Brendan R. Roberts

Vertical evolution of potential energy density and vertical wave number spectrum of Antarctic gravity waves from 35 to 105 km at McMurdo (77.8°S, 166.7°E)

We report the first characterization of potential energy densities and vertical wave number spectra of Antarctic gravity waves (GWs) from 35 to 105 km, derived from Fe lidar temperature measurements at McMurdo (77.8°S, 166.7°E) in 2011–2013 winters. For GWs with periods of 2–10 h, the potential energy density per unit volume (Epv) decreases by 2 orders of magnitude from 35 to 105 km, while that per unit mass (Epm) increases from several to hundreds of J/kg. Epm increases with a mean scale height of ~10.4 km in the Rayleigh region (35–65 km) and of ~13.2 km in the Fe region (81–105 km), and of particular interest is the inferred severe dissipation in between (65–81 km). Overall, the vertical evolutions of Epv and Epm indicate considerable wave energy loss from the stratosphere to the lower thermosphere. The vertical wave number spectra exhibit power law forms for vertical wavelengths λz   10 km in 35–60 km. PSDs increase by 1 order of magnitude from the stratosphere to the lower thermosphere. Using higher temporal resolution data to include 0.5–2 h waves increase Epm by ~25–45% and increase PSDs of 2–5 km waves by a factor of 2 and of >10 km waves by less than 50%.

Lidar observations of persistent gravity waves with periods of 3–10 h in the Antarctic middle and upper atmosphere at McMurdo (77.83°S, 166.67°E)

Persistent, dominant, and large-amplitude gravity waves with 3–10 h periods and vertical wavelengths ~20–30 km are observed in temperatures from the stratosphere to lower thermosphere with an Fe Boltzmann lidar at McMurdo, Antarctica. These waves exhibit characteristics of inertia-gravity waves in case studies, yet they are extremely persistent and have been present during every lidar observation. We characterize these 3–10 h waves in the mesosphere and lower thermosphere using lidar temperature data in June from 2011 to 2015. A new method is applied to identify the major wave events from every lidar run longer than 12 h. A continuous 65 h lidar run on 28–30 June 2014 exhibits a 7.5 h wave spanning ~60 h, and 6.5 h and 3.4 h waves spanning 40 and 45 h, respectively. Over the course of 5 years, 323 h of data in June reveal that the major wave periods occur in several groups centered from ~3.5 to 7.5 h, with vertical phase speeds of 0.8–2 m/s. These 3–10 h waves possess more than half of the spectral energy for ~93% of the time. A rigorous prewhitening, postcoloring technique is introduced for frequency power spectra investigation. The resulting spectral slopes are unusually steep (−2.7) below ~100 km but gradually become shallower with increasing altitude, reaching about −1.6 at 110 km. Two-dimensional fast Fourier transform spectra confirm that these waves have a uniform dominant vertical wavelength of 20–30 km across periods of 3.5–10 h. These statistical features shed light on the wave source and pave the way for future research.

Winter temperature tides from 30 to 110 km at McMurdo (77.8°S, 166.7°E), Antarctica: Lidar observations and comparisons with WAM

We provide the first characterization of diurnal and semidiurnal thermal tides in temperature from 30 to 110 km in the winter season (May through August) at McMurdo (77.8°S, 166.7°E), Antarctica. The observations were made with an Fe Boltzmann temperature lidar in 2011 and 2012. Over 330 h of winter data are compiled into a composite day of temperature perturbations that significantly reduce the incoherent wave effects while preserving the coherent tidal signatures. Both diurnal and semidiurnal tides have small amplitudes (less than 3 K) below 100 km with vertical wavelengths of ~29 and ~23 km, respectively. A new finding of this study is the fast growth of diurnal and semidiurnal tidal amplitudes above 100 km to at least 15 K near 110 km, exceeding that of the freely propagating tides originating from the lower atmosphere. Such fast growth exists for all Kp index cases and diurnal amplitude increases to 15–30 K at 110 km with larger Kp indices corresponding to larger tidal amplitudes and faster growth rates. The slopes of diurnal tidal phases become steeper above 100 km, and the tidal phases barely change with altitude from 100 to 106 km. The tidal growth behavior is reproduced in the Whole Atmosphere Model (WAM) with phases comparable to the observations but magnitudes significantly underestimated. WAM compares reasonably well with the observations below 100 km. The observed significant amplitude increases and phase structure changes suggest additional tidal sources near or above 100 km, which deserve future investigation.

Lidar observations of stratospheric gravity waves from 2011 to 2015 at McMurdo (77.84°S, 166.69°E), Antarctica: 1. Vertical wavelengths, periods, and frequency and vertical wave number spectra

Five years of atmospheric temperature data, collected with an Fe Boltzmann lidar by the University of Colorado group from 2011 to 2015 at Arrival Heights, are used to characterize the vertical wavelengths, periods, vertical phase speeds, frequency spectra, and vertical wave number spectra of stratospheric gravity waves from 30 to 50 km altitudes. Over 1000 dominant gravity wave events are identified from the data. The seasonal spectral distributions of vertical wavelengths, periods, and vertical phase speeds in summer, winter, and spring/fall are found obeying a lognormal distribution. Both the downward and upward phase progression gravity waves are observed by the lidar, and the fractions of gravity waves with downward phase progression increase from summer ~59% to winter ~70%.

Lidar Observations of Stratospheric Gravity Waves From 2011 to 2015 at McMurdo (77.84°S, 166.69°E), Antarctica: 2. Potential Energy Densities, Lognormal Distributions, and Seasonal Variations

Abstract Five years of Fe Boltzmann lidars Rayleigh temperature data from 2011 to 2015 at McMurdo are used to characterize gravity wave potential energy mass density (E pm), potential energy volume density (E pv), vertical wave number spectra, and static stability N 2 in the stratosphere 30–50 km. E pm (E pv) profiles increase (decrease) with altitude, and the scale heights of E pv indicate stronger wave dissipation in winter than in summer. Altitude mean E¯pm and E¯pv obey lognormal distributions and possess narrowly clustered small values in summer but widely spread large values in winter. E¯pm and E¯pv vary significantly from observation to observation but exhibit repeated seasonal patterns with summer minima and winter maxima. The winter maxima in 2012 and 2015 are higher than in other years, indicating interannual variations. Altitude mean N2¯ varies by ~30–40% from the midwinter maxima to minima around October and exhibits a nearly bimodal distribution. Monthly mean vertical wave number power spectral density for vertical wavelengths of 5–20 km increases from summer to winter. Using Modern Era Retrospective Analysis for Research and Applications version 2 data, we find that large values of E¯pm during wintertime occur when McMurdo is well inside the polar vortex. Monthly mean E¯pm are anticorrelated with wind rotation angles but positively correlated with wind speeds at 3 and 30 km. Corresponding correlation coefficients are −0.62, +0.87, and +0.80, respectively. Results indicate that the summer‐winter asymmetry of E¯pm is mainly caused by critical level filtering that dissipates most gravity waves in summer. E¯pm variations in winter are mainly due to variations of gravity wave generation in the troposphere and stratosphere and Doppler shifting by the mean stratospheric winds.