Studies of relative contributions of internal gravity waves and 2‐D turbulence to tropospheric and lower‐stratospheric temporal wind spectra measured by a network of VHF windprofiler radars using a decade‐long data set in Canada

Abstract

Tropospheric and lower-stratospheric motions at mesoscales and larger are a mixture of waves and two-dimensional (2-D) turbulence. Determining their relative importance is necessary, since waves are capable of coordinated systematic momentum transport accompanying the wave propagation, and associated wind forcing, in ways that 2-D turbulence is not. This can impact weather forecasting. Using a network of ten windprofiler radars in eastern Ontario and western Quebec in Canada, plus an additional one in the Arctic, the relative roles of internal gravity (buoyancy) waves and two-dimensional turbulence are examined at temporal scales from about 3-4 hrs to several tens of hours (horizontal spatial scales of typically one or two hundred kilometres to a few thousand kilometres), with the purpose of investigating the respective roles of these two distinct characteristic fluid motions as functions of location, season and year. The emphasis is on studies of spectral slope variability, rather than absolute spectral magnitudes, giving a perspective not previously substantially presented. In particular, we have found a frequency band in which gravity-wave Doppler shifting produces distinctly different spectral slopes than those predicted for 2-D turbulence, and these differences are employed to distinguish the flow fields. The network used (excluding the Arctic site) covers an area of ∼10 6 km 2 and includes a variety of different terrains. Radial velocities have been recorded on time scales of minutes for data lengths covering durations of up to 12 years. Altitude coverage is from 1 km to typically 14 km, at 500 m resolution. Results suggest a region from ∼2 to ∼5 km altitude (deeper for some radars) where waves are weaker and 2-D turbulence appears to be generally more significant, but where occasional bursts of gravity-wave activity can occur, while above typically 6-8 km, gravity waves increase in significance. There are distinct site-to-site variations.

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

“…It can be seen in the composite mean of zonal and meridional winds (Figure 2) that the wind velocity becomes nearly 0-2 ms −1 during summer months (in the whole altitude range of 1.80-18.00 km), while they are comparatively larger in the other months (around 10 ms −1 mostly). Contrary to our observation, Hocking et al (2021) observed a shallower slope in the summer (May-August) than in the nonsummer (October-March) months, which they assumed due to the increased convective activity during summer. Therefore, it can be assumed that the background mean winds substantially influence the spectral slopes.…”

“…But in the case of IGW, the frequency spectrum does not show any −3 spectral slope as it gets concealed by the effects of IGW dispersion relations. The IGW spectrum gets enhanced due to the presence of IGWs although Q2DT always exists in the atmosphere (Hocking et al., 2021). Therefore, the Q2DT −3 spectral slope gets suppressed and is usually not detectable even though it influences the linear fitting of the spectral slope.…”

“…The additional complexity for the interpretation of slopes arises when both the Q2DT and IGW (with and without possible Doppler shifting) mechanisms are involved. It becomes challenging to distinguish between the mechanisms responsible and their relative contributions for the observed spectral slopes (Hocking et al., 2021). Consequently, Hocking et al.…”

“…The additional complexity for the interpretation of slopes arises when both the Q2DT and IGW (with and without possible Doppler shifting) mechanisms are involved. It becomes challenging to distinguish between the mechanisms responsible and their relative contributions for the observed spectral slopes (Hocking et al, 2021). Consequently, Hocking et al (2021) proposed some criteria for the distinction of the mechanisms responsible for the wind fluctuations whereby: (a) a log-log spectral frequency slope less than −2.1 indicates a strong Q2DT, (b) spectral slopes greater than −1.5 would imply significant and strong Doppler-shifted IGWs, (c) intermediate slope values indicate supplementary investigations to resolve the probable mechanism, and (d) if both categories (a) and (b) spectral slopes are visible from different sites or single station at different times, then it is conspicuous that the atmosphere encompasses both the Q2DT and IGWs though they may not exist at the same time.…”

Mid-Latitude Detection of High Schmidt-Number Turbulent Echoes, and Comparison to PMSE and Geomagnetic Variations

“…[34][35], and Supplementary Section 1 of Ref. [36], among others.) We will not delve further into the radar properties, as these can be found in such earlier references.…”