Joseph R. Isler

Gravity wave breaking in two and three dimensions: 2. Three‐dimensional evolution and instability structure

A companion paper by Andreassen et al. (this issue) introduced and used a nonlinear, compressible, spectral collocation code to address the relative evolutions of two-dimensional motions obtained in two- and three-dimensional simulations of gravity wave breaking. That study illustrated the effects of instability on the wave field and mean flow evolution and suggested that two-dimensional models are unable to fully describe the physics of the wave breaking process. The present paper examines in detail the structure, evolution, and energetics of the three-dimensional motions accounting for wave instability as well as their associated transports of momentum and heat. It is found that this instability comprises counterrotating vortices which evolve very rapidly within the convectively unstable region of a breaking wave. Instability scales are selected based on wave geometry and vortices are elongated in the streamwise direction (horizontal wavenumber in the spanwise direction) and result in the rapid collapse of superadiabatic regions within the wave field. The resulting spectra show clearly the transition from gravity wave forcing of harmonics of the incident wave to instability onset and evolution. Fluxes of momentum and heat by the instability reveal the manner in which the gravity wave amplitude is constrained and the influences of instability on the wave transports of these quantities. The breakdown of the instability structure and its evolution toward isotropic small-scale structure is the subject of the companion paper by Isler et al. (this issue).

Observational evidence of wave ducting and evanescence in the mesosphere

A collaborative radar and imaging study of gravity waves over the Hawaiian Islands was performed during October 1993 as part of the Airborne Lidar and Observations of Hawaiian Airglow 1993/Coupling and Dynamics of Regions Equatorial (ALOHA-93/CADRE) campaign to investigate the propagation characteristics of short-period (<1 hour) waves at nightglow altitudes. The horizontal wavelengths and apparent phase speeds of quasi-monochromatic wave events were measured in four separate nightglow emissions using data obtained by a high-resolution CCD imager. This information was correlated with simultaneous MF radar wind measurements over the same height interval (∼80–100 km) to infer intrinsic wave parameters in each case. Correlating the two data sets allowed the determination of the local vertical wavenumber for each event, in particular whether it be real (indicative of freely propagating waves) or imaginary (indicative of ducted or evanescent waves). The results of this study indicate a preponderance of ducted or evanescent waves at 80–100 km during the time of the observations, with up to ∼75% of the events recorded exhibiting ducted or evanescent behavior. Also noted was a tendency for ducted behavior to be more prevalent among waves with shorter horizontal wavelengths, in agreement with Doppler ducting theory. These results suggest that ducted waves are relatively common in the upper mesosphere and lower thermosphere region, at least over the mid-Pacific Ocean. As small-scale waves which are ducted have the potential to travel much longer horizontal distances than freely propagating waves, the frequency of their occurrence should be taken into account in efforts to quantify gravity wave effects at these altitudes.

Long‐term MF radar observations of solar tides in the low‐latitude mesosphere: Interannual variability and comparisons with the GSWM

Long-term MF radar wind measurements in the 80–100 km height range made at three equatorial and subtropical sites (Adelaide, Christmas Island, and Kauai) are used to produce climatologies and to study interannual variability of solar tides. Twelve years of data were available for Adelaide and up to 6 years at the other sites and are analyzed in 30-day intervals. The climatological values are compared with the Global Scale Wave Model (GSWM). Good agreement between the measured and model amplitudes and phases is found for the diurnal tide, but the semidiurnal model values agree less well with the observations. The diurnal tidal amplitudes and phases show strong seasonal variability. Maximum amplitudes are attained in March, and subsidiary maxima are observed in July/August and October, while the phase shows an annual cycle at Adelaide and Kauai, with the phase advancing by ∼4–6 hours from summer to winter. Amplitudes of the semidiurnal tide rarely exceed 10 m s−1. The phases undergo rapid shifts around the equinoxes at Adelaide and Kauai, but there is a more complicated phase variation at Christmas Island. The diurnal tide shows strong interannual variability in amplitude, especially near the March equinox. There appears to be an association with the quasi-biennial oscillation (QBO) in zonal winds in the equatorial stratosphere, with the amplitudes being larger than the climatological average in years when the stratospheric winds are eastward and smaller than average when the QBO is in its westward phase. In contrast, the phase of the diurnal tide, as well as the semidiurnal tide, shows little systematic interannual variability.

