Thermal Tides in the Atmosphere of Venus

Citation

Pechmann, Judith Burt (1983) Thermal Tides in the Atmosphere of Venus. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5cd0-3q63. https://resolver.caltech.edu/CaltechTHESIS:10312023-202011054

Abstract

The daily variation in the absorption of sunlight by the atmosphere provides the forcing for thermal tides. In this thesis the response of the Venus atmosphere as a function of height and latitude to the diurnal and semidiurnal components of the forcing is calculated using a linearized primitive equation model. We specify the atmospheric mean state using data from the Pioneer Venus probes and orbiter, and solve for the first order tidal perturbation. Our forcing function is based on data returned by the solar flux radiometer on the Pioneer Venus sounder probe. The perturbation variables are discretized horizontally by spherical harmonics and vertically by finite elements. A semi­ implicit time-stepping algorithm is used.

The model results for Venus thermal tides are in agreement with the solar-fixed component of diurnal and semidiurnal brightness temperature fluctuations determined from Pioneer Venus orbiter infrared radiometer (OIR) data. Contrary to the prediction of classical tidal theory, the observed semidiurnal brightness temperature maxima occur before the forcing maxima from about 60 to 80 km. In the model, this phase lead is due to a long vertical wavelength (~30 km) from the cloud tops to 80 km. Also, the data unexpectedly show that the semidiurnal amplitude is larger than the diurnal over much of the region observed, even though the diurna1 forcing is about twice as great. The model's diurnal temperature amplitude is larger than that observed in the brightness temperature. However, convolution with the OIR weighting functions results in model brightness temperature amplitudes which are as small as the observed values because the vertical wavelength of the diurnal tide is shorter than the width of the weighting functions.

The success of the model increases our confidence in our knowledge of the mean state of the Venus atmosphere and provides us with the opportunity to determine the importance of energy and angular momentum transport by the tides. The zonally averaged vertically integrated tidal energy flux is significant compared to the zonally averaged imbalance in incoming solar radiation and outgoing infrared radiation from equatorial to mid-latitudes. However, the tides do not in general tend to reduce the imbalance by transporting heat poleward. The mean meridional circulation driven by this imbalance and the tidal energy flux consists of stacked direct and indirect Hadley cells. The angular momentum transport is upward and poleward in the direct cells. Since the transport is not upward at all altitudes, some other processes must be involved in maintaining the large shear in the mean zonal wind. The vertical transport by the tides is small compared to that of the mean circulation. However, the tides do transport a significant amount of angular momentum equatorward from mid-latitudes.

Thesis Files