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Hilding Sundqvist


In using σ as the vertical coordinate there is a need for interpolation of analyzed observations from p- to σ-surfaces in the process of establishing initial fields for a forecast. By employing an equation that establishes balance between the mass and wind fields directly on σ-surfaces (here called alternative b), we need to interpolate temperature alone from p to σ. This procedure appears preferable to the alternative of balancing on p-surfaces by the conventional balance equation followed by an interpolation of both mass and wind fields (called alternative a); how to preserve the established balance in the interpolation procedure is not obvious.

The divergence equation is derived from the equations of motion with σ [=p/ps] as the vertical co-ordinate. A comparison of the order of magnitude of the terms is made with the aid of scale analysis. This shows that when the streamfunction for the non-divergent mass flow is introduced in the divergence equation, a consistent approximation to a balance equation results regardless of whether the earth's surface is level or not.

A comparison is made between forecasts started from initial fields obtained by the two methods. In case a, pronounced gravity oscillations are present at least during the first 8–12 h, while in case b such oscillations are hardly detectable at any time during the forecast.

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Xiang-Yu Huang and Hilding Sundqvist


An initialization scheme for numerical models containing treatment of cloudiness is presented. The dynamic type of initialization scheme is based on the digital filtering technique, which requires integration of the model backward and forward about the analysis time. As the numerical model contains an advanced condensation-cloud parameterization, the initialization procedure renders initial cloud water and cloud cover fields, yet no cloud observations have been available. The initial cloud fields obtained are in a good agreement with the weather situations as they appear on satellite imagery and in synoptic analyses. The cloud water content is of the same order of magnitude as the one obtained from a 24-h forecast with the model. Improvements are observed in the spinup of the model cloud and of the precipitation rate.

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Hilding Sundqvist, Erik Berge, and Jón Egill Kristjánsson


This paper presents the implementation of a parameterization scheme for convective and stratiform condensation (with cloud water as a prognostic variable) into a fine mesh numerical weather prediction model.

The results from a 36 h integration of the model, with grid distance 50 km, indicate that the new condensation scheme contributes to an improved forecast compared to that obtained by the original model. Furthermore, from a qualitative comparison with satellite pictures, it is found that the prediction of condensation-cloud parameters is quite realistic.

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