Abstract: Following a scheme of discrete space in a prediction model for tropical limited areas (TL-6) by the institute, appropriate equations of horizontal and vertical structures are derived for the motion of tropical atmosphere and their vertical and horizontal modes are computed. In the normal modes obtained here, which are similar to any analytic results for the motion, there are well-defined properties of frequency sepatation in the gravity waves and Rossby waves. Based on the conputation, separating the equatorial Kelvin waves from the mixed Rossby waves is discussed. As the last discussion, the set of normal modes are used to filter meteorological fields.