Seasonal variations of the gravity wave momentum flux in the Antarctic mesosphere and lower thermosphere

Abstract

[1] Airglow imager and dynasonde/imaging Doppler interferometer (IDI) radar wind measurements at Halley Station, Antarctica (75.6°S, 26.6°W) have been used to estimate the seasonal variation of the vertical fluxes of horizontal momentum carried by highfrequency atmospheric gravity waves. The cross-correlation coefficients between the vertical and horizontal wind perturbations were calculated from sodium (Na) airglow imager data collected during the austral winter seasons of 2000 and 2001. These were combined with wind velocity variances from coincident radar measurements to estimate the daily averaged upper limit of the vertical flux of horizontal momentum due to gravity waves. The resulting momentum flux at the Na airglow altitudes, while displaying a large day-to-day variability, showed a marked rotation from the northwest to the southeast throughout the winter season. Calculations show that this rotation is consistent with seasonal changes in the wind field filtering of gravity waves below the Na airglow region. The calculations also indicate that while the magnitude of the meridional wind is small, this filtering leads to the observed seasonal changes in the meridional momentum flux.

Characteristics and sources of gravity waves observed in noctilucent cloud over Norway

“…Although there are a number of gravity-wave studies at high latitudes, most of these rely upon analyzing the wave structures induced by the waves in the night airglow (e.g., Taylor and Henriksen, 1989;Pautet et al, 2005;Nielsen et al, 2006Nielsen et al, , 2009Espy et al, 2004;, Suzuki et al, 20092013). However, these observations are not possible during the high latitude polar summer when the mesosphere remains sunlit.…”

Regional variations of mesospheric gravity-wave momentum flux over Antarctica

“…As part of a collaboration between Utah State University (USU) and the British Antarctic Survey (BAS), observations of the gravity-wave induced intensity fluctuations of Na night airglow in two-dimensions were made using a monochromatic imaging system developed by USU. The camera used in this investigation, which has been described in detail elsewhere (Espy et al, 2004), consisted of a cooled (-45 • C), bare, 512×512 (effective) charge coupled device (CCD) array. This was coupled to a fast, f /4, telecentric lens system to produce an all-sky image.…”

“…Combining these with wind-velocity variances from coincident radar measurements, the daily averaged upper-limit of the vertical flux of horizontal momentum due to gravity waves may be inferred. The measurements were undertaken at Rothera for comparison with the climatology of momentum flux reported from Halley (Espy et al, 2004). The two Antarctic stations are separated by 8 • of latitude, and each is subject to extremely different wind regimes and orography.…”

Gravity wave reflection and its influence on the consistency of temperature- and wind-based momentum fluxes simulated above Typhoon Ewiniar

“…Note that the sign of m is negative for upward propagating waves and positive for downward propagating waves, with the sign convention of non-negative ω. For the calculation of F T in this study, we take the sign of m to be negative, based on the assumption that the temperature variance stems from upward propagating waves, which is often made in other studies as well (e.g., Espy et al, 2004;Boccara et al, 2008;Hertzog et al, 2008;Li et al, 2011).…”

“…The assumption has been used for the momentumflux estimations of the observed gravity waves not only by the satellites but by other measurement methods as well (e.g., Espy et al, 2004;Hertzog et al, 2008;Li et al, 2011). However, in the middle atmosphere downward propagation of gravity waves occurs by reflection of the upward propagating waves due to the changes in the vertical wavenumber with height (e.g., Hines and Reddy, 1967;Gossard and Hooke, 1975) or from the in situ wave sources in the upper level (e.g., Holton and Alexander, 1999;Zhou et al, 2002;Chun and Kim, 2008).…”