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Mesospheric data were analyzed by a composite method combining phase and group velocity tracing technique and the spectra method of Stokes parameter analysis to obtain the propagation parameters of atmospheric gravity waves (AGW) in the height ranges between 63.6 and 99.3 km, observed using the MU radar at Shigaraki in Japan in the months of November and July in the years 1986, 1988 and 1989. The data of waves with downward phase velocity and the data of waves with upward phase velocity were independently treated. First, the vertical phase velocity and vertical group velocity as well as the characteristic wave period for each wave packet were obtained by phase and group velocity tracing technique. Then its horizontal wavelength, intrinsic wave period and horizontal group velocity were obtained by the dispersion relation. The intrinsic frequency and azimuth of wave vector of each wave packet were checked by Stokes parameters analysis. The results showed that the waves with intrinsic periods in the range 30 min–4.5 h had horizontal wavelength ranging from 25 to 240 km, vertical wavelength from 2.5 to 12 km, and horizontal group velocities from 15 to 60 m s−1. Both upward moving wave packets and downward moving wave packets had horizontal group velocities mostly directed in the sector between directions NNE (north-north-east) and SEE in the month of November, and mostly in the sector between directions NW and SWS in the month of July. Comparing with mean wind directions, the gravity waves appeared to be more likely to propagate along with mean wind than against it. This apparent prevalence for downstream wave packets was found to be caused by a systematic filtering effect existing in the process of phase and group velocity tracing analysis: A significant portion of upstream wave packets might have been Doppler shifted out of the vertical range in phase and group velocity tracing analysis.

Three among the existing methods of obtaining the properties (intrinsic period, wavelength, propagation direction) of atmospheric gravity waves (AGWs) were compared and studied by numerical method to simulate radar data. Three-dimensional fluctuation velocity satisfying dispersion equation and polarization relation of atmospheric gravity wave were generated, then the numerical data were analysed by these methods to obtain the properties of waves. We found that, hodograph analysis was accurate for a monochromatic wave in obtaining its wave period and propagation direction, but the analysis became erratic for the case of multiple waves' superposition. The error was especially large when data consisted of both upward propagating waves and downward propagating waves. The hodograph method became meaningful again if all the component waves propagated in the same direction and the resulting period was dominantly decided by the lowest frequency wave. Stokes parameters method would obtain statistically meaningful values of wave period and azimuth if the spreading of the azimuths among the component waves did not exceed 90° and the resulting period and azimuth were dominated by the lowest frequency wave component as well, irrespective of the vertical sense of propagation. Another method called phase and group velocity tracing technique was reconfirmed to be meaningful in measuring the characteristic wave period and vertical group and phase velocities of a wave packet: the characteristic wave period and vertical wavelength was dominated by the wave with the highest frequency among the component waves in the wave packet. Based on these numerical results, a composite procedure of data analysis for wave propagation was proposed and an example of real data analysis was presented.

International audience; The wave packets of atmospheric gravity waves were numerically generated, with a given characteristic wave period, horizontal wave length and projection mean wind along the horizontal wave vector. Their projection phase and group velocities along the oblique radar beam (vpr and vgr), with different zenith angle ? and azimuth angle ?, were analyzed by the method of phase- and group-velocity tracing. The results were consistent with the theoretical calculations derived by the dispersion relation, reconfirming the accuracy of the method of analysis. The RTI plot of the numerical wave packets were similar to the striation patterns of the QP echoes from the FAI irregularity region. We propose that the striation range rate of the QP echo is equal to the radial phase velocity vpr, and the slope of the energy line across the neighboring striations is equal to the radial group velocity vgr of the wave packet; the horizontal distance between two neighboring striations is equal to the characteristic wave period ?. Then, one can inversely calculate all the properties of the gravity wave responsible for the appearance of the QP echoes. We found that the possibility of some QP echoes being generated by the gravity waves originated from lower altitudes cannot be ruled out.

Numerical simulations have been carried out to study and compare the mechanisms of the formations of the topside and bottomside ionospheric irregularities. As is well-known, the topside ionospheric irregularities have always originated from the bottomside ionosphere due to Gravitational Rayleigh-Taylor (GRT) instability. From the penetration process we found that the primary bubble with a horizontal scale of tens of kilometers always has a smooth shape in the bottomside ionosphere and breaks into much smaller-scale structures after penetrating into deep topside ionosphere. Since the plasma bubble is unlikely to break into irregularities in the bottomside ionosphere, the bottomside irregularities probably result from the turbulence in the neutral atmosphere. Examples of simulations are presented to demonstrate that bottomside irregularities may exist in a region with a strong neutral wind jet and seeding gravity wave motion. A strong neutral jet stream always has a strong vertical wind shear, which provides a broad height range of dynamic instability. When the atmospheric gravity wave approaches its critical level, its amplitude will continually grow to break into smaller-scale waves and turbulence. Then all the waves of different scales as well as the turbulence will become the seeds of the bottomside irregularities.

