4-8 day planetary waves in the middle and upper atmosphere: What can theory and modeling tell us about the observations we make.

Planetary waves with periods ranging from four to eight days are often observed in the mesosphere and lower thermosphere. At times these global scale waves exhibit characteristics like those of the 1st symmetric Rossby normal mode for zonal wave number one, often referred to as the 5-day wave. At other times the period and seasonality of these waves suggest behavior unexpected from a normal mode propagating upward from the troposphere. Planetary wave propagation theory and a series of modeling experiments will be used to study global scale oscillations with periods around 5 days. Vertical propagation characteristics will be examined over the course of a year using realistic representations of the atmosphere, dissipation, and of lower atmospheric forcing. Mechanisms other than lower atmospheric forcing, such as an in-situ realization of normal mode in the face of broad-band forcing, and instabilities of the mean state will also be examined.