31st Annual Meeting of the DPS, October 1999
Session 53. Outer Planet Physics I Posters
Poster Group II, Thursday-Friday, October 14, 1999, Kursaal Center

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[53.07] Computation of Gravity Waves near the Tropopause

J.P. McHugh (UNH)

A difference between observed temperatures and predicted temperatures in the atmospheres of Neptune and the other outer planets has been previously documented. The difference is most significant above the tropopause. Gravity waves, often observed in the atmospheres of the outer planets, are likely to be responsible. The gravity waves are assumed to be created by convected motion of the atmosphere at a lower elevation, the waves propagate vertically, and then break near the tropopause, resulting in turbulence and heating.

Similar effects occur on Earth, although the heating of the atmosphere by gravity waves on Earth has not been verified. Balloon observations of Earth show strong patches of turbulence (often called blini's) in the vicinity of the tropopause. The cause of the blinis is not well understood, but again, gravity waves are the most likely source. Observations of Earth show that the turbulence occurs in patches with large but finite horizontal extent, and small vertical thickness. The horizontal size has been theorized to be dictated by the rate of rotation of the planet, indicating that there may be a difference between equatorial and polar regions.

The present work computationally studies waves propagating near the topopause of the outer planets. The computations show that a distinct tropopause causes the waves to partially reflect of the gravity wave, reducing the effect of breaking above the tropopause. This reflection is strongly influenced by the magnitude of the Brunt-Vaisala frequency in the stratosphere. Wave energy that does pass through the tropopause experiences a rapid change in vertical wavenumber, indicating a tendency for Kelvin-Helmholtz instabilities. However the similations do not produce such instabiilities. Heating rates are estimated based on an eddy viscosity model.

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