[A-AS04] Evolution of Global Environmental Research based on Atmospheric Vertical Motions

[AAS04-01] Statistical Analysis and frequency Spectra of Vertical Air Velocities from MU Radar Data in the Tropopause-Stratosphere (1987-2024)

★Invited Papers

*Hubert LUCE¹, Noriyuki Nishi², Hiroyuki Hashiguchi¹ (1.Research Institute for Sustainable Humanosphere, 2.Fukuoka University)

Keywords:vertical velocity, frequency spectra, statistics, lower atmosphere

Vertical air motion is essential in various meteorological processes, but measuring it presents significant challenges. VHF Stratosphere-Troposphere radars, such as the 46.5 MHz Middle and Upper atmosphere (MU) radar at the Shigaraki MU Observatory in Japan, provide a method for measuring vertical air velocities in the tropopause-stratosphere. The MU radar has been operating since 1984, collecting routine data each month over several days. This study presents an extensive statistical analysis of vertical air velocity measurements (W ) in the altitude range of 2 to 20 km, spanning 38 years (1987–2024). We analyze mean, standard deviation, percentiles, skewness, kurtosis, and intermittency from three independent measurement methods: direct vertical measurements (W), and measurements derived from two pairs of oblique beams (W_NS and W_EW ), symmetrically oriented toward the North-South and East-West directions, respectively, at 10 degrees off zenith. Below 10 km, all three methods exhibit similar seasonal variations in the statistical parameters. Above 10 km, slight differences emerge, likely linked to increased static stability. Additionally, we analyze the mean slopes S_w of the frequency spectra of W within the [0.5-6 hours] period range, corresponding to the core frequency range of internal gravity waves at the MU radar latitude (36°N) (Luce et al., 2024). After accounting for previously overlooked instrumental effects, we find that: (1) in the troposphere (N²<2×10^-4 rad²s^-2), S_w remains nearly seasonally invariant (-0.8), despite significant variability in horizontal wind speed U , and (2) in the stratosphere (N²>2×10^-4 rad²s^-2), S_w varies between 0 and -1.2 and shows a clear exponential dependence on U up to 70 ms^-1 . Numerical simulations show that Doppler shift effects cannot explain the U dependence. Thus, the variation of S_w with U likely reflects a real change in the intrinsic frequency spectrum's shape, which is not accounted for in current theoretical models.

Luce, H., Nishi N., Hashiguchi H. (2024) A Climatological Study of the Frequency Spectra of Vertical Winds From MU Radar Data (1987–2022), J. Geophys. Res., 129, e2024JD041677, https://doi.org/10.1029/2024JD041677