Intradiurnal wind variations observed in the lower thermosphere over the South Pole

Abstract

Abstract. The ®rst meteor radar measurements of meridional winds in the lower thermosphere (about 95 5 km), along four azimuth directions: 0°, 90°E, 180°and 90°W; approximately 2°from the geographic South Pole were made during two observational campaigns: January 19, 1995±January 26, 1996, and November 21, 1996±January 27, 1997. Herein we report analyses of the measurement results, obtained during the ®rst campaign, which cover the whole one-year period, with particular emphasis on the transient nature and seasonal behavior of the main parameters of the intradiurnal wind oscillations. To analyze the data, two complementary methods are used: the well-known periodogram (FFT) technique and the S-transform technique. The most characteristic periods of the intradiurnal oscillations are found to be rather uniformly spread between about 7 h and 12 h. All of these oscillations are westward-propagating with zonal wave number s 1 and their usual duration is con®ned to several periods. During the austral winter season the oscillations with periods less than 12 h are the most intensive, while during summer season the 12-h oscillations dominate. Lamb waves and internal-gravity wave propagation, non-linear interaction of the short-period tides, excitation in situ of the short period waves may be considered as possible processes which are responsible for intradiurnal wind oscillations in the lower thermosphere over South Pole.

Middle atmosphere dynamics with gravity wave interactions in the numerical spectral model: Tides and planetary waves

“…The PIGW in Fig. 18a are apparently generated like the non-migrating tides through nonlinear coupling between PW and migrating tides, as had been suggested in several papers (e.g., Manson et al, 1982;Manson and Meek, 1990;Forbes et al, 1999;Portnyagin et al, 2000).…”

“…The inferred large vertical wavelength indicated that a Lamb wave was observed. At the South Pole (88°S ), westward propagating wind oscillations with periods 7 -12 hours were observed with radar measurements (Forbes et al, 1999;Portnyagin et al, 2000). Lamb waves and "normal modes" were invoked, but inertio gravity waves, nonlinear coupling between tides and planetary waves were also suggested as possible explanations.…”

High- and mid-latitude quasi-2-day waves observed simultaneouslyby four meteor radars during summer 2000

“…6 and 7. It is worth remembering here that all periods and time moments of maximum activity are found by wavelet spectra with definite errors (see Portnyagin et al, 2000), and the significance of an oscillation depends on its duration. All of these wind variations have small amplitudes and are presented because they are features of the spectra, which are significant.…”

“…Hence, we can treat the S-transform spectra of the day-to-day wind variations as if they had been calculated from hourly data. Then we can estimate the peak variance as Portnyagin et al (2000), but should take for an estimation of the primary data variance only independent values of the filtered wind. Thus, we can find significance levels for every frequency in the S-transform spectra.…”

On the longitudinal structure of the transient day-to-day variation of the semidiurnal tide in the mid-latitude lower thermosphere − I. Winter season

“…The S-transform is closely related to the continuous wavelet transform and allows one to determine the absolute phase of an oscillation. The significance level of peaks obtained by using the S-transform may be estimated according to a method presented by Portnyagin et al (2000). To obtain the 4-hourly mean parameters of the semidiurnal tide a sliding harmonic analysis was performed on 48 hour and 72 hour intervals of the hourly-mean wind velocities.…”