Interactions between synoptic and planetary scales of motion

Abstract

The effects of synoptic waves on the dynamics of planetary waves are investigated using normal mode analysis. Initialized analyses of the Navy Operational Global Atmospheric Prediction System (NOGAPS) for 19 days between January and April 1986 are projected onto the normal modes of a linearized version of the model. For each analysis, the different terms (adiabatic nonlinear, linear and diabatic) which affect the time tendency of planetary-scale modes are determined by a one-time step integration of the NOGAPS model. The effect of synoptic scales on planetary scales is determined by computing the difference between the adiabatic nonlinear term computed from the NOGAPS analyses and analyses for the same period that have been spectrally filtered to remove most of the synoptic-scale waves. The energy tendency due to the nonlinear adiabatic term and the synoptic-scale contribution to this term are also computed. It is shown that the synoptic-scale contribution to the adiabatic nonlinear term and the time tendency of planetary-scale modes can be a very large percentage of these terms. By eliminating momentum advections in the model and computing the adiabatic nonlinear term for the filtered and unfiltered analyses, the relative importance of interactions through mass field interactions or momentum field interactions are determined. It is shown that synoptic-scale interactions which affect the planetary­ scale barotropic modes are primarily through the momentum advections, while mass and momentum interactions are possible for the baroclinic modes. The importance of mass field interaction generally increases as the vertical scale of the wave decreases. Because of the importance of synoptic waves to the dynamics and energetics of planetary waves, errors in the forecasts of planetary waves may in part be due to the synoptic-scale forecast errors.