Planetary-Scale Waves in the Venus Atmosphere

Abstract

A numerical model of planetary-scale waves in Venus? atmosphere is used to simulate observed wave-like cloud features such as the dark horizontal Y. The model is based on the linearized primitive equations. Observed variations of static stability and mean zonal wind as a function of altitude are included in the basic state. Preferred modes of oscillation are found by imposing forcing over a range of frequencies, and determining the frequencies at which atmospheric response is greatly enhanced. Preferred responses exist at frequencies which are observed for the Y and other wave-like features. The Y shape can be produced by a linear combination of two model output waves: a midlatitude Rossby wave and an equatorial Kelvin wave. In order to preserve the relative phase between the waves and maintain the Y, nonlinear coupling between the waves is needed. Both waves are upward propagating, similar to the upward propagating planetary waves in Earth's stratosphere. The Kelvin wave may be forced at any altitude, but the Rossby wave must be forced at cloud heights to avoid absorption at a critical level. The Kelvin wave transports westward momentum upward, and thus can act to maintain the strong westward zonal winds on Venus. The Rossby wave acts to decrease the equator-pole temperature difference and therefore would decelerate the zonal wind.