Normal mode initialization
From AMS Glossary
normal mode initialization
A method that uses model normal modes to adjust the initial conditions of an NWP model so that high-frequency oscillations are removed from the subsequent forecast.
The most successful procedure is known as nonlinear normal mode initialization, in which the normal modes are used to filter out high frequencies from the time derivative of the initial conditions.
Daley, R. 1991. Atmospheric Data Analysis. 263–321.