Lipps, F. B., 1970: Barotropic stability and tropical disturbances. Monthly Weather Review, 98 (2), 122-131.

Abstract: This paper attempts to determine under what conditions horizontal shear in the mean zonal flow can provide the initial source of energy for the traveling disturbances of low latitudes. A three-zone barotropic model is constructed in order to examine the stability of an idealized mean zonal current. The width and total wind shear associated with this mean current are varied. The form of growing disturbances and their amplification rates are found.
A stability analysis is also carried out for a basic flow which has a hyperbolic tangent variation with latitude. Results obtained by numerical integration for this basic flow are similar to those found previously with the three-zone model. In discussing his easterly wave model, Yanai indicates a basic flow which has a total wind shear of about 8 m sec^-1 occurring over approximately 6 degrees of latitude. Results obtained for a basic flow with these characteristics show that the fastest growing wave has a wavelength near 2500 km and an e-folding time of about 7 days.