We present a study of horizontal winds in the mesosphere and lower thermosphere (MLT) during the boreal winters of 2009-2010 and 2012-2013 produced with a new high-altitude numerical weather prediction (NWP) system. This system is based on a modified version of the Navy Global Environmental Model (NAVGEM) with an extended vertical domain up to ∼116 km altitude coupled with a hybrid four-dimensional variational (4DVAR) data assimilation system that assimilates both standard operational meteorological observations in the troposphere and satellite-based observations of temperature, ozone and water vapor in the stratosphere and mesosphere.

[1] An 8 year series of 965 high-resolution radiosonde soundings over Rothera (67°S, 68°W) on the Antarctic Peninsula are used to study gravity wave characteristics in the lower stratosphere. The gravity wave energy is shown to have a seasonal variation with peaks at the equinoxes; the largest peak is around the spring equinox. During the winter months and extending into the spring, there is both an enhancement in the downward propagating wave activity and a reduction in the amount of critical-level filtering of upward propagating mountain waves. The horizontal propagation directions of the gravity waves were determined using hodographs. It was found that there is a predisposition toward northward and westward propagating waves above Rothera. This is in agreement with previous observations of gravity wave momentum flux in the wintertime mesosphere over Rothera. These results are consistent with a scenario whereby the stratospheric gravity wavefield above Rothera is determined by a combination of wind flow over topography-generating waves from below, and sources such as the edge of the polar stratospheric vortex-generating waves from above, especially during winter and spring.Citation: Moffat-Griffin, T., R. E. Hibbins, M. J. Jarvis, and S. R. Colwell (2011), Seasonal variations of gravity wave activity in the lower stratosphere over an Antarctic Peninsula station,

[1] This paper describes the use of EOS Microwave Limb Sounder (MLS) data to observe the field of traveling planetary waves with quasi 16 day periods. This study utilizes MLS v2.2 temperature and geopotential data between 1 January 2005 and 31 December 2008 in the range 316 hPa to 0.001 hPa (approximately 8 to 97 km) to examine these waves. Analysis demonstrates that the quasi 16 day wavefield is made up of a number of components with westward and eastward propagating s = 1 and s = 2 waves generally dominant. In the Northern Hemisphere the westward and eastward propagating s = 1 waves have similar magnitudes and are larger than the other modes, while in the Southern Hemisphere, the eastward propagating s = 1 and s = 2 waves are larger than the westward propagating wave modes. All of the modes examined display strong seasonal patterns in the temperature amplitude, significant variability in the wave activity from year to year, and the presence of strong pulse-like patterns in the activity. All of the modes also display large median temperature amplitudes poleward of 40 degrees in both hemispheres. Our analysis also demonstrates that the variability in winter from year to year is larger in the Northern Hemisphere than the Southern Hemisphere. Detailed study also suggests that the exclusion of waves from regions of negative refractive index squared likely forms much of the seasonal pattern observed. Thus, regions of strong westward wind speeds effectively exclude vertically propagating waves as expected from theory. The reflection and absorption of waves associated with critical lines is also likely to explain the frequent occurrence of standing wave patterns in the EOS MLS temperature observations. This study highlights the potential of MLS observations for observing waves from the upper troposphere to the lower mesosphere.

A B S T R A C TUsing simulations with a whole-atmosphere chemistry-climate model nudged by meteorological analyses, global satellite observations of nitrogen oxide (NO) and water vapour by the Sub-Millimetre Radiometer instrument (SMR), of temperature by the Microwave Limb Sounder (MLS), as well as local radar observations, this study examines the recent major stratospheric sudden warming accompanied by an elevated stratopause event (ESE) that occurred in January 2013. We examine dynamical processes during the ESE, including the role of planetary wave, gravity wave and tidal forcing on the initiation of the descent in the mesosphere-lower thermosphere (MLT) and its continuation throughout the mesosphere and stratosphere, as well as the impact of model eddy diffusion. We analyse the transport of NO and find the model underestimates the large descent of NO compared to SMR observations. We demonstrate that the discrepancy arises abruptly in the MLT region at a time when the resolved wave forcing and the planetary wave activity increase, just before the elevated stratopause reforms. The discrepancy persists despite doubling the model eddy diffusion. While the simulations reproduce an enhancement of the semi-diurnal tide following the onset of the 2013 SSW, corroborating new meteor radar observations at high northern latitudes over Trondheim (63.4°N), the modelled tidal contribution to the forcing of the mean meridional circulation and to the descent is a small portion of the resolved wave forcing, and lags it by about ten days.

