R. A. Vincent

Empirical wind model for the upper, middle and lower atmosphere

Abstract The HWM90 thermospheric wind model has been revised in the lower thermosphere and extended into the mesosphere, stratosphere and lower atmosphere to provide a single analytic model for calculating zonal and meridional wind profiles representative of the climatological average for various geophysical conditions. Gradient winds from CIRA-86 plus rocket soundings, incoherent scatter radar, MF radar, and meteor radar provide the data base and are supplemented by previous data driven model summaries. Low-order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, local time (tides), and longitude (stationary wave 1), with a cubic spline interpolation in altitude. The model represents a smoothed compromise between the original data sources. Although agreement between various data sources is generally good, some systematic differences are noted, particularly near the mesopause. Overall root mean square differences between dar.a and model values are on the order of 15 m/s in the mesosphere and 10 m/s in the stratosphere for zonal winds, and 10 m/s and 5 m/s respectively for meridional winds.

Mesospheric Momentum Flux Studies at Adelaide, Australia: Observations and a Gravity Wave–Tidal Interaction Model

Abstract We present here the results of an analysis of gravity wave momentum fluxes in the mesosphere and lower thermosphere, inferred using a dual-beam Doppler radar near Adelaide, Australia during June 1984. Our analysis reveals that over 70% of the momentum flux and of the inferred zonal drag was due to gravity waves with observed periods less than one hour. This suggests that it is the gravity waves with high intrinsic frequencies and small horizontal scales that are most effective at transporting momentum into the middle atmosphere. The temporal variations in the momentum flux and flux divergence due to high-frequency motions were also examined in detail. In addition to daily variability, a strong diurnal modulation was observed to occur. This was found to be correlated with the phase of large-amplitude diurnal tidal motions. As a result of these observations, a gravity wave–tidal interaction model was proposed which accounts for all of the major features of the observed data, including a reduction i...

HF Doppler Measurements of Mesospheric Gravity Wave Momentum Fluxes

Abstract Recent theoretical studies have emphasized the probable importance of internal gravity waves in balancing the momentum budget of the mesosphere. In this paper, we propose a method by which the vertical flux of horizontal momentum can be measured by ground based radars. The method uses two or more radar beams each offset from the vertical to measure the atmospheric motions by the Doppler technique. Provided there is horizontal homogeneity, the momentum flux is proportional to the difference of the variances of the Doppler velocities measured in each beam. The flux convergence and, hence, the associated body force acting on the atmosphere can be inferred by measuring the flux as a function of height. It is shown that mean wind components can also be measured by this method and, under certain circumstances, so can the horizontal wavelengths and phase velocities of the internal waves. Observations of the vertical flux of zonal momentum made with this technique using an HF radar located near Adelaide,...

Gravity wave activity in the lower atmosphere: Seasonal and latitudinal variations

A climatology of gravity wave activity in the lower atmosphere based on high-resolution radiosonde measurements provided by the Australian Bureau of Meteorology is presented. These data are ideal for investigating gravity wave activity and its variation with position and time. Observations from 18 meteorological stations within Australia and Antarctica, covering a latitude range of 12°S – 68°S and a longitude range of 78°E–159°E, are discussed. Vertical wavenumber power spectra of normalized temperature fluctuations are calculated within both the troposphere and the lower stratosphere and are compared with the predictions of current gravity wave saturation theories. Estimates of important model parameters such as the total gravity wave energy per unit mass are also presented. The vertical wavenumber power spectra are found to remain approximately invariant with time and geographic location with only one significant exception. Spectral amplitudes observed within the lower stratosphere are found to be consistent with theoretical expectations but the amplitudes observed within the troposphere are consistently larger than expected, often by as much as a factor of about 3. Seasonal variations of stratospheric wave energy per unit mass are identified with maxima occurring during the low-latitude wet season and during the midlatitude winter. These variations do not exceed a factor of about 2. Similar variations are not found in the troposphere where temperature fluctuations are likely to be contaminated by convection and inversions. The largest values of wave energy density are typically found near the tropopause.

