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Showing papers by "David C. Fritts published in 1999"


Journal ArticleDOI
Joseph Werne1, David C. Fritts1
TL;DR: In this article, the highest-resolution 3D direct numerical simulations to date of Kelvin-Helmholtz instability are reported, and the resulting turbulence spans a broad range of spatial scales and exhibits Kolmogorov spectra in horizontal planes.
Abstract: The highest-resolution 3D direct numerical simulations to date of Kelvin-Helmholtz instability are reported. The ensuing turbulence spans a broad range of spatial scales and exhibits Kolmogorov spectra in horizontal planes. Non-Gaussian statistics are observed, with highly intermittent entrainment zones at the edges of the resulting shear layer. Profiles of the local gradient-Richardson number are presented and shown to remain less than 1/4 throughout the entire evolution of the turbulence.

151 citations



Journal ArticleDOI
TL;DR: In this paper, the authors used the hydrodynamic equations to model entrainment of the mixing ratios of ionized constituents hypothesized to be present in the upper polar mesosphere to calculate the spectra and cospectra of ions and electrons.
Abstract: The temporal evolution of a turbulent layer is calculated in detail by solving the hydrodynamic equations. The turbulence is initiated by a Kelvin-Helmholtz instability. The field of potential-temperature fluctuations serves as a tracer for modeling entrainment of the mixing ratios of ionized constituents hypothesized to be present in the upper polar mesosphere. This entrainment modeling provides the input to a turbulence advection model capable of calculating the spectra and cospectra of ions and electrons. The turbulence advection model is used as a subgrid-scale model and is required because, given present or foreseeable computer capabilities, numerical solutions cannot span the enormous range of spatial scales from the depth of the shear layer to the smallest scales on which the most massive ions diffuse. The power spectrum of electron number-density fluctuations obtained from the turbulence advection model is compared with that measured by a rocket during the STATE (Structure and Atmospheric Turbulence Environment) experiment; agreement is found for a case of massive ions. The radar cross section for Bragg scattering is calculated from the electron number-density power spectrum and is used to calculate the signal-to-noise ratio (S/N) for the Poker Flat 50 MHz radar. The resultant S/N is then compared with the radar measurements obtained during the STATE experiment. These comparisons support the hypothesis that massive ions can cause polar mesosphere summer echoes from turbulent layers. Large-scale morphology of the turbulent layer obtained from rocket and radar measurements is reproduced by the hydrodynamic solution.

61 citations


Journal ArticleDOI
TL;DR: In this paper, the authors examined the structure, wave-mean flow interactions, and potential sources of the 2-day wave in the middle atmosphere during three southern hemisphere summers using four MF and meteor radars at equatorial and subtropical sites and with the High Resolution Doppler Imager (HRDI) instrument aboard the UARS satellite.
Abstract: 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.

54 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present radar measurements of the 12-hour wave, a zonal wavenumber 1 westward propagating wave that exists in the southern polar mesopause region winds.
Abstract: We present radar measurements of the 12-hour wave, a zonal wavenumber 1 westward propagating wave that exists in the southern polar mesopause region winds (Hernandez et al., 1993; Forbes et al., 1995). MF radar measurements of the horizontal winds at McMurdo (77.8°S, 166.67°E) show that the 12-hour wave is highly seasonal, occurring during the austral summer solstice. During these seasonal occurrences, the wave is highly intermittent with amplitude peaks of ≳30 m s−1. The burst-like occurrences of large 12-hour wave amplitudes are highly correlated between the zonal and meridional direction. The diurnal tide over McMurdo has a more constant amplitude, but it is also an almost exclusively summertime phenomenon. Inertia-gravity wave activity is evident at periods less than 12 hr during the austral winter months. The weakening of gravity wave activity during the summer is probably due to critical layer filtering by the zonal mean wind, 12-hour wave and diurnal tide which are all strong during this season. The 12-hour wave is confined in height to the vicinity of the zero crossing in the zonal winds above the westward jet. Extreme distortion is observed in the vertical phase fronts of the 12-hour wave which could signify either refraction or in situ forcing. The distortion in the phase fronts and localization of the 12-hour wave in time and height is apparently responsible for departures in period from the nominal 12 hours. We do not find the wave period to be systematically different from 12 hours. The association of the 12-hour wave events with shear in the mean wind suggests that refractive effects could conceivably cause a dilation in wave amplitude. However, the shear is of the opposite sign to cause this dilation unless the wave originates at higher altitudes and propagates downward into the mesosphere. Investigations are made of the zonal structure of the 12-hour wave by comparing phases of the 12-hour wind component between McMurdo and the dynasonde at Halley (75.8°S, 26.4°W). The phase is found to be stable and consistent with a westward propagating zonal wavenumber 2 structure during seasons when the 12-hour wave is weak. The migrating semidiurnal tide evidently dominates during these times of the year. During seasons when the 12-hour wave amplitude is large, the zonal structure is highly unstable and there is not an obvious dominant zonal wavenumber.

