Investigations of problems related to the choice of the initial state for numerical weather prediction models

• In numerical prediction models, where the wind and mass fields are described as free variables (so called primitive equation models), the initial state has to be carefully chosen, to avoid spurious oscillations in the forecast. This thesis deals with this initialization problem. The general approach has been to use simplified models in the investigations.In one of the papers an investigation is performed, where the quasigeostrophic divergence is included in the initial state. This is done in a two-layer linearized model, and the results show a generally lower level of the gravity modes than that of the geostrophic initialization.The other papers deal with various aspects of the non-linear normal mode initialization. One of the papers is a description of a derivation of a shallow water model on the equatorial ß-plane. This model is used to examine the generation and non-linear interactions of gravity modes, as a function of physical parameters, such as the Rossby-number and the depth of the fluid.It is found that the level of the gravity amplitudes, created by the model (with zero initial gravity amplitudes), is a little higher than the corresponding value for non-linear normal mode initialization. The gravity amplitudes get larger when the fluid is shallower, or if the Rossby-number increases.In the last paper another model is used. This is a mid-latitude ß-plane model, with only longitudinal dependence in the variables. With this model experiments, connected to the convergence problem of non-linear normal mode initialization, are performed. The results show that, if no forcing is included, the lack of convergence, for small depths is due to the non-linear interactions between the gravity modes. Experiments are also made where a linear forcing is present. It is shown that linearizing around a basic state, where the forcing is included is preferable to letting the forcing act on the non-linear side of the equations of the system.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Meteorologi och atmosfärforskning (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Meteorology and Atmospheric Sciences (hsv//eng)

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