Climatology of the semiannual oscillation of the tropical middle atmosphere

We have used a variety of satellite, ground-based, and in situ observations to construct a climatology of the semiannual oscillation (SAO) of the tropical middle atmosphere. The sources of data include rocketsonde observations of winds and temperature, MF radar wind observations, and observations of winds and temperatures made from space by the High Resolution Doppler Imager (HRDI) and the Solar Mesosphere Explorer (SME). These data sets provide a generally consistent picture of the SAO, of the relationship between its stratospheric and mesospheric manifestations, and of its apparent modulation by the stratospheric quasi-biennial oscillation (QBO). In agreement with earlier studies, we find that the first cycle of the stratospheric SAO (which begins with the stratopause easterly phase in northern winter) is stronger than the second cycle (beginning with the easterly phase in southern winter). Similar behavior is apparent in the mesosphere, where the easterly phase is stronger during the first cycle than during the second cycle. HRDI and MF radar are capable of observing the seasonal cycle well into the lower thermosphere. Data from these two sources indicate that a strong SAO is present up to about 90 km, giving way above this altitude to time mean easterly winds with a weak semiannual variation. Between 105 and 110 km, HRDI data indicate the presence of a westerly wind layer with almost no seasonal variation. Apparent modulation of the stratospheric SAO by the QBO is found in rocketsonde data, while HRDI and MF radar observations suggest a correlation between the QBO and the easterly phase of the mesospheric SAO. We discuss the implications of these observations for the wave processes that drive the SAO.

A Climatology of Gravity Wave Motions in the Mesopause Region at Adelaide, Australia

Abstract A statistical study of gravity wave motions in the mesosphere and lower thermosphere measured with a MF partial reflection radar located at Buckland Park new Adelaide (35°S, 138°E) in the period November 1933 to December 1984 is presented. The analyses am confined to waves with ground based periods between 1 and 24 h. Time-height cross sections show that the mean square amplitudes u′2 and v′2, of the zonal and meridional perturbation velocities, respectively, vary in a predominantly semiannual manner such that the minima in wave activity coincide with the reversals in the zonal circulation in the middle atmosphere. In most instances, v′2 is greater than u′2 which, together with the small but nonzero u′v′ fluxes shows that the gravity wave field is partially polarized. A technique similar to that used to analyse partially polarized electromagnetic waves suggests that on a seasonal basis, the wave field is polarized by about 10% to 20% but for shorter periods the degree of polarization may be signi...

Gravity waves in the tropical lower stratosphere: An observational study of seasonal and interannual variability

Radiosonde observations made at Cocos Islands (12°S, 97°E) in the Indian Ocean between September 1992 and June 1998 are used to study seasonal and interannual variations in gravity wave activity in the lower stratosphere (18–25 km). The islands are located in a region of generally strong convection that occurs at all times of the year, with the period of strongest convective activity between December and July (wet season). The prevailing zonal winds during the observational period and height range are westward with a quasi-biennial oscillation (QBO) superimposed. Time series of wave energy show that largest wave amplitudes occur during the wet season when convection is strongest, but a QBO-like variation is also apparent. Maximum energy densities of about 25 J kg−1 occur early 1993, 1995, and 1997 at the times when the westward shears are largest. Wave energy is found to be propagating upward, and in the horizontal there is considerable azimuthal anisotropy, with predominate eastward propagation against the prevailing wind. Upward fluxes of zonal momentum flux (u′w′¯) are estimated by combining the temperature and wind information. Fluxes show a similar temporal behavior to the energy. The motion and temperature fields are dominated by waves with vertical wavelengths ∼2 km. Using a Stokes parameter analysis, it is found that the intrinsic frequencies are, on average, 2–3 times the inertial frequency, corresponding to intrinsic periods of 20–25 hours. Horizontal wavelengths between 200 and 2000 km are inferred, with a mean value of about 1000 km. The mean intrinsic phase speeds are about 10 ms−1, but ground-based phase speeds are centered on 0 ms−1.

