wave Generation around the Polar Vortex in the Stratosphere Revealed by 3-Hourly Radiosonde Observations at Syowa Station

Abstract

Intensive radiosonde observations were performed at Syowa Station (69.0°S, 39.6°E) over about 10 days in each of March, June, October, and December 2002 to examine inertia-gravity wave characteristics in the Antarctic lower stratosphere. Based on the 3-hourly observation data, two-dimensional (i.e., vertical wavenumber versus frequency) spectra of wind fluctuations were examined, utilizing a double Fourier transform method. Clear signals of gravity waves whose phases propagate upward, suggesting downward energy propagation, are detected in June and October when the polar night jet (PNJ) was present. On the other hand, downward phase propagation (i.e., upward energy propagation) components are dominant in all months. There is a spectral peak around the inertial frequency in a wide range of vertical wavenumbers in December when the background wind was weak, whereas large spectral densities are distributed over lower-frequency regions in June and October. These spectral characteristics are consistent with the results obtained using a gravity wave-resolving global circulation model (GCM) by Sato et al. Dynamical characteristics are examined separately for upward-and downward-propagating gravity waves in June, using a hodograph analysis method. As a result, it is found that upward-and downward-propagating wave packets observed simultaneously in the same height regions have similar horizontal wavelengths and phase velocities. This fact suggests that these gravity waves are generated from the same source with a similar mechanism. When the wave packets were observed, both the local Rossby number and the residual in the nonlinear balance equation estimated using NCEP-NCAR reanalysis data are large around the PNJ situated slightly to the lower latitudes of Syowa Station. Therefore, it is likely that the observed inertia-gravity waves are generated by a spontaneous adjustment around the geostrophically unbalanced PNJ and propagate toward Syowa Station. The possibility of spontaneous gravity wave generation around the PNJ is confirmed by comparison with the GCM simulation by Sato et al.

Global distribution of vertical wavenumber spectra in the lower stratosphere observed using high-vertical-resolution temperature profiles from COSMIC GPS radio occultation

“…Murayama et al (1994) reported a clear correlation between GW activity and the magnitude of subtropical jet streams using Middle and Upper Atmosphere Radar (MU) observations. Orographic generation of GWs has previously been observed with balloon soundings and satellite measurements over the Andes (Alexander et al, 2008a) and Antarctica (Baumgaertner and McDonald, 2007;Sato and Yoshiki, 2008). Effects of the polar night jet and sudden stratospheric warming events on GW excitation have also been studied (Yoshiki et al, 2004;Hei et al, 2008).…”

Gravity wave reflection and its influence on the consistency of temperature- and wind-based momentum fluxes simulated above Typhoon Ewiniar

“…The downward propagating gravity waves in the stratosphere have been detected from in situ measurements (e.g., Yoshiki and Sato, 2000;Yamamori and Sato, 2006;Sato and Yoshiki, 2008) utilizing analysis methods such as hodograph or two-dimensional vertical wavenumber-frequency spectrum. Using radiosonde measurements, Yoshiki and Sato (2000) and Sato and Yoshiki (2008) observed the downward propagating gravity waves around the stratospheric polar night jet that are generated from the upper-stratosphere jet by a spontaneous adjustment process. Recently, Sato et al (2012) proposed the partial reflection mechanism by a rapid change in the static stability for the downward propagating waves near the stratospheric polar jet simulated in a high-resolution general circulation model.…”

Spontaneous Gravity Wave Radiation in a Shallow Water System on a Rotating Sphere

“…Observational studies reported that inertial gravity waves are radiated from strong rotational flows, such as polar night jets (Yoshiki and Sato 2000;Sato and Yoshiki 2008), sub-tropical jets (Uccelini and Koch 1987;Kitamura and Hirota 1989;Sato 1994;Plougonven et al 2003), and cyclones (Pfister et al 1993;May 1996;Wu and Eckermann 2008). However, because of the limited number of observations, the characteristics of radiated inertial gravity waves (hereafter called as gravity waves in this study) and background flow fields have been mainly reported.…”