Damian J. Murphy

MF radar observations of seasonal variability of semidiurnal motions in the mesosphere at high northern and southern latitudes

Abstract The semidiurnal tide (SDT) is investigated through comparative analysis of horizontal winds measured at Poker Flat (65°N, 147°W), Andenes (69°N, 16°E), Davis (69°S, 78°E), and Rothera (68°S, 69°W). At the northern hemisphere sites the SDT maximizes around the autumn equinox. Poker Flat and Andenes results from 1999–2001 are used to demonstrate that there is a clear repeatable enhancement in SDT amplitudes around the autumn equinox, and that the maximum is localized in height around 86 km . In the southern hemisphere seasonal dependence of the SDT during 1997–1998 is more complicated, and the autumn enhancement is less pronounced. Many competing mechanisms might contribute to the observed seasonal dependence of the SDT, but this study focuses on the refractive effects of shears in the mean zonal wind and gradients in temperature. The main evidence for a refractive influence is that the seasonal enhancement in the SDT amplitude is accompanied by a dramatic shortening in the waves vertical scale. This shortening of the vertical scale is consistent with refraction of the SDT energy into the horizontal wind component. Simplified linear tidal theory equations are used to estimate the expected magnitude of the refractive effects using wind and temperature fields observed over Andenes, Norway. The predicted refractive effects are shown to be potentially significant and qualitatively consistent with the observations. In addition to a seasonal dependence, the SDT amplitudes obtained at all the radar sites exhibit a deep amplitude modulation on a time scale characteristic of planetary waves. This sort of modulation has most often been attributed to nonlinear interactions between the tides and planetary waves. We suggest that refraction might instead produce, or at least contribute to, the observed modulation. Although the planetary waves are of weak ( m s −1 ) amplitude, the SDT (particularly the gravest S(2,2) mode) is only marginally propagating at high latitudes. Thus, small perturbations to the background are enough to periodically inhibit propagation of the SDT to higher levels.

The Deep Propagating Gravity Wave Experiment (DEEPWAVE): An Airborne and Ground-Based Exploration of Gravity Wave Propagation and Effects from Their Sources throughout the Lower and Middle Atmosphere

AbstractThe Deep Propagating Gravity Wave Experiment (DEEPWAVE) was designed to quantify gravity wave (GW) dynamics and effects from orographic and other sources to regions of dissipation at high altitudes. The core DEEPWAVE field phase took place from May through July 2014 using a comprehensive suite of airborne and ground-based instruments providing measurements from Earth’s surface to ∼100 km. Austral winter was chosen to observe deep GW propagation to high altitudes. DEEPWAVE was based on South Island, New Zealand, to provide access to the New Zealand and Tasmanian “hotspots” of GW activity and additional GW sources over the Southern Ocean and Tasman Sea. To observe GWs up to ∼100 km, DEEPWAVE utilized three new instruments built specifically for the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) Gulfstream V (GV): a Rayleigh lidar, a sodium resonance lidar, and an advanced mesosphere temperature mapper. These measurements were supplemented by in situ probes, dropson...

A climatology of tides in the Antarctic mesosphere and lower thermosphere

[1] A function that approximates atmospheric tidal behavior in the polar regions is described. This function is fitted to multistation radar measurements of wind in the mesosphere and lower thermosphere with the aim of obtaining a latitude-longitude-height description of the variation of tides over the whole Antarctic continent. Archival wind data sets are combined with present-day ones to fill the spatial distribution of the observations and to reduce the potential effects of spatial aliasing. Multiple years are combined through the compilation of monthly station composite days, yielding results for each month of the year. Despite potential problems associated with year-to-year variations in the tidal phase, a useful climatology of Antarctic zonal and meridional tidal wind components is compiled. The results of the fits reproduce the major features of the high-latitude tidal wind field: the dominance of the semidiurnal migrating mode in the winter months and the presence of a semidiurnal zonal wave number one component in the summer months. It is also found that the summer semidiurnal tide contains a zonal wave number zero component.

The large‐scale dynamics of the mesosphere–lower thermosphere during the Southern Hemisphere stratospheric warming of 2002

An unprecedented major stratospheric warming occurred in the Antarctic winter of 2002. We present measurements of winds in the mesosphere-lower thermosphere (MLT) made with MF radars located at Davis (69degreesS, 78degreesE), Syowa (69degreesS, 40degreesE) and Rothera (68degreesS, 68degreesW). The mesospheric wind field in 2002 was found to be considerably different to other years due to increased planetary wave activity throughout the winter. Zonal winds were weaker than usual during the 2002 winter and also during the transition to the summer circulation. The MLT zonal winds showed a reversal about one week earlier than the stratospheric reversal associated with the warming. Meridional winds showed oscillations consistent with the presence of traveling wave-1 planetary waves with periods similar to14 days. The results are compared with similar mesospheric observations made during northern hemisphere stratospheric warmings. Some similarities between hemispheres were found, notably that the reversal in the mesospheric winds precedes the warming events.

A comparison of mean winds and gravity wave activity in the northern and southern polar MLT

Mean winds and waves observed in the mesosphere and lower thermosphere with MF radars located at Davis (69oS, 78oE) and Poker Flat (65oN, 147oW) are compared. Measurements covering the pe- riod from 1999 to mid 2000 show differences in the strength of the horizontal wind fields. In the southern hemisphere the zonal and meridional winds reach their maximum values near the summer solstice, but are de- layed by 2-3 weeks in the northern hemisphere. Gravity wave variances also show significant differences, as do the strength of vertical velocities.

