Momentum Flux Spectra of a Mountain Wave Event Over New Zealand

Abstract

During the Deep Propagating Gravity Wave Experiment (DEEPWAVE) 13 July 2014 research flight over the South Island of New Zealand, a multiscale spectrum of mountain waves (MWs) was observed. High‐resolution measurements of sodium densities were available from ~70 to 100 km for the duration of this flight. A comprehensive technique is presented for obtaining temperature perturbations, T′, from sodium mixing ratios over a range of altitudes, and these T′ were used to calculate the momentum flux (MF) spectra with respect to horizontal wavelengths, λH, for each flight segment. Spectral analysis revealed MWs with spectral power centered at λH of ~80, 120, and 220 km. The temperature amplitudes of these MWs varied between the four cross‐mountain flight legs occurring between 6:10UT and 9:10UT. The average spectral T′ amplitudes near 80 km in altitude ranged from 7–13 K for the 220 km λH MW and 4–8 K for the smaller λH MWs. These amplitudes decayed significantly up to 90 km, where a critical level for MWs was present. The average MF per unit mass near 80 km in altitude ranged from ~13 to 60 m2/s2 across the varying spectra over the duration of the research flight and decayed to ~0 by 88 km in altitude. These MFs are large compared to zonal means and highlight the importance of MWs in the momentum budget of the mesosphere and lower thermosphere at times when they reach these altitudes.

Large‐Amplitude Mountain Waves in the Mesosphere Observed on 21 June 2014 During DEEPWAVE: 1. Wave Development, Scales, Momentum Fluxes, and Environmental Sensitivity

“…Little is currently known of the MW amplitudes and their associated MFs at mesopause heights. A key goal of the DEEPWAVE mission was to identify distinct MW events and measure their MFs and potential impacts on the MLT region (Bossert et al, 2018;Eckermann et al, 2016;Fritts et al, 2018;Kaifler et al, 2015;Pautet et al, 2016). Our combined measurements of the 21 June event provide an exceptional resource for quantifying the MFs accompanying one very prominent MLT event and investigating its variability.…”

Large‐Amplitude Mountain Waves in the Mesosphere Accompanying Weak Cross‐Mountain Flow During DEEPWAVE Research Flight RF22

“…We assumed dR Na /dt~0, implying that R Na is a good tracer of MW ζ 0 (x,z). This was justified by detailed Na chemistry modeling of MW modulations of the Na layer, where R Na was predicted to be elevated by at most~20% due to adiabatic warming accompanying downward ζ 0 of~5-6 km over three MW cycles (also see Bossert et al, 2018). Hence, to be conservative, the deepest downward MW ζ 0 were estimated from measured R Na 50% higher than the lowest contour.…”

“…These data were used to provide estimates of the mean and perturbation Na mixing ratios, R Na0 ( z ) and R Na ′( x , z ) along each flight leg (see Figure ), in order to explore MW dynamics in the mesosphere with the highest possible resolution, ~3.6 km along track and 1.8 km in altitude. Laser locking was sporadic on Legs 2 and 3, however; hence a low‐pass filter having a passband of 100 s (~24 km) and a stop band of 50 s (~12 km) retained MWs at λ x ~ 30 km and larger (see Bossert et al, , for further details). These data enabled identification of MW λ x , vertical and horizontal parcel displacements, ζ ′( x , z ), and regions of overturning within the MW field.…”

“…We also assumed that d θ /d t = 0, given the short MW intrinsic period, T MW = 2 π / ω i ~ 20 min, and T 0 ( z , t ), H ( z , t ), N 0 ( z , t ), U 0 ( z , t ), and other parameters estimated from SABER T ( z ) and NAVGEM reanalysis fields. Together, these enabled estimates of MW (1) T ′( x , z ) and λ x from R Na ′( x , z ); (2) λ z from λ x , N 0 ( z , t ), and U 0 ( z , t ); (3) u ′ and w ′ from these quantities using equations and ; and (4) peak < u ′ w ′> for several specific MW features discussed in section 6.3 below (see Bossert et al, , for a more complete description of the Na chemistry modeling).…”

VHF radar measurements of momentum flux using summer polar mesopause echoes

“…More generally, Gardner and Taylor (1998) consider the observational limitations of gravity wave observations using lidar, radar and airglow observations. More recently, Bossert et al (2018) have considered the measurement of momentum flux using a Na density lidar and included the chemistry of the Na response to the mountain waves. We note that the VR83 approach is not valid in the case of stationary waves.…”