Showing papers by "David C. Fritts published in 2008"

Gravity wave penetration into the thermosphere: sensitivity to solar cycle variations and mean winds

Abstract: We previously considered various aspects of grav- ity wave penetration and effects at mesospheric and ther- mospheric altitudes, including propagation, viscous effects on wave structure, characteristics, and damping, local body forcing, responses to solar cycle temperature variations, and filtering by mean winds. Several of these efforts focused on gravity waves arising from deep convection or in situ body forcing accompanying wave dissipation. Here we generalize these results to a broad range of gravity wave phase speeds, spatial scales, and intrinsic frequencies in order to address all of the major gravity wave sources in the lower atmosphere potentially impacting the thermosphere. We show how pen- etration altitudes depend on gravity wave phase speed, hor- izontal and vertical wavelengths, and observed frequencies for a range of thermospheric temperatures spanning realistic solar conditions and winds spanning reasonable mean and tidal amplitudes. Our results emphasize that independent of gravity wave source, thermospheric temperature, and fil- tering conditions, those gravity waves that penetrate to the highest altitudes have increasing vertical wavelengths and decreasing intrinsic frequencies with increasing altitude. The spatial scales at the highest altitudes at which gravity wave perturbations are observed are inevitably horizontal wave- lengths of 150 to 1000 km and vertical wavelengths of 150 to 500 km or more, with the larger horizontal scales only becoming important for the stronger Doppler-shifting conditions. Observed and intrinsic periods are typically 10 to 60 min and 10 to 30 min, respectively, with the intrinsic periods shorter at the highest altitudes because of preferen- tial penetration of GWs that are up-shifted in frequency by thermospheric winds.

Gravity wave and tidal influences on equatorial spread F based on observations during the Spread F Experiment (SpreadFEx)

Abstract: The Spread F Experiment, or SpreadFEx, was per- formed from September to November 2005 to define the po- tential role of neutral atmosphere dynamics, primarily grav- ity waves propagating upward from the lower atmosphere, in seeding equatorial spread F (ESF) and plasma bubbles ex- tending to higher altitudes. A description of the SpreadFEx campaign motivations, goals, instrumentation, and structure, and an overview of the results presented in this special issue, are provided by Fritts et al. (2008a). The various analyses of neutral atmosphere and ionosphere dynamics and structure described in this special issue provide enticing evidence of gravity waves arising from deep convection in plasma bub- ble seeding at the bottomside F layer. Our purpose here is to employ these results to estimate gravity wave characteristics at the bottomside F layer, and to assess their possible con- tributions to optimal seeding conditions for ESF and plasma instability growth rates. We also assess expected tidal influ- ences on the environment in which plasma bubble seeding occurs, given their apparent large wind and temperature am- plitudes at these altitudes. We conclude 1) that gravity waves can achieve large amplitudes at the bottomside F layer, 2) that tidal winds likely control the orientations of the gravity waves that attain the highest altitudes and have the greatest effects, 3) that the favored gravity wave orientations enhance most or all of the parameters influencing plasma instability growth rates, and 4) that gravity wave and tidal structures act- ing together have an even greater potential impact on plasma instability growth rates and plasma bubble seeding.

Latitudinal wave coupling of the stratosphere and mesosphere during the major stratospheric warming in 2003/2004

Abstract: The coupling of the dynamical regimes in the high- and low-latitude stratosphere and mesosphere during the major SSW in the Arctic winter of 2003/2004 has been studied. The UKMO zonal wind data were used to explore the latitudinal coupling in the stratosphere, while the coupling in the mesosphere was investigated by neutral wind measurements from eleven radars situated at high, high-middle and tropical latitudes. It was found that the inverse relationship between the variability of the zonal mean flows at high- and low-latitude stratosphere related to the SSW is produced by global-scale zonally symmetric waves. Their origin and other main features have been investigated in detail. Similar latitudinal dynamical coupling has been found for the mesosphere as well. Indirect evidence for the presence of zonally symmetric waves in the mesosphere has been found.

Dual-beam measurements of gravity waves over Arecibo: Reevaluation of wave structure, dynamics, and momentum fluxes

Abstract: [1] A previous study by Zhou and Morton (2006) employed dual-beam incoherent scatter radar measurements of radial velocities at the Arecibo Observatory to study the structure, dynamics, and momentum fluxes of gravity waves in the mesosphere and lower thermosphere for ∼8 h on 28 July 2001. Because of erroneous assumptions about wave character and inferences of the relationship between radial velocities, however, the advertised results of this previous study are largely in error. The purposes of the present study are both to point out these errors to help avoid such pitfalls in the future and to provide a new interpretation of these data, which represent a very interesting case study of gravity wave dynamics at these altitudes. Specific findings of the present study (largely in contradiction to the previous analysis) include (1) the ∼15-min oscillation was apparently a large-amplitude Doppler-ducted gravity wave structure propagating at one or two maxima of the westward large-scale wind present during the event; (2) the gravity wave exhibited a deep and coherent vertical phase structure, except between the two westward wind maxima at later times, entirely inconsistent with proximity to a critical level; (3) the dominant motions within the gravity wavefield were vertical velocities up to ∼10 m s–1, except for inferred horizontal motions where the vertical motions changed phase and above and below the vertical velocity maxima, as dictated by the continuity equation; (4) there were likely no regions of dynamical instability accompanying these ducted wave motions; and (5) momentum fluxes due to this wave motion were small, despite its very large amplitude.