Abstract
Abstract. 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.
Highlights
There are many methods to obtain propagation properties of atmospheric gravity waves (AGWs), three of them hold our current interest: hodograph method, Stokes parameters method and the technique of phase and group velocity tracing
According to polarization relation of gravity wave, if the wave has a downward phase velocity, its hodograph-ellipse will have a clockwise rotation, the ratio of the major to minor axis equals the frequency ratio ω f of intrinsic frequency ω to inertial frequency f, the major axis lies along the horizontal propagation direction of the wave (Hirota and Niki, 1985; Nakamura et al, 1993; Tsuda et al, 1990), and the 180◦-ambiguity of the horizontal propagation direction can be solved by a relation between the horizontal and vertical perturbation velocities (Tsuda et al, 1990)
Separation of data into sets of upward propagating waves and downward propagating waves before doing analysis is essential for hodograph analysis
Summary
There are many methods to obtain propagation properties of atmospheric gravity waves (AGWs), three of them hold our current interest: hodograph method, Stokes parameters method and the technique of phase and group velocity tracing. Hodograph analysis on a single monochromatic atmospheric gravity wave is accurate in obtaining its intrinsic frequency and propagation direction. The most probable characteristic intrinsic frequency, horizontal wavelength and azimuth of each wave packet can be found by fitting the dispersion equation and its related formula for vertical group velocity (Kuo et al, 2009). We shall compare the merit and demerit of each method and try to find a composite procedure for obtaining the most probable propagation parameters of AGWs. One example of wave packet analysis of real radar data will be presented to demonstrate the procedure. We would focus on the cases with negligible background wind in which u0 ∼= 0 and ω ∼= σ
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