Global free oscillations of the atmosphere and secondary planetary waves in the mesosphere and lower thermosphere region during August/September time conditions

Abstract

[1] Wind measurements made using ground-based mesosphere and lower thermosphere radar systems at Sheffield, UK (2°W, 53°N) and Saskatoon, Canada (107°W, 52°N) show an increase in long-period planetary wave (PW) activity at the end of August until the middle of September 1993. Some of these waves can be identified with the well-known normal atmospheric modes; however, the westward propagating wave with zonal wave number one (m = 1) and a period of about 6.5 days observed during this time interval cannot be explained using the classical theory of free atmospheric oscillations. Analysis of the UKMO assimilated fields shows that in the stratosphere the 7-day wave with m = 2 increases in amplitude during the time interval considered, and there exists a strong stationary planetary wave (SPW) with m = 1. A steady state two-dimensional numerical model of planetary waves has been used to simulate the free atmospheric oscillations and secondary planetary wave, which arises from a nonlinear interaction between the second asymmetric normal mode with m = 2 (the 7-day wave) and SPW with m = 1. The results of simulation show that an increase in long-period PW activity during August/ September can be explained by seasonal changes of the zonal mean flow, which influence the wave propagation conditions, and that one of the possible sources for the 6.5-day wave is a nonlinear interaction between the 7-day wave and SPW with m = 1. It is suggested that the 6.5-day wave with m = 1 be referred to as a ''secondary normal mode,'' which has been generated by random meteorological motions in the lower atmosphere and involving interaction with a SPW of wave number 1.

The 16-day planetary waves: multi-MF radar observations from the arctic to equator and comparisons with the HRDI measurements and the GSWM modelling results

“…periods near 2 and 7 days, produced by nonlinear interaction between the primary PWs (i.e. resonant or Rossby normal modes) (Pancheva et al, 2000;Pogoreltsev et al, 2002).…”

Wave activity (planetary, tidal) throughout the middle atmosphere (20-100km) over the CUJO network: Satellite (TOMS) and Medium Frequency (MF) radar observations

“…As in the earlier paper we show only 85-km spectra, as the spectral changes from 79-88 km (the height range with best data for spectra) are usually quite modest. We have used the same location for the TOMS data as for the MFR for several reasons: we follow Pancheva et al (2003) at this stage for reasons of comparison; and the low frequency (temporal and spatial) PW are expected to be present over a significant range of latitudes and longitudes near the radar sites (they are often described by global Hough modes). Uncertainties about the latter expectation are the reason for the formation of "regional" harmonic means for wavelets from the 3 radar locations of the CUJO network (North AmericanJapanese 40 • N locations); this was mentioned in the Analysis Sect.…”

“…Shorter period waves (4-to 5-d) were dominantly westward propagating and while they were seen throughout the year, they increased in amplitude during the seasonal transitions. Pancheva et al (2003) assessed the variability of the semidiurnal (12-h) tide (circa 92 km altitude) due to fluctuations in solar activity and total (column) ozone. The latter is of interest to us here.…”

Interhemispheric structure and variability of the 5-day planetary wave from meteor radar wind measurements

“…If the 6.5-day wave is generated by the nonlinear interaction between the s = 2 7-day wave and s = 1 stationary planetary wave (Pogoreltsev, 2002), interannual variability of the stationary planetary wave may also result in interannual variability of the 5DW.…”

“…Talaat et al (2001Talaat et al ( , 2002 proposed that the 5DW is generated by nonlinear interactions among large-scale waves in the troposphere and stratosphere and propagates upward to the MLT. Pogoreltsev et al (2002) proposed that nonlinear interaction between the anti-symmetric normal mode 7-day wave with s = 2 and the stationary planetary wave with s = 1 generates a 6.5-day wave with s = 1. Meyer and Forbes (1997) proposed that the 6.5-day wave is instead generated via baroclinic instability in the upper mesosphere.…”