Variational Normal Mode Initialization for a Multilevel Model

Abstract

The application of variational normal mode initialization to the ECMWF multilevel grid-point model is described. The technique involves minimizing a variational integral of the changes made by the initialization to the analyzed mass and wind fields, suitably weighted with a view to exerting some control over the relative magnitudes of the adjustments to these fields. We show how to construct an appropriate three-dimensional integral, and describe an efficient procedure for solving the minimization problem in the simple case where the weights depend only on latitude. Experimental results are presented to demonstrate that, in comparison with unconstrained initialization, the changes made to the analyzed mass field can be considerably reduced without undue damage to the wind field, and without compromising the benefits of initialization in providing a forecast free of high-frequency oscillations. In the case studied here, the use of constrained normal mode initialization has no significant impact on the results of a subsequent 5-day forecast.