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Vivek Hardiker

Abstract

A conformal transformation suggested by F. Schmidt is followed to implement a global spectral model with variable resolution. A conformal mapping is defined from a physical sphere (like the earth) to a transformed (computational) sphere. The model equations are discretized on the computational sphere, and the conventional spectral technique is applied to march forward in time.

Two types of transformations are investigated in the present study, namely the rotation and the stretching transformation. Application of the stretching transformation leads to finer resolution in the meridional direction; however, due to the spherical geometry, the resolution becomes finer in the latitudinal direction also, and furthermore, the rotation can be used to relocate the model poles. The idea is now to rotate the north pole and refine the resolution around the new north pole by applying the stretching transformation.

A multilevel global spectral model is formulated from the current Florida State University global spectral model to implement the total (rotation followed by stretching) transformation. The control run in this study is a conventional T-170 resolution global spectral model. The transformed T-83 resolution global spectral model is used to study Hurricane Andrew. The performance of the transformed model is clearly seen to be improved in describing the structure, intensity, and motion of the hurricane over the conventional T-85 resolution spectral model. The computational cost for the transformed model is approximately one-half the cost for the conventional T-170 model. The conformal transformation technique can be thus used as a viable alternative to the limited-area models.

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T. N. Krishnamurthi, H. S. Bedi, Darlene Oosterhof, and Vivek Hardiker

Abstract

A high-resolution global model forecast of the formation of Hurricane Frederic of 1979 is analyzed by means of several diagnostic computations on the model's output history. The formation is addressed from an analysis of limited-area energetics where the growth of eddy kinetic energy is examined. The question on internal versus external forcing during the formative stage of the hurricane is explored by means of the Kuo-Eliassen framework for the radial-vertical circulation of the hurricane. The intensity of the predicted hurricane is diagnosed from a detailed angular momentum budget following the three-dimensional motion of parcels arriving at the maximum wind belt. Overall, the successful simulation of the hurricane has enabled us to make such a detailed diagnosis of the predicted hurricane at a high resolution. The principal findings of this study are that a north-south-oriented beating function maintained a zonal easterly flow that supplied energy barotropically during the growth of an African wave. The growth of eddy kinetic energy is somewhat monotonic and slow throughout the history of the computations. The initial development of the easterly wave appears to be related to the widespread weak convective heating that contributes to a covariance of heating and temperature and of temperature and vertical velocity. The hurricane development period is seen as one where both the barotropic and convective processes contribute to the growth of eddy kinetic energy. During this developing stage, the growth of radial-vertical circulation is largely attributed to convective, radiative, and frictional forcings. The role of eddy convergence of momentum flux appears to be insignificant. The intensity issue of the storm (maximum wind of the order of 37 m s−1) was addressed by means of a detailed angular momentum budget following parcel motion. The pressure torque in the model simulation had a primary role in explaining the intensity of the predicted storm. It is only in the storm's inner rain area where the frictional stress becomes quite large. But at these small radii the frictional torque is still smaller compared to the contribution from the (small but significant) azimuthal asymmetries of the pressure field and the resulting pressure torques.

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