Baroclinic waves in a shallow saturated layer

Humidity stratification may affect baroclinic wave growth by inducing vertical confinement in the layer where small amplitude updrafts are saturated. By means of a two-dimensional numerical model the authors show that small amplitude modes are trapped in saturated regions, exhibiting horizontal and vertical scales that are smaller and growth rates that are larger than those of Eady waves in a dry atmosphere. Then the small amplitude is contrasted with the finite amplitude growth: whereas in the former a clear-cut modal structure is attained, in the latter the eventual mixing of moisture alters the initial environment to the point that shallow waves are no longer supported. When the alteration of the basic state takes place the amplitude of the shallow wave, although finite, is small enough that the approximations adopted to represent meridional advection of moisture in this tu two-dimensional model are still valid. The effect of surface drag is also discussed and it is shown that it does not prevent modal and finite amplitude growth in the moist atmosphere.