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Comparative study of different orographic representations with respect to the Indian summer monsoon simulation

Dash S. K., Mohandas S.

Acta Geophysica Polonica

2005

Vol. 53, nr 3

325-340

In spectral General Circulation Models that are now widely used in opera- tional weather forecasting and research, the time dependent atmospheric parameters and orography are represented in the form of double series. In such spectral transform methods, Gibbs oscillations appear due to sharp gradients in terrain height, moisture, and cloud fields. The present paper shows the usefulness of different digital filters in reducing the negative values of orography. The application of filters also reduces the heights of the mountain peaks. This affects the Indian region the most, because of the presence of the Himalayas and the Western Ghats. In this paper, an attempt has been made to represent the orography by a new method, called the Filtered Modified Orography (FMO), in which a two dimen- sional Lanczos filter has been applied in the spectral domain globally with a subsequent local enhancement of the Himalayas and the Western Ghats. The dual advantages of reduction in negative orography values and enhancement of mountain peaks were achieved. A comparison with the envelope orography, where the mean orography is enhanced globally, shows that the new method is able to reduce some of the errors and disparities associated with the envelope technique while retaining some of the advantages of the barrier effect regionally. Results show reasonably good representation of global winds, geopotential and rainfall in FMO representation in T80 model of the National Centre for Medium Range Weather Forecasting.

A non-singular description of parametrical resonance

Romanów F., Hańćkowiak J.

Journal of Theoretical and Applied Mechanics

2000

Vol. 38 nr 4

879-892

Employing the Mathieu equations we present a method for construction of Ince-Strutt diagrams, in which no cut-off of the infinite chain of equations for Fourier coefficients is necessary.

Niesingularny opis rezonansu parametrycznego. Na przykładzie równania Mathieu przedstawiamy niesingularną metodę konstrukcji diagramów Incea-Strutta, które obrazują na płaszczyźnie F parametrów równania obszary stabilności i niestabilności. Niesingularność metody została osiągnięta dzięki nie obrywaniu nieskończonych łańcuchów równań dla współczynników Fouriera. Pozwala to na zachowanie pierwotnego charakteru rozważanych równań (zagadnienie początkowe). Przedstawione rozważania sugerują poszerzenie oszarów niestabilności.