Abstract
As a continuation of previous studies (Yamasaki, 1969; Ooyama, 1971), the properties of wave disturbances superimposed upon zonal current in a conditionally unstable model tropics are studied by solving linearized primitive equations on the spherical coordinates. The effect of moist convection is incorporated by the use of the assumption that non-adiabatic heating due to convective clouds is specified by the horizontal convergence in the boundary layer, as Ooyama (1964) proposed in the study of tropical cyclones.
In the absence of zonal current, surface friction and heat release, the model includes barotropic Rossby mode discussed by Haurwitz (1940b) in addition to baroclinic Rossby mode, gravity mode and Kelvin mode discussed by Matsuno (1966). It is found that the Haurwitz type waves of the lower meridional modes (n=0, 1) become unstable when surface friction, heat release and vertical shear of a zonal current are incorporated. The preferred wavelength increases from about 2, 000 km to 6, 000 km with the increase in vertical shear. Comparisons with observations show that the lowest meridional mode (n=0) is similar to the equatorial easterly wave described by Palmer (1952) and that analyzed by Chang et al. (1970). Since frictional convergence is a maximum at latitudes where the absolute value of the Coriolis parameter is equal to the frequency of this wave relative to the basic zonal current (Holton et al., 1971; Yamasaki, 1971), the stability properties of this wave appears to be strongly controlled by heat release associated with the frictional convergence around this latitude.
The properties of other kinds of unstable waves are also discussed briefly.
