Showing papers by "Michael P. Hickey published in 2014"

Thermospheric Dissipation of Upward Propagating Gravity Wave Packets

Abstract: We use a nonlinear, fully compressible, two-dimensional numerical model to study the effects of dissipation on gravity wave packet spectra in the thermosphere. Numerical simulations are performed to excite gravity wave packets using either a time-dependent vertical body forcing at the bottom boundary or a specified initial wave perturbation. Three simulation case studies are performed to excite (1) a steady state monochromatic wave, (2) a spectrally broad wave packet, and (3) a quasi-monochromatic wave packet. In addition, we analyze (4) an initial condition simulation with an isothermal background. We find that, in cases where the wave is not continually forced, the dominant vertical wavelength decreases in time, predominantly due to a combination of refraction from the thermosphere and dissipation of the packets' high frequency components as they quickly reach high altitude. In the continually forced steady state case, the dominant vertical wavelength remains constant once initial transients have passed. The vertical wavelength in all simulations increases with altitude above the dissipation altitude (the point at which dissipation effects are greater than the wave amplitude growth caused by decreasing background density) at any fixed time. However, a shift to smaller vertical wavelength values in time is clearly exhibited as high-frequency, long vertical wavelength components reach high altitudes and dissipate first, to be replaced by slower waves of shorter vertical wavelength. Results suggest that the dispersion of a packet significantly determines its spectral evolution in the dissipative thermosphere.

Numerical Simulation of the Long-Range Propagation of Gravity Wave Packets at High Latitudes

Abstract: We use a 2-D, nonlinear, time-dependent numerical model to simulate the propagation of wave packets under average high latitude, winter conditions. We investigate the ability of waves to propagate large horizontal distances, depending on their direction of propagation relative to the average modeled ambient winds. Wave sources were specified to represent the following: (1) the most common wave parameters inferred from observations of Nielsen et al. (2009) (18 km λx, 7.5 min period), (2) waves consistent with the average phase speed observed (40 m/s) but outlying horizontal wavelength and period values (40 km λx, 17 min period), and (3) waves which would be subject to strong ducting as suggested by Snively et al. (2013) (25 km λx, 6.7 min period). We find that wave energy density was sustained over large horizontal distances for waves ducted in the stratosphere. Waves traveling against winds in the upper stratosphere/lower mesosphere are more likely to be effectively ducted in the stratosphere and travel large horizontal distances, while waves which escape in the form of leakage are more likely to be freely propagating above 80 km altitude. Waves propagating principally in the direction of the stratopause winds are subject to weaker stratospheric ducting and thus increased leakage of wave energy density from the stratosphere. However, these waves are more likely to be subject to reflection and ducting at altitudes above 80 km based upon the average winds chosen. The wave periods that persist at late times in both the stratosphere and the mesosphere and lower thermosphere (MLT) range from 6.8 to 8 min for cases (1) and (3). Shorter-period waves tend to become trapped in the stratosphere, while longer-period waves can dissipate in the thermosphere with little reflection or trapping. It is suggested that the most common scenario is of partial ducting, where waves are observed in the airglow after they leak out of the stratosphere, especially at large horizontal distances from the source. Stratospheric ducting and associated leakage can contribute to a periodic and horizontally distributed forcing of the MLT.