A spectral model for Cisk‐barotropic energy sources for tropical waves

Abstract

AbstractA non‐linear, three‐dimensional spectral model on a β‐plane is developed to study the relative rǒles of important energy sources for tropical waves. Both the north‐south shear of the zonal wind and Cisk (conditional instability of the second kind) are included. Energy exchanges and wave growth rates are studied in the presence of various specified vertical profiles of latent heat. Radiation is parameterized by Newtonian cooling; it plays a minor rǒle. Latent heat is parameterized in terms of large‐scale boundary layer flow. Dissipation is also included.The model includes three layers in the troposphere and one in the stratosphere. The top of the boundary layer is taken at 900 mb and its effect is described by frictionally induced vertical velocity and surface dissipation. The initial state is an idealized form of observations taken in the Marshall Islands.Important results show that when latent heat release decreases with height and is large at low tropospheric levels, the eddies are initially generated and maintained barotropically and then Cisk assumes a dominating rǒle. When heat release increases with height, Cisk dominates at all stages of the wave development; the barotropic mechanism, however, is still necessary.It is also found that when latent heat release is maximum at the low levels, the most dominant wave is 3,000 km with an initial growth rate of 3 days in e‐folding time. The longer waves assume greater dominance at the upper levels, especially when latent heat increases with height.