H. Y. Lue

Vertical phase and group velocities of internalgravity waves derived from ionograms during the solareclipse of 24 October 1995

Abstract A procedure is developed to derive the vertical phase and group velocities of wavesfrom measurements of ionograms. We apply the developed procedure to a sequence of ionogramsrecorded by the digisonde portable sounder in Taiwan and find numerous waves occuring in theionosphere during the solar eclipse of 24 October 1995. A detailed analysis of an 87-min periodwave shows that the vertical phase velocities of the wave below and above the F1 ledgeare about 100 m/s in the downward and upward directions, respectively. The associated groupvelocities below and above the ledge are found to be about 10 m/s in the downward and upwarddirections, respectively, which indicates that during the solar eclipse the wave source is near the F1-ledge.

Numerical simulations of the saturated gravity wave spectra in the atmosphere

Abstract A two dimensional numerical model is used to compute the saturation of small scale gravity waves in the region near the critical level. The vertical wave number spectrum of horizontal velocity fluctuations in the unstable region (USR) where shear instability develops is found to be governed by wave-shear interaction and follows a theoretical saturation spectrum ~ ω b 2 /2 m 3 . Wave-shear interaction is also found to be responsible for the observed fact that the variance of vertical velocity fluctuations is significantly lower than the level predicted by linear gravity wave theory. On the other hand, the corresponding spectrum in the stable region (SR) following a much shallower spectrum ~m −2 is found to result from the combined effects of wave-wave interactions and eddy diffusion. The key step in our simulation is the separate parameterization of horizontal and vertical eddy diffusion coefficients instead of a constant molecular viscosity coefficient.

Measurement of vertical phase and group velocities of atmospheric gravity waves by VHF radar

Abstract A systematic method of deriving from MST radar data the group velocity and phase velocity of the atmospheric wave along the radar beam direction is proposed and verified by a series of numerical simulations. We apply the method to two data sets measured by Chung-Li radar under different background wind conditions. It is found that the vertical group velocity and phase velocity are mostly in the opposite direction when the background wind is weak. The energy source of downgoing wave packets was evidently related to the instability in the upper height range (10.5–11.7 km) where strong wind shear existed. When the background wind and wind shear are stronger, the vertical group and phase velocities may propagate in the same direction. We also found from numerical simulation and data analysis that the wave packet of gravity waves following power law spectrum are short-lived. A by-product of the group velocity measurement is that the horizontal wavelength may also be deduced from a vertical radar beam measurement from the dispersion relation if it is valid.

A study of velocity fluctuation spectra in the troposphere and lower stratosphere using MU radar

Abstract We present an analysis of the vertical wave number and frequency spectra of atmospheric motions in the height ranges between 5 and 25 km observed using the Shigaraki, Japan, MU radar during a 4-day period in January 1988. The vertical wave number spectrum of the horizontal velocity fluctuation is found to saturate at large wave numbers satisfying power law ~ N 2 2K 3 z while departing from this −3 power law at small wave numbers. Frequency spectra of the oblique radial velocity fluctuations can be fitted by a Garrett-Munk gravity wave model spectrum. However, the vertical velocity fluctuation cannot be fitted simultaneously. The observed spectra are too steep and their energy levels are too low compared with the results from model prediction. Also, the vertical profiles of the energy densities of the horizontal velocity fluctuations are found to be positively correlated to the background wind velocity profile. These characteristics of the observed spectra are satisfactorily explained by dynamic instability and wave-wave interactions in the regions below the critical layer through nonlinear numerical simulations. The correlation between the background wind and the horizontal velocity fluctuations is shown to result from wave-shear interaction.

Effect of the wave-shear interaction on gravity wave activity in the lower and middle atmosphere

Abstract Observational results of the wave activities and the vertical wave number spectra of the horizontal wind fluctuations and the temperature/density fluctuations in the lower and middle atmosphere obtained by various groups using different instruments at different locations are reviewed and summarized. Then, we use a simple analytic model of wave-shear interaction to explain the wave-energy dissipation observed in the stratosphere/lower mesosphere, the east-west anisotropy of the wave propagation, and the deceleration of the zonal mean flow, in summer and in winter in the middle mesosphere, the annual variation in the upper troposphere/stratosphere/lower mesosphere, and the semi-annual variation in the middle mesosphere. We also point out that the saturation spectra observed in the middle mesosphere and the winter troposphere are caused by wave motions in the strong background wind shear and the low stability temperature profile, and that the saturation spectrum is universally N 2 2m 3 (where N is the Brunt Vaisala frequency and m is the vertical wave number).

Physical parameters of gravity wave packet propagation contained in radar RTI plots

Abstract By studying the oscillation profile of numerically generated wave packets, it is found that the patterns of their gray scale maps of local power fluctuations are totally dictated by the group velocity of the wave packet, the phase velocity and the frequency of the central component wave. A method of quick estimation of these three parameters of a wave packet is presented. Prospective applications of this method to the observational data taken by MST-radars on the ionosphere and lower atmosphere are briefly discussed.