Momentum flux estimates accompanying multiscale gravity waves over Mount Cook, New Zealand, on 13 July 2014 during the DEEPWAVE campaign

Observations performed with a Rayleigh lidar and an Advanced Mesosphere Temperature Mapper aboard the National Science Foundation/National Center for Atmospheric Research Gulfstream V research aircraft on 13 July 2014 during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) measurement program revealed a large-amplitude, multiscale gravity wave (GW) environment extending from ~20 to 90 km on flight tracks over Mount Cook, New Zealand. Data from four successive flight tracks are employed here to assess the characteristics and variability of the larger- and smaller-scale GWs, including their spatial scales, amplitudes, phase speeds, and momentum fluxes. On each flight, a large-scale mountain wave (MW) having a horizontal wavelength ~200–300 km was observed. Smaller-scale GWs over the island appeared to correlate within the warmer phase of this large-scale MW. This analysis reveals that momentum fluxes accompanying small-scale MWs and propagating GWs significantly exceed those of the large-scale MW and the mean values typical for these altitudes, with maxima for the various small-scale events in the range ~20–105 m2 s−2.

Radiosonde observations of gravity waves in the lower stratosphere over Davis, Antarctica

Radiosonde observations made from Davis station, Antarctica, (68.6°S, 78.0°E) between 2001 and 2012 are used to compile a climatology of lower stratosphere inertial gravity wave characteristics. Wavelet analysis extracts single wave packets from the wind and temperature perturbations. Wavelet parameters, combined with linear gravity wave theory, allow for the derivation of a wide range of wave characteristics. Observational filtering associated with this analysis preferentially selects inertial gravity waves with vertical wavelengths less than 2–3 km. The vertical propagation statistics show strong temporal and height variations. The waves propagate close to the horizontal and are strongly advected by the background wind in the wintertime. Notably, around half of the waves observed in the stratosphere above Davis between early May and mid-October propagate downward. This feature is distributed over the observed stratospheric height range. Based on the similarity between the upward and downward propagating waves and on the vertical structure of the nonlinear balance residual in the polar winter stratosphere, it is concluded that a source due to imbalanced flow that is distributed across the winter lower stratosphere best explains the observations. Calculations of kinetic and potential energies and momentum fluxes highlight the potential for variations in results due to different analysis techniques.

The effects of deionization processes on meteor radar diffusion coefficients below 90 km

The decay times of VHF radar echoes from underdense meteor trails are reduced in the lower portions of the meteor region. This is a result of plasma neutralization initiated by the attachment of positive trail ions to neutral atmospheric molecules. Decreased echo decay times cause meteor radars to produce erroneously high estimates of the ambipolar diffusion coefficient at heights below 90 km, which affects temperature estimation techniques. Comparisons between colocated radars and satellite observations show that meteor radar estimates of diffusion coefficients are not consistent with estimates from the Aura Microwave Limb Sounder satellite instrument and that colocated radars operating at different frequencies estimate different values of the ambipolar diffusion coefficient for simultaneous detections of the same meteors. Loss of free electrons from meteor trails due to attachment to aerosols and chemical processes were numerically simulated and compared with observations to determine the specific mechanism responsible for low-altitude meteor trail plasma neutralization. It is shown that three-body attachment of positive metal ions significantly reduces meteor radar echo decay times at low altitudes compared to the case of diffusion only that atmospheric ozone plays little part in the evolution of low-altitude underdense meteor trails and that the effect of three-body attachment begins to exceed diffusion in echo decay times at a constant density surface.

Variations in the phase of the semidiurnal tide over Davis, Antarctica

Abstract The amplitude and time of maximum of the semidiurnal tide in the mesosphere and lower thermosphere above Davis varies through the summer months. In particular, the time of maximum can oscillate around a fixed value or, at some heights, shift by a whole cycle of local time. Recent studies of the semi-diurnal tide at the South Pole have suggested that an s =1 mode is common at high-southern latitudes. At mid-latitudes, the s =2 mode is thought to dominate suggesting that a region where these modes overlap is a possibility. Although it is not possible to discern which of these modes are present using single station data, the concept that more than one 12-h wave is present allows a new interpretation of the Davis semi-diurnal amplitudes and times of maximum. In this study, wind data obtained at Davis, Antarctica, during the summer of 1996/97 are used to show that the complex variations in time of maximum and amplitude can be explained by a sum of two waves. The simplest of models combines an invariant and a varying 12-h wave and yields characteristics for each component that are less complex than the original observation. There is also potential for changes to the interpretation of time of maximum vs. height profiles.

Coordinated global radar observations of tidal and planetary waves in the mesosphere and lower thermosphere during January 20–30, 1993

A multi-instrument global campaign involving incoherent scatter, medium frequency, and meteor wind radars was conducted during the period of January 20–30, 1993, to study the dynamics of the mesosphere and lower thermosphere. Data obtained from 15 radar stations covering a wide latitude range have been used to determine the global distribution of planetary and tidal waves during this 10-day campaign. Spectral analysis of the neutral winds measured by the radars in the altitude range from 80 to 130 km indicates the existence of a strong 48-hour wave near 90 km at latitudes between 40°N and 40°S that is present up to 108 km at 18°N. The semidiurnal tide is large at middle and high latitudes near 90 km and is predominant above 110 km, while the diurnal tide is observed to be particularly important in the upper mesosphere near 40° latitude. A least squares fit to the radar data is performed to obtain the amplitudes and phases of the tidal and 48-hour waves. Comparison with National Center for Atmospheric Research thermosphere-ionosphere-mesosphere general circulation model shows that the predictions from the model agree reasonably well with the observed global morphology of tidal wave amplitudes.