Showing papers by "David C. Fritts published in 2011"

Observation of a mesospheric front in a thermal-doppler duct over King George Island, Antarctica

Abstract: © Author(s) 2011. This work is distributed under the Creative Commons Attribution 3.0 License.

Computation of clear‐air radar backscatter from numerical simulations of turbulence: 2. Backscatter moments throughout the lifecycle of a Kelvin‐Helmholtz instability

Abstract: [1] Franke et al. (2011) describe a numerical simulation of the instability and turbulent breakdown of Kelvin-Helmholtz (KH) billows at a high Reynolds number, numerical assessment of radar backscatter, and accuracies of inferred Doppler spectral moments for one test volume. Those results suggest a potential for significant measurement biases for radars that obtain backscatter from refractive index fluctuations. We present in this paper the morphology of computed radar moments throughout the KH instability lifecycle for two radar configurations in order to reveal the evolving character of radar backscatter and compare the radar velocity estimates with true velocities throughout the evolution, and to provide guidance, and cautions, for the interpretation of these dynamics in observational data. Results reveal strong variations in backscatter moments and character, and dependence on radar measurement parameters, that should be beneficial in the interpretation of such measurements in the atmosphere. Backscatter power predictions agree reasonably with observations of such events and their temporal evolutions. Our results also reveal a potential for significant measurement or sensitivity biases, some of which were predicted previously. Examples include a lack of significant backscatter power in well-mixed billow cores, suggesting possibly weak turbulence where in fact it may be strongest, maximum backscatter power in the billow exteriors, where refractive index fluctuations are large but turbulence is weak, underestimated vertical velocities within the KH billows at early times, and inferred significant vertical velocities where true vertical velocities are near zero at late stages of restratification, especially in the edge regions of the turbulence layer.

Comparisons of predicted bore evolutions by the Benjamin-Davis-Ono and Navier-Stokes equations for idealized mesopause thermal ducts

Abstract: [1] Numerical simulations are performed employing two numerical models to contrast nonlinear bore evolutions predicted by the Benjamin-Davis-Ono (BDO) equation with evolutions described by the Navier-Stokes (NS) equations. The first model is a simple one-dimensional solver of the BDO equation; the second describes the nonlinear two-dimensional dynamics of the NS equations. Both utilize the Boussinesq approximation. Owing to their simpler, horizontally isotropic nature, only isolated thermal ducts are considered in this study. Simulations assume an initial long-wave perturbation and address the influences of perturbation amplitude and wavelength, viscosity, and nonzero background stability on the resulting evolutions. Results indicate that the BDO equation provides reasonable predictions of bore character and evolution for conditions that satisfy its underlying assumptions. BDO predictions fail to describe bore character and evolution in cases where either the initial perturbations or the thermal environment differs significantly from BDO assumptions. Predictions employing the NS equations will thus provide more realistic guidance in the interpretation and understanding of bores observed in the mesopause region for general environments.

Drake Antarctic Agile Meteor Radar (DrAAMER) First Results: Configuration and Comparison of Mean and Tidal Wind and Gravity Wave Momentum Flux Measurements with SAAMER

Abstract: A new-generation meteor radar was installed at the Brazilian Antarctic Comandante Ferraz Base (62.1degS) in March 2010. This paper describes the motivations for the radar location, its measurement capabilities, and comparisons of measured mean winds, tides, and gravity wave momentum fluxes from April to June of 2010 and 2011 with those by a similar radar on Tierra del Fuego (53.8degS). Motivations for the radars include the "hotspot" of small-scale gravity wave activity extending from the troposphere into the mesosphere and lower thermosphere (MLT) centered over the Drake Passage, the maximum of the semidiurnal tide at these latitudes, and the lack of other MLT wind measurements in this latitude band. Mean winds are seen to be strongly modulated at planetary wave and longer periods and to exhibit strong coherence over the two radars at shorter time scales as well as systematic seasonal variations. The semidiurnal tide contribute most to the large-scale winds over both radars, with maximum tidal amplitudes during May and maxima at the highest altitudes varying from approx.20 to >70 m/s. In contrast, the diurnal tide and various planetary waves achieve maximum winds of approx.10 to 20 m/s. Monthly-mean gravity wave momentum fluxes appear to reflect the occurrence of significant sources at lower altitudes, with relatively small zonal fluxes over both radars, but with significant, and opposite, meridional momentum fluxes below approx.85 km. These suggest gravity waves propagating away from the Drake Passage at both sites, and may indicate an important source region accounting in part for this "hotspot".

Mesosphere–Ionosphere Coupling Processes Observed in the F Layer Bottom-Side Oscillation

Abstract: During the Spread FEx campaign, under the NASA Living with a star (ILWS) program which was carried out in the South American Magnetic Equatorial region from September to November 2005, we observed formation of the bottom-type spread F and simultaneous occurrence of mesospheric gravity wave events. The events were monitored by the ionosonde, coherent radar and airglow OI 630.0 nm and OH imager. It is found that the bottom-type scattering layer has a wave form generated most probably by local gravity waves. Reverse ray-tracing of the observed gravity waves indicate their possible sources in the troposphere or thermosphere. Forward ray-tracing indicates their penetration into the ionosphere. The present work summarizes the observational evidence and results of the data analysis and discusses the mesosphere–ionosphere coupling processes.