Analysis and interpretation of airglow and radar observations of quasi-monochromatic gravity waves in the upper mesosphere and lower thermosphere over Adelaide, Australia (35°S, 138°E)

Abstract Observations of wave-driven fluctuations in emissions from the OH Meinel (OHM) and O2 Atmospheric band were made with a narrow-band airglow imager located at Adelaide, Australia (35S, 138E) during the period April 1995 to January 1996. Simultaneous wind measurements in the 80–100 km region were made with a co-located MF radar. The directionality of quasi-monochromatic (QM) waves in the mesopause region is found to be highly anisotropic, especially during the solstices. During the summer, small-scale QM waves in the airglow are predominately poleward propagating, while during winter they are predominately equatorward. The directionality inferred from a Stokes analysis applied to the radar data also indicates a strong N–S anisotropy in summer and winter, but whether propagation is from the north or south cannot be determined from the analysis. The directionality of the total wave field (which contains incoherent as well as coherent features) derived from a spectral analysis of the images shows a strong E–W component, whereas, an E–W component is essentially absent for QM waves. The prevalence of QM waves is also strongly seasonally dependent. The prevalence is greatest in the summer and the least in winter and correlates with the height of the mesopause; whether it is above or below the airglow layers. The height of the mesopause is significant because for nominal thermal structures it is associated with a steep gradient in the Brunt-Vaisala frequency that causes the base of a lower thermospheric thermal duct to be located in the vicinity of the mesopause. We interpret the QM waves as waves trapped in the lower thermosphere thermal duct or between the ground and the layer of evanescence above the duct. Zonal winds can deplete the thermal duct by limiting access to the duct or by negating the thermal trapping. Radar measurements of the prevailing zonal wind are consistent with depletion of zonally propagating waves. During winter, meridional winds in the upper mesophere and lower thermosphere are weak and have no significant effect on meridionally propagating waves. However, during summer the winds in the duct region can significantly enhance ducting of southward propagating waves. The observed directionality of the waves can be explained in terms of the prevailing wind at mesopause altitudes and the seasonal variation of distant sources.

Gravity-wave motions in the mesosphere

Partial reflection measurements of horizontal winds in the mesosphere and lower thermosphere made at Adelaide (35° S, 138° E) and Townsville (19°S, 147°E) have been spectrally analyzed as functions of frequency and wavenumber. The spectral densities of gravity wave motions observed at 85 km are found to follow a power law relation of the form f−k with k ~ 1.5, where f is the frequency. The functional form of the frequency spectra and the inferred rms amplitudes (~ 30 ms−1) agree well with other reported observations, which suggests there is some ‘universality’ to the gravity wave field in the mesosphere. A vector spectral decomposition of gravity wave wind vs height profiles shows that at least 65% of the vertical energy flux of long period gravity waves is upgoing. The wave number spectra show that the mean vertical wavelength is ~ 12 km, which implies horizontal scales ranging between about 4000 km for the quasi-inertial waves to ~ 50 km for the short period waves. The estimated vertical energy flux at the mesopause is ~ 10−2 Wm−2, a large fraction of which appears to be carried by the short period (< 1 h) waves. There is strong evidence for wave breaking and from the observed wave amplitudes upper limits of about 280 m2 s−1 and 350 m2 s−1 are placed on the mesospheric eddy diffusivities at Townsville and Adelaide, respectively.

Dynamics of the equatorial mesosphere: First results with a new generation partial reflection radar

The first observations of mesospheric winds made between January–August 1990 with an MF partial reflection radar located on Christmas Island (2°N, 157°W) in the central Pacific are described. The mean zonal winds are in general westward, but show clear evidence for a wave-driven circulation. Power spectral studies indicate that waves are present over a wide range of periods. Ultra-fast Kelvin waves are especially evident in January–March, with peak amplitudes ∼20 ms−1, and intrinsic phase speeds of ∼150 ms−1 indicated. The Kelvin waves are estimated to contribute an eastward acceleration of up to 10 ms−1 day−1. Gravity wave amplitudes are also found to be almost as large as those observed at mid-latitude sites, which suggests that convection is a major source of gravity wave activity.