Gravity wave breaking in two and three dimensions: 1. Model description and comparison of two-dimensional evolutions

A nonlinear, compressible, spectral collocation code is employed to examine gravity wave breaking in two and three spatial dimensions. Two-dimensional results exhibit a structure consistent with previous efforts and suggest wave instability occurs via convective rolls aligned normal to the gravity wave motion (uniform in the spanwise direction). Three-dimensional results demonstrate, in contrast, that the preferred mode of instability is a series of counterrotating vortices oriented along the gravity wave motion, elongated in the streamwise direction, and confined to the region of convective instability within the wave field. Comparison of the two-dimensional results (averaged spanwise) for both two- and three-dimensional simulations reveals that vortex generation contributes to much more rapid wave field evolution and decay, with rapid restoration of near-adiabatic lapse rates and stronger constraints on wave energy and momentum fluxes. These results also demonstrate that two-dimensional models are unable to describe properly the physics or the consequences of the wave breaking process, at least for the flow parameters examined in this study. The evolution and structure of the three-dimensional instability, its influences on the gravity wave field, and the subsequent transition to quasi-isotropic small-scale motions are the subjects of companion papers by Fritts et al. (this issue) and Isler et al. (this issue).

Mean Motions and Tidal and Two-Day Structure and Variability in the Mesosphere and Lower Thermosphere over Hawaii

Abstract An overview of the motion field and an analysis of the tidal and 2-day wave motions observed in the mesosphere and lower thermosphere over the central Pacific from 1 October 1990 through 19 August 1992 is presented. Characteristics and interactions of motions at lower and higher frequencies will be addressed elsewhere. Wind measurements were obtained with an MF radar operating on Kauai, Hawaii (22°N, 160°W), using the partial reflection drift technique. Results presented in this paper reveal a zonal mean motion reflecting the mesopause semiannual oscillation (MSAO) observed at more equatorial latitudes from ∼ January to July, coinciding with the period during which the MSAO and the annual cycle of the zonal mean wind at higher latitudes are in phase. Eastward and westward maxima are 55 m s−1 below 80 km and 45 m s−1 near 85 km during the first year, with maxima of 57 and 53 m s−1 during the second year and evidence of substantial interannual variability. The second MSAO cycle is greatly suppresse...

Wave breaking signatures in sodium densities and OH nightglow. 2. Simulation of wave and instability structures

Measurements of atmospheric structure and dynamics near the mesopause were performed using a sodium lidar, an MF radar, and a night-glow CCD camera during the CORN campaign performed in central Illinois during September 1992. The major features of the observed structure on September 27/28 include a low-frequency, large-scale wave accounting for persistent overturning of the temperature and sodium density fields, superposed higher-frequency motions, small-scale transient ripples in the nightglow images suggestive of instability structures, and large-scale wind shear near the height of apparent instability. We describe four simulations of wave breaking with a three-dimensional model designed to assist in the interpretation of these observations. Two simulations address the instability of a low-frequency wave in a background shear flow with and without higher-frequency modulation. These show higher-frequency motions to be important in assigning the spatial and temporal scales of instability structures. Two other simulations examine the instabilities accompanying a convectively unstable inertia-gravity wave with and without higher-frequency modulation without mean shear. These show the instability structure to remain aligned in the direction of wave propagation, with only weak influences by the high-frequency motion. Our results suggest that instability due to a superposition of waves accounts best for the nightglow features observed during the CORN campaign and that streamwise convective instabilities observed due to wave breaking at higher intrinsic frequencies continue to dominate instability structure for internal waves for which inertial effects are important.

Short-period fluctuations of the diurnal tide observed with low-latitude MF and meteor radars during CADRE : Evidence for gravity wave/tidal interactions

MF and meteor radar data from four equatorial and subtropical sites (Hawaii, Christmas Island, Jakarta, and Adelaide) are used to examine diurnal tide amplitude and phase variability at mesosphere and lower thermosphere altitudes. All sites exhibit significant seasonal variability, with the largest amplitude fluctuations occurring at Hawaii and Adelaide. Shorter-term variability is found to occur primarily on timescales of ∼5 to 30 days. Amplitude and phase fluctuations are well correlated among different sites on occasion, but in general, the amplitude and phase coherences are low and suggest significant local influences on the tidal structures. The temporal behavior of height variations of the diurnal tide amplitude and phase is also examined. Cross correlations and cross spectra of these tidal parameters, especially between the amplitude and phase, are examined closely. The tendency for phase maxima to lead amplitude maxima is consistent with tidal modulation of gravity wave propagation and momentum fluxes, with a corresponding feedback by the gravity wave momentum flux divergences on the observed tidal structures. These results substantially extend previous more limited studies of gravity wave/tidal interactions and provide a statistical basis for the possible importance of this interaction and its influences on the diurnal tidal structure.

