The spectrum of atmospheric motions is vast, encompassing phenomena having periods ranging
from seconds to millennia. The motions of interest to the forecaster typically have time-scales of a day or longer, but the mathematical models used for numerical prediction describe a broader span
of dynamical features than those of direct concern. For many purposes these higher frequency
components can be regarded as noise contaminating the motions of meteorological interest.
The elimination of this noise is achieved by adjustment of the initial fields, a process called