40 citations


Journal ArticleDOI
TL;DR: In this paper, the frequency spectra and spectral variances of horizontal motions of gravity waves are used to provide information on the predominant azimuthal directions of propagation for the waves.
Abstract: Observations of winds and gravity waves (GW) by MF radars from the Arctic to the Equator are used to provide frequency spectra and spectral variances of horizontal motions, and information on the predominant azimuthal directions of propagation for the waves. The years used are mainly 1993/4; the height layer 76–88 km; and the GW bands 10 100 min. and 1–6 hrs. The high/mid-latitude locations of Tromso, Saskatoon, London/Urbana, Yamagawa, generally demonstrate similar behaviour: the monthly spectra have slopes near −5/3 in winter months, but smaller (absolute) slopes at higher frequencies (<2 hrs.) in summer. Corresponding to this, the spectral densities (10–100 min.) are larger for conditions of higher mean background windspeed—this is related by means of a new correlation-vector technique to GW propagating anti-parallel to the mean zonal winds, and the closure of the solstitial mesospheric jets. Also consistent with this, the sizes and orientations of perturbation ovals (fitted to the wind variations), demonstrate strong semi-annual-oscillations (SAO), and generally similar monthly and latitudinal directions. This suggests strong control, especially of the high-frequency GW band, by the dominant zonal wind-structures of the mesosphere. In contrast the low-latitude locations of Hawaii and Christmas Island demonstrate uniquely different behaviours, with indications of significant inter-annual variability. The frequency spectra for all months tend to have smaller slopes at higher frequencies. Also the dependence of spectral density in both GW bands, upon background wind speed, is negative rather than positive, and is shown to be generally consistent with GW propagating parallel to the mean-global winds. This is consistent with weaker vertical shears in the zonal winds (76–88 km), and lower GW momentum depositions. The perturbation ovals reveal much weaker SAO, and more variable orientations, consistent with more dependency upon GW sources, and less control by the mean winds of the mesosphere.

35 citations


Journal ArticleDOI
TL;DR: In this paper, a three-dimensional simulation of a breaking internal gravity wave in a stratified, compressible, and sheared fluid is performed to investigate the vorticity dynamics accompanying the transition from laminar to turbulent flow.
Abstract: We perform a three-dimensional simulation of a breaking internal gravity wave in a stratified, compressible, and sheared fluid to investigate the vorticity dynamics accompanying the transition from laminar to turbulent flow. Baroclinic sources contribute preferentially to eddy vorticity generation during the initial convective instability of the wave field, yielding counter-rotating vortices aligned with the external shear flow. These vortices enhance the spanwise vorticity of the shear flow via stretching and distort the spanwise vorticity via advective tilting. The resulting vortex sheets undergo a dynamical (Kelvin-Helmholtz) instability which rolls the vortex sheets into tubes which link, in turn, with the original streamwise convective rolls to produce a collection of intertwined vortex loops. Following the formation of discrete vortex loops, the most important interactions are the self-interactions of single vortex tubes and the mutual interactions of adjacent vortex tubes in close proximity. The initial formation of vortex tubes from the roll-up of localized vortex sheets imposes axial vorticity variations having both axisymmetric and azimuthal wavenumber two components. Axisymmetric variations excite axisymmetric twist waves, or Kelvin vortex waves, which propagate along the tubes, drive axial flows, and deplete and fragment the tubes. Azimuthal wavenumber two variations excite m = 2 twist waves on the vortex tubes which amplify and unravel single vortex tubes into pairs of intertwined helical tubes. Other interactions, judged less fundamental to the turbulence cascade, include reconnection among tube fragments, mutual stretching of orthogonal tubes in close proximity, excitation of azimuthal wavenumber one twist waves, and the continual roll-up of weaker vortex sheets throughout the evolution. Collectively, these vortex interactions result in a rapid cascade of energy and enstrophy toward smaller scales of motion.

10 citations