Characteristics of Frequency‐Power Spectra in the Troposphere and Lower Stratosphere Over Andøya (Norway) Revealed by MAARSY

Abstract

This study investigates the tropospheric and lower stratospheric 3D wind frequency spectra at Andøya, Norway (69.30°N and 16.04°E) during the years 2017–2020 obtained with the Middle Atmosphere Alomar Radar System. We produce power spectral density S(f) in a broad frequency range (3.5 days−1 > f > 1h−1). We categorize the spectra into different ranges: two frequency ranges (lower and higher than (13 hr)−1 according to the local inertial period of 13 hr), four altitude ranges (lower troposphere, middle troposphere, tropopause region, and lower stratosphere), and four seasons (spring, summer, autumn, and winter). We approximate the power law S(f) ∝ fβ through a least‐square regression. Our results indicate that (a) the spectra of the horizontal winds follow a power law with slopes of about β = −5/3 and −2 at the high and low frequency, respectively, (b) the slopes get steeper around the tropopause and seasonally during the summer (which we report for the first time), and (c) the slope β in the vertical wind is shallow 0 > β > −1, which flattens with altitude.

The Role of Stratified Turbulence in the Cold Summer Mesopause Region

“…Thus, there is one joint field of velocity fluctuations and a single field of buoyancy fluctuations. This theory was already successfully used by the authors in other observational and numerical studies of the lower and middle atmosphere (Avsarkisov et al, 2022;Chau et al, 2020;Ghosh et al, 2022;Poblet et al, 2022Poblet et al, , 2023.…”

Momentum Flux and Vertical Wind Power Spectral Characteristics in the Troposphere and Lower Stratosphere Over Andøya, Norway as Observed by MAARSY

“…The standard deviation in the zonal (meridional) wind is ∼0–9 ms −1 (∼±0–5 ms −1 ) below 4.00 km and ∼10–15 ms −1 in both the zonal and meridional winds above 4.00 km [Figures 2d and 2e; Ghosh et al. (2022)]. Our observation of peaks in the MF spectra in the troposphere could be due to the variations in the background wind and the buoyancy frequency (modulated by the wave thermodynamics) for the gravity waves crossing the tropopause or frontal systems (Wei & Zhang, 2015).…”

“…It should be noted that the peak in the height profile of VWP ∼2-3 km is not considered for any interpretation as it could be due to measurement errors. Contamination of vertical winds by horizontal winds due to the finite half-power beam width (3.6°) during 2017-2020 was also checked in Ghosh et al (2022). The correlation coefficient of ∼0.3-0.4 is observed between absolute horizontal winds versus absolute vertical winds (2017-2020), which is noticeable at lower heights.…”

“…The composite mean zonal wind during 2017-2020 is ∼0-6 ms −1 below ∼8.00 km (Figure 2a; Ghosh et al (2022)), corresponds to the very low magnitude of zonal winds in the lower troposphere, indicating critical-level filtering of orography generated waves at the radar site with nearly zero phase speed. The standard deviation in the zonal (meridional) wind is ∼0-9 ms −1 (∼±0-5 ms −1 ) below 4.00 km and ∼10-15 ms −1 in both the zonal and meridional winds above 4.00 km [Figures 2d and 2e;Ghosh et al (2022)]. Our observation of peaks in the MF spectra in the troposphere could be due to the variations in the background wind and the buoyancy frequency (modulated by the wave thermodynamics) for the gravity waves crossing the tropopause or frontal systems (Wei & Zhang, 2015).…”

“…We found the spectra in the high-frequency (<13 h) range nearly flat, which may be apparent due to the influence of the vertical shear of horizontal winds leading to critical level interactions. The composite mean zonal wind during 2017-2020 is ∼0-6 ms −1 below ∼8.00 km (Figure 2a; Ghosh et al (2022)), corresponds to the very low magnitude of zonal winds in the lower troposphere, indicating critical-level filtering of orography generated waves at the radar site with nearly zero phase speed. The standard deviation in the zonal (meridional) wind is ∼0-9 ms −1 (∼±0-5 ms −1 ) below 4.00 km and ∼10-15 ms −1 in both the zonal and meridional winds above 4.00 km [Figures 2d and 2e;Ghosh et al (2022)].…”