Convectively generated internal gravity waves in Venus's middle atmosphere : momentum transport and radio scintillations

Citation

Leroy, Stephen Sylvain (1994) Convectively generated internal gravity waves in Venus's middle atmosphere : momentum transport and radio scintillations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/msgs-7b48. https://resolver.caltech.edu/CaltechETD:etd-06182008-135507

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. This work is divided into two papers. Firstly, we theoretically calculate how gravity waves are emitted from the neutrally buoyant dry convection in Venus's middle atmosphere and investigate whether such waves play a significant role in supporting the superrotation of Venus's atmosphere. Secondly, we attempt to explain the radio scintillations seen in the occultations of many spacecraft, particularly Pioneer Venus, as caused by convectively generated gravity waves. Below are the abstracts of the two papers presented in this thesis. Paper I: We calculate the emission of internal gravity waves from neutrally buoyant dry convection embedded within a stable atmosphere with static stability and zonal winds varying in height. We apply this theory to Venus's middle atmosphere to investigate whether these waves can help support the superrotation of Venus's atmosphere. Furthermore, we model the radio scintillation data obtained for Venus as caused by such internal gravity waves. The emission mechanism is similar to that suggested for driving the gravity modes of the Sun. We assume a background atmosphere on which we have superimposed linear wave propagation. Waves are damped by reabsorption by the convection, wavebreaking in the stable atmosphere, critical layer absorption, and by wave radiation to space. Wavebreaking is imposed wherever the waves become convectively unstable. Inertial effects are neglected and plane parallel geometry is assumed. Propagation of the waves is handled using a second order WKBJ approximation. A complete three dimensional ensemble of waves is retained. We show that both westward and eastward propagating waves exert strong accelerations on the mean flow. The westward propagating carry enough momentum to support the westward superrotation between the convection and the cloud-tops; however, the bulk of the wave momentum flux is critically absorbed and deposited within a kilometer of the convection because most of the waves propagate slowly in the horizontal. The eastward propagating waves are found to exert large decelerations above the zonal wind maximum. The decelerations are larger than 20 m [...] day [...], similar to wave drag in the Earth's mesosphere. In the course of evaluating our model, we have found that dry convection must have "penetrative" mixed layers above and below it. This arises from wavebreaking of gravity waves immediately after their generation. The effect of the penetrative layer is to filter wave emission and to create a discontinuity in the background temperature lapse rate dT/dz. The latter may be observable in atmospheres. Paper II: We simulate radio scintillations as they would appear in Pioneer Venus radio occultation data assuming that the index of refraction fluctuations in Venus's atmosphere responsible for the scintillations are directly caused by gravity wave fluctuations. We assume that the gravity waves are created by a global convection layer between 50 and 55 km altitude in Venus's atmosphere and propagate vertically. Associated with the gravity waves are density fluctuations which create the index of refraction variations. We compare the simulated scintillations with data and argue that this theory for the radio scintillations is preferable to the theory that the scintillations are caused by clear air turbulence in Venus's atmosphere. We show that these gravity waves can explain the shape and amplitude of the radio scintillation variance spectra in frequency. The shape of the simulated radio scintillation variance spectra in frequency is nearly a direct result of a saturated spectrum of breaking gravity waves. This saturated spectrum is the spectrum of breaking gravity waves in the vertical wavenumber. On the other hand, the overall amplitude is subject to parameters such as the intensity of the convection, the angle between the zonal winds and the beam path, and the zonal wind profile at polar latitudes. Limits can be placed, though, on the intensity of the convection which generates the waves and on the angle between the radio beam path and the winds in Venus's atmosphere. We find that the convection in Venus's middle atmosphere, even in polar regions, must transport 1 W/[...] to create gravity waves strong enough to break. This result is dependent on the amplitude of the zonal winds at polar latitudes.

Thesis Files