First observations of mesospheric dynamics with a partial reflection radar in Hawaii (22°N, 160°W)

We present here an overview of the wind measurements made during the first year of operation of a partial reflection radar system in Hawaii. The system operates at 1.98 MHz and provides wind estimates between ∼60 and 100 km at 2-km intervals. Our wind observations reveal a zonal mean motion that has several features in common with the mesopause semiannual oscillation (MSAO) seen at more equatorial latitudes, but which is influenced as well by the seasonal variations of the mesospheric jet structure at higher latitudes. Major features of the zonal mean wind structure include a downward progression of strong eastward and westward phases of the MSAO from ∼January to July, coinciding with the period during which the MSAO and the annual cycle of the zonal mean wind at higher latitudes are in phase. This results in an eastward maximum of ∼60 ms−1 near 80 km during January and February which descends rapidly and a westward maximum of ∼50 ms near 85 km during March and April which descends much more slowly. The second MSAO cycle is greatly suppressed relative to the first due to the reversal of the correlation between this and the annual cycle at higher latitudes from ∼July to December. Mean eastward motions occur primarily above ∼85 km early in this cycle and throughout the height range at later times. Planetary wave activity is seen to contribute substantial variability during times of eastward mean flow and to have a smaller role when zonal mean motions are westward. Tidal and 2-day motions are found to be large and variable, with the maximum 2-day amplitudes occurring during February and August. The largest diurnal tides were observed during October and November, March and April, and July and August, while the semi-diurnal amplitudes exhibited a minimum from ∼April to June.

Two‐day wave structure and mean flow interactions observed by radar and High Resolution Doppler Imager

Data obtained with four MF and meteor radars at equatorial and subtropical sites and with the High Resolution Doppler Imager (HRDI) instrument aboard the UARS satellite were used to examine the structure, wave-mean flow interactions, and potential sources of the 2-day wave in the middle atmosphere during three southern hemisphere summers. The three wave events were highly transient, having typical durations of 20 to 30 days and exhibiting modulation at shorter periods. Temporal variations were found to exhibit good correlations between radar and HRDI data. Radar and HRDI data were used to estimate those components of the Eliassen-Palm flux that could be assessed with these data. Meridional fluxes of momentum and heat were computed using HRDI data and agree reasonably with the momentum fluxes computed from radar data at discrete locations. These fluxes were found to exhibit consistent latitudinal structures each year, suggesting systematic wave excitation and wave-mean flow interactions. Meridional momentum flux gradients were seen to be anticorrelated with zonal wind accelerations in a manner consistent with wave forcing of the large-scale circulation. The apparent wave-mean flow interactions suggest that the 2-day wave could be a transient response to baroclinic instability of the summer hemisphere mesospheric jet. A calculation of the meridional gradient of quasi-geostrophic potential vorticity using HRDI winds and the COSPAR International Reference Atmosphere (CIRA 1986) temperatures exhibits a region of instability in the lower and middle mesosphere extending into subtropical latitudes and provides additional evidence of a possible source of this motion via baroclinic instability of the summer hemisphere jet structure.

Gravity wave variability and interaction with lower-frequency motions in the mesosphere and lower thermosphere over Hawaii

Abstract Radar observations of the mesosphere and lower thermosphere over Kauai, Hawaii, have provided measurements of the horizontal wind field since September 1990. Horizontal velocity variances in the gravity wave band of frequencies were computed for several averaging intervals over the entire dataset. The results, correlated with tidal and lower-frequency winds, show evidence of filtering of gravity waves by tidal and other lower-frequency motions. Spectra of gravity wave variances for two years of data display peaks at planetary wave as well as tidal periods, primarily near 12-, 24-, 48-h, and 16-day periods. The modulation of gravity wave variance during four distinct background regimes is examined: one in which the diurnal tide predominates; another in which the 2-day wave is large, a third in which the zonal mean wind and tidal amplitudes are large; and a fourth in which the zonal mean, tidal, and 2-day amplitudes are all small. Intervals in which the diurnal tide or the 2-day wave predominates s...