The neutral wind field effects on the development of the equatorial plasma bubbles have been simulated by a two-dimensional time-dependent model similar to that developed by Zalessak and Ossakow. The results indicate that when there exists no neutral wind, any perturbation on the bottom-side of the ionosphere density profile will be amplified by the gravitational Rayleigh-Taylor (GRT) instability and an upwelling bubble will form as expected. When a zonal neutral wind field exists, the uplift velocity of the bubble will be enhanced by a uniform neutral wind, but suppressed by vertical shear of the wind field. Secondary structures called plumes will grow out from the side walls of the primary bubble if some secondary perturbation signal with vertical structure is seeded. We notice that patches and multiple plume structures have been observed in the mid-latitude and low-latitude ionosphere. So we have developed a two-dimensional local theory of the generalized GRT instability to calculate the growth rate of a small-scale perturbation signal in the vicinity of the primary bubble and found that the growth rate increases with time along with the growth of the bubble. It is proved that the irregularities generated by some of our simulations should be able to cascade into observable (e.g., by radar) turbulence in a reasonable period of time after their generation. Similar to the effect on the growth of the primary bubble, a strong uniform wind field may enhance the growth rate of a perturbation signal in the vicinity of the bubble, while a strong vertical wind shear will suppress the growth rate. The main point of this theory is that the growth rate of the GRT instability is controlled by the density gradients and the magnitudes of the velocity of the plasma motion relative to the neutral wind in both directions. In addition, our simulations have proved that a seeding wave with phase velocity matching the background wind speed will generate the fastest growing bubble, disapproving the theory of spatial resonance.

The particular characteristics of the VHF mesosphere-stratosphere-troposphere (MST) radar and wind profiler system, operated at the National Central University in Chung-Li, Taiwan, Republic of China, are described. These are the dual-mode applications of the Doppler and the spaced antenna technique, where the latter is used in the conventional mode to determine the wind velocity as well as in the novel interferometer mode. A brief outline of the scientific rationale and the technical layout of this particular configuration of the Chung-Li VHF radar is given. A generally applicable method for system performance checks by deducing principle radar data parameters before the data analysis has been developed and is briefly described. Receiver system and antenna calibration measurements with celestial radio sources are reported and prove the appropriateness of the antenna system. Radar reflectivity profiles, measured with vertical as well as with off-vertical antenna beams, are presented and discussed. We also present height-time-reflectivity plots for quiet and disturbed weather conditions and reflect on their peculiarities. We show that the wind profiles measured with the conventional Doppler method and the spaced antenna method are in good agreement with radiosonde measurements. The spatial correlation parameters, deduced with the spaced antenna method, are consistent with results from other radars. We also discuss measurement capabilities to deduce the tropopause height with the Chung-Li VHF radar. We also describe echo power variations due to focusing from reflectivity structures modulated by gravity waves and a brief case study of Kelvin-Helmholtz instability. We finally outline future research directions which could be performed with the Chung-Li VHF radar.

[1] The middle- and upper-atmosphere (MU) radar (34.85°N, 136.10°E) was operated in the incoherent scatter power-only mode to observe the ionosphere during 17 June 2001. Pronounced 200- to 300-min. waves in the echo power appeared in the F2 region during 0240–1250 local time on 17 June 2001. A procedure of Fourier analyses is conducted to derive power spectra, vertical phase, and group velocities of the pronounced waves. Results show that the center frequency of the waves is a function of the wave number. The opposite directions of the vertical phase and group velocities imply the presence of atmospheric gravity waves.

Signal statistics of data from the SOUSY-VHF radar were analyzed. Mean, variance, skewness and kurtosis of the distribution of the quadrature components of the signals as well as Nakagami m coefficient of the amplitude distribution were calculated. We found a systematic behavior in the correlation between kurtosis and Nakagami coefficient, and we propose a stochastic model for its explanation. These stochastic effects on the signal are generated in the variable echoing process itself. Numerical simulation and analytical calculation of the model were carried out for comparison with the data, and the results are encouraging.