Studies of vertical fluxes of horizontal momentum in the lower atmosphere using the MU-radar

Abstract

Abstract.We study the momentum flux of the atmospheric motions in the height ranges between 6 and 22 km observed using the MU radar at Shigaraki in Japan during a 3 day period in January 1988. The data were divided by double Fourier transformation into data set of waves with downward-phase-velocity and data set of waves with upward-phase-velocity for independent momentum flux calculation. The result showed that both the 72 h averaged upward flux and downward flux of zonal momentum were negative at nearly each height, meaning that the upward flux was dominated by westward propagating waves while the downward flux was dominated by eastward propagating waves. The magnitude of the downward flux was approximately a factor of 1.5 larger than the upward flux for waves in the 2∼7 h and 7∼24 h period bands, and about equal to the upward flux in the 10-30 min and 30 min-2 h period bands. It is also observed that the vertical flux of zonal momentum tended to be small in each frequency band at the altitudes below the jet maximum (10∼12 km), and the flux increased toward more negative values to reach a negative maximum at some altitude well above the jet maximum. Daily averaged flux showed tremendous variation: The 1st 24 h (quiet day) was relatively quiet, and the fluxes of the 2nd and 3rd 24 h (active days) increased sharply. Moreover, the upward fluxes of zonal momentum below 17 km in the quiet day for each period band (10∼30 min, 30 min∼2 h, 2∼7 h, and 7∼24 h) were dominantly positive, while the corresponding downward fluxes were dominantly negative, meaning that the zonal momentum below 17 km in each period band under study were dominantly eastward (propagating along the mean wind). In the active days, both the upward fluxes and Correspondence to: F. S. Kuo (fskuo@msa.vnu.edu.tw) downward fluxes in each frequency band were dominantly negative throughout the whole altitude range 6.1-18.95 km.

Statistical characteristics of AGW wave packet propagation in the lower atmosphere observed by the MU radar

“…When the background wind is not negligible, such as the case of the winter data in this study, the azimuth angle of the horizontal wave vector 3748 F. S. Kuo et al: Statistical characteristics of AGW wave packet propagation no longer equals to the azimuth angle of the horizontal group velocity (see Appendix B), we must take horizontal group velocity into consideration. Phase-and-group-velocity-tracing technique had been proved to be efficient to estimate the vertical group-andphase-velocities (v gz and v pz ) as well as the period τ of a wave packet (Kuo et al, 2003(Kuo et al, , 2007(Kuo et al, , 2008. In this study, we further proposed a process to calculate the horizontal group velocity v gh from the dispersion equations using the measured values of v gz , v pz , and τ .…”

“…So from Table 1, we can readily identify T3c for example, to represent the time range of 4∼24 h and Z4 to represent the height range of 5.9∼17.7 km. Different windows in Table 1 have different upper frequency and wave number boundaries, but may have the same lower frequency and wave number boundaries, because only the high frequency end (high wave number end) dictates the characteristic wave period (wavelength) of the wave packets as we had frequently explained in our previous papers (Kuo et al, 2003(Kuo et al, , 2008Kuo and Röttger, 2005). The number of gravity wave packets (in bold) along with the original number of wave packets being measured (the number under the slash) in different window were listed in Table 2 (for winter data set) and Table 3 (for summer data set), where the content in the 5th column of the 4th row represents the content of window T3cZ5, and the 2nd column of the 3rd row represents the content of window T2Z4, and so on.…”

“…Before doing data analysis, the missing data was filled by interpolation from its neighbouring good data. By dual beam method, the zonal-velocity u, meridional velocity v, and vertical velocity w were obtained from the Doppler velocities measured by the four oblique beams as described in our previous paper (Kuo et al, 2008). Figure 1 shows the 72 h averaged wind velocity profiles of winter data set and the 96 h averaged wind velocity profiles of summer data set.…”

“…The zonal wind of the summer data was much weaker than that in the winter data, while the strength of the meridional wind of the summer data was comparable with that of the winter data. The frequency and vertical wave number spectra of this winter data were studied and presented in an earlier paper (Kuo et al, 1992); while the study of its vertical flux of horizontal momentum was presented in a recent paper (Kuo et al, 2008).…”

“…Then the vertical group velocity v gz , vertical phase velocity v pz , and the characteristic wave period τ of a wave packet were measured directly by the technique of phase and group velocity tracing (Kuo et al, 1998(Kuo et al, , 2003(Kuo et al, , 2007(Kuo et al, , 2008, and the observed wave frequency σ and vertical wave number m were obtained readily by σ =1 τ and m=σ v pz , respectively. For the readers' convenience, we put the method of up-down wave separation in Sect.…”