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Y. J. Lin and P. T. Chang


A three-dimensional severe thunderstorm model is employed to study some effects that shearing and veering environmental winds exert on the structure and internal dynamics of a typical supercell rotating storm during its quasi-steady mature stage. These environmental winds are analytically formulated to conform with observations deduced from seven well-documented supercell storms. Horizontal relative winds are generated using the Rankine vortex concept for the inner core region (radii 0–4 km), the potential flow concept for the outer portion (radii 8–25 km) and the transition zone in between (radii 4–8 km). The temperature field is considered to conform with the observed warm-core structured storm. Using these semi-realistic data as input, six numerical experiments are conducted allowing the environmental wind to veer and to shear systematically from one case to another. Vertical velocities are obtained by solving the scaled mass continuity equation. Values of total pressure and perturbation pressure are computed from the diagnostic pressure equation obtained from the horizontal momentum equations using the sequential relaxation method. Results show that fields of perturbation pressure and vertical velocity are quite sensitive to the veering and shearing environmental wind in the region surrounding the central updraft core. Specifically, pronounced upward and downward motions are found on the right and left flank of a storm's updraft core, respectively. The magnitude of these induced vertical velocities increases in proportion to the vertical wind shear and is found to be closely related to perturbation pressure gradients. These findings are in good qualitative agreement with observational evidence reported in the literature. The role of these perturbation pressure forces in protecting the storm's main updraft is emphasized.

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Y. K. Sasaki and L. P. Chang


In a diagnostic study by expanding global data in normal mode functions, Kasahara and Puri found that for zonal wavenumber one, even the seventh vertical mode (the highest mode they presented) contains about 50% of the energy of the external mode. The vertical normal modes are eigensolutions of the vertical structure equation, and each mode is associated with well‐defined physical significance. Consequently, it is of interest to look into the accuracy of representation of, say, the first ten vertical modes in a discretized model because seriously misrepresented normal mode functions may not be able to honestly express the physics embedded in the data to be expanded. Along this line, a systematic method of obtaining matching eigensolutions of the vertical structure equation of a multilayered stratified atmosphere was developed. The resultant eigensolutions were used to investigate the influence of the upper boundary condition, the judicious method of the vertical grid levels and the relative accuracy of a finite‐difference and a finite‐element method in obtaining the discretized vertical normal mode functions. An important conclusion of this study is that in a discretized model, an inadequate grid resolution in the upper domain may result in considerable misrepresentation of the vertical structure functions even in the lower part of the domain for vertical modes higher than mode 5.

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G. T. J. Chen, Y. J. Wang, and C-P. Chang


This study compares the systematic errors of 36-h surface cyclone and anticyclone forecasts for two operational numerical weather prediction models over East Asia and the western North Pacific Ocean: the U.S. Navy's Operational Global Atmospheric Prediction System (NOGAPS), and Japan Meteorological Agency's Fine-mesh Limited Area Model (JFLM). The study is carried out for the 1983 Mei-Yu season (May–July), which is the wettest season over East Asia based on nontyphoon-produced rainfall. All available 0000 and 1200 GMT forecast runs are evaluated against an independent dataset of subjective analysis produced operationally by the Central Weather Bureau, Taipei. The mean position errors, mean central pressure errors and forecast skill indices for both cyclones and anticyclones in the NOGAPS and JFLM models are examined.

Both NOGAPS and JFLM models are more likely to underforecast than to overforecast the existence and/or genesis of both cyclones and anticyclones. However, over the Tibetan Plateau and its vicinity, both models tend to overforecast the existence and/or genesis of cyclones. They also forecast both cyclones and anticyclones too slow and too far to the north.

Diurnal variations in central pressure errors suggest that the error source is the lack of radiation processes in the JFLM and too strong a diurnal cycle of radiation processes in NOGAPS. Also, the failure to treat adequately the bulk effects of cumulus convection seems to be primarily responsible for the poor forecasts of oceanic cyclone development.

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