Abstract. This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW) activity in the mesosphere-lower thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain of eight SuperDARN radars at high northern latitudes that span longitudes from 150 • W to 25 • E and latitudes from 51 to 66 • N. Zonal wave number 1 and 2 components were extracted from the meridional wind for the years 2000-2008. The observed wintertime PW activity shows common features associated with the stratospheric wind reversals and the accompanying stratospheric warming events. Onset dates for seven SSW events accompanied by an elevated stratopause (ES) were identified during this time period using the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). For the seven events, a significant enhancement in wave number 1 and 2 PW amplitudes near 95 km was found to occur after the wind reversed at 50 km, with amplitudes maximizing approximately 5 days after the onset of the wind reversal. This PW enhancement in the MLT after the event was confirmed using SD-WACCM. When all cases of polar cap wind reversals at 50 km were considered, a significant, albeit moderate, correlation of 0.4 was found between PW amplitudes near 95 km and westward polar-cap stratospheric winds at 50 km, with the maximum correlation occurring ∼ 3 days after the maximum westward wind. These results indicate that the enhancement of PW amplitudes near 95 km is a common feature of SSWs irrespective of the strength of the wind reversal.

Abstract. The Microwave Limb Sounder (MLS) on the Aura satellite has been used to measure temperatures in the stratosphere, mesosphere and lower thermosphere. The data used here are from August 2004 to December 2010 and latitudes 75 • N to 75 • S. The temperature data reveal the regular presence of a westward-propagating 16-day planetary wave with zonal wavenumber 1. The wave amplitudes maximise in winter at middle to high latitudes, where monthly-mean amplitudes can be as large as ∼8 K. Significant wave amplitudes are also observed in the summer-time mesosphere and lower thermosphere (MLT) and at lower stratospheric heights of up to ∼20 km at middle to high latitudes. Wave amplitudes in the Northern Hemisphere approach values twice as large as those in the Southern Hemisphere. Wave amplitudes are also closely related to mean zonal winds and are largest in regions of strongest eastward flow. There is a reduction in wave amplitudes at the stratopause. No significant wave amplitudes are observed near the equator or in the strongly westward background winds of the atmosphere in summer. This behaviour is interpreted as a consequence of wave/mean-flow interactions. Perturbations in wave amplitude summer MLT are compared to those simultaneously observed in the winter stratosphere of the opposite hemisphere and found to have a correlation coefficient of +0.22, suggesting a small degrees of inter-hemispheric coupling. We interpret this to mean that some of the summer-time MLT wave may originate in the winter stratosphere of the opposite hemisphere and have been ducted across the equator. We do not observe a significant QBO modulation of the 16-day wave amplitude in the polar

Studies of vertical and interhemispheric coupling during Sudden Stratospheric Warmings (SSWs) suggest that gravity wave (GW) momentum flux divergence plays a key role in forcing the middle atmosphere, although observational validation of GW forcing is limited. We present a whole atmosphere view of zonal winds from the surface to 100 km during the January 2013 major SSW, together with observed GW momentum fluxes in the mesopause region derived from uninterrupted high‐resolution meteor radar observations from an All‐Sky Interferometric Meteor Radar system located at Trondheim, Norway (63.4°N, 10.5°E). Observations show GW momentum flux divergence 6 days prior to the SSW onset, producing an eastward forcing with peak values of ∼+145 ± 60ms−1d−1. As the SSW evolves, GW forcing turns westward, reaching a minimum of ∼−240 ± 70ms−1d−1∼+18 days after the SSW onset. These results are discussed in light of previous studies and simulations using the Whole Atmosphere Community Climate Model with Specified Dynamics.

High-resolution and high-S/N observations of the strong and relatively narrow j5797 di †use interstellar absorption band, recorded with the UltraÈHigh-Resolution Facility at the 3.9 m Anglo-Australian Telescope and the 0.9 m coude feed of the Kitt Peak National Observatory, are reported. For selected lines of sight and using resolving powers of up to 600,000, a remarkable degree of intrinsic ultraÐne structure is found within the band. There exists an almost exact correspondence between the structure in the spectra recorded toward k Sgr, f Per, and f Oph, including one particularly narrow component with a width that is comparable to the widths of atomic and molecular absorption lines along these lines of sight. The results provide a new and extremely exacting test against which theoretical or laboratory candidates can be assessed, and they point to a new approach to studies of di †use band carrier distributions through their velocity signatures.