Insensitivity of the Nonlinear Normal Mode Initialization of a Limited Area Model to the Inclusion of Nonstationary Rossby Modes

Abstract

Application of the nonlinear normal mode initialization method requires the construction of the normal modes of the linearized model equations. In the case of a limited area model the Coriolis parameter in this linear system is usually assumed to be constant, so that the Rossby modes are stationary. Since inclusion of the beta terms might be important in practical applications of nonlinear normal mode initialization, we investigate this effect by including specific beta terms so that the Rossby modes are nonstationary. Results of the methods with stationary and nonstationary Rossby modes are compared. They show that the two methods are virtually identical in their effect upon the initial fields and upon their development during the first hours of the forecast.