A Climatological Study of the Frequency Spectra of Vertical Winds From MU Radar Data (1987–2022)

Abstract In this paper, we present a statistical analysis of the frequency spectra of vertical velocity W based on data collected by the VHF middle and upper atmosphere (MU) radar from 2.025 to 19.875 km altitude during several days per month over 36 years (1987–2022). We analyzed the mean spectral slopes in the band [0.5–6 hr], which is in the core of the period range for internal gravity waves at the MU radar latitude (∼36° N). First, we tested the performance of several spectral estimators when data are missing to minimize estimation bias on and variance. Second, we analyzed the properties of . We found a dependence on radar echo power aspect ratio and power imbalance between oblique beams maybe due to contaminations of measurements by the horizontal wind . After corrections, we found that: (a) in the troposphere ( is almost seasonally invariant (∼−0.8) despite the strong variability of and (b) is highly variable in the stratosphere ( from ∼0 to −1.2 and shows a clear exponential dependence on at least up to . An empirical expression of the apparent frequency spectra in the stratosphere is with, for example, and . From numerical simulations, we show that Doppler shift effects cannot explain the dependence on . Therefore, the variation of with may represent a real change in the shape of the intrinsic frequency spectrum, which is not considered in the theoretical models.