On the propagation of pressure-patterns: Part I

Summary The propagation speed of sinoidal troughs and wedges in a steady state flow is determined from consideration of the mass transport due to the bodily motion of the system. Fundamental propositions are established regarding the mutual motion of wind-, pressure-, temperature-, and density-fields.It is found that in a frictionless barotropic general flow, all perturbations are propagated with the same speed—the speed of the general current. In a baroclinic general flow a perturbation will only be propagated without dispersion if it has a specific (sinoidal) horizontal structure.When a sinoidal perturbation is embedded in a baroclinic general flow-field, it will be propagated as though by a barotropic flow with the sameeffective speed. The effective speed can be computed when the vertical structure of the perturbation and of the mean flow are known.It is frequently assumed that the speed of mean flow at some particular level (500 mb is often assumed) gives the «steering» of the surface perturbation by a baroclinic general flow, that is to say, a baroclinic flow steers a perturbation with the speed of an equivalent barotropic field. The present paper provides a rational basis for the concept of an equivalent barotropic flow, but it is to be remembered that the «steering level» does not depend uniquely on the vertical structure of the mean flow-field, but varies from perturbation to perturbation, being lower for shallow perturbations than for (vertically) deep ones.