African Easterly Waves and Convection. Part I: Linear Simulations

A linearized version of a nonhydrostatic model is used to study the normal-mode selection and the structure of the African easterly waves in dry and moist environments associated with an idealized African easterly jet structure. The dry mode reproduces the principal observed characteristics of the lower troposphere, in particular the low-level ascent and convergence in the trough. A simple CISK-type parameterization improves the upper-level circulation. A budget of kinetic energy shows barotropic and baroclinic contributions and adequately reproduces the analysis of Norquist et al. The scale selection and the structure of the most unstable mode appears sensitive to the jet structure, in particular to the meridional and vertical shear. Several discrepancies between observations and these linear modes emphasize the importance of an accurate description of convection.