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C. P. Chang, F. T. Jacobs, and B. B. Edwards

Abstract

A diagnostic model is proposed to use digitized satellite cloud brightness data to estimate objectively the large-scale flow patterns over data-void tropical regions. The model utilizes a linear barotropic vorticity equation with two primary assumptions: 1) that the area-averaged cloud brightness is positively correlated with large-scale divergence in the tropical upper troposphere; and 2) that the large-scale tropical flow is quasi-barotropic and quasi-non-divergent. It is designed to be used at any upper tropospheric level where divergence is important in determining the vorticity field. Three types of information are required: 1) boundary conditions determined from surrounding wind reports, 2) a mean zonal flow determined from climatology, and 3) an equivalent divergence forcing function constructed empirically from the brightness data.

The model is tested daily over a western North Pacific region for July-August 1971. Results for an 8-day representative period are presented and discussed. In general for 25% of the days tested, the model produces a flow field which accurately resembles the major features of the streamfunction field analyzed by the National Meteorological Center. In another 30% of the days it provides some valuable information about the flow patterns which would be difficult to obtain from boundary information alone. Experiments are also performed for two days in which the brightness data are enhanced by time-interpolated satellite infrared data. The resultant flow fields bear better resemblance to the NMC analysis. It is thus suggested that improved results may be expected when infrared and other types of advanced satellite data are available.

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C. B. Chang, D. J. Perkey, and C. W. Kreitzberg

Abstract

Results of moist and dry fine-mesh (∼140 km) numerical simulations of the 6 May 1975 Omaha squall line are presented. The moist fine-mesh simulation reproduced several observable features of the squall system and was then used to study other unobservable features. Differential thermal advection was responsible for the creation of potential instability. Low-level horizontal thermal advection contributed to pressure falls which, in turn, enhanced the convergence into the warm tongue. These processes initiated a band of convection. Results of the dry simulation suggested that the location and orientation of the initial low-level convergence was determined by dry mechanisms, that the convective latent heat release increased the rate of cyclonic-scale occlusion and that the occlusion process was followed by the dissipation of the convection.

In addition to the fine-mesh simulations, a meso-mesh (∼35 km) simulation was conducted. Because of the increased resolution, this simulation was able to reproduce the narrowing of the convective band from 400 to 100 km. This narrowing and intensification generated large values of low-level convergence and cyclonic vorticity. These processes produced a band of intense upward vertical velocity which, along with the convective transports of heat and moisture, stabilized the static energy profiles.

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C-B. Chang, D. J. Perkey, and C. W. Kreitzberg

Abstract

The sensitivity of a numerical simulation of a severe storm environment in the southwestern United States to a missing wind sounding is investigated. The case is the AVE-SESAME '79 storm of 10 April 1979. On that day a major outbreak of severe local storms took place in Oklahoma and Texas. Two 24 h fine-mesh forecasts wore conducted using the Drexel Limited Area Mesoscale Prediction System (LAMPS). The initial wind fields of thew two forecast were significantly different in the speed and structure of the upper-level jet around the base of a sharp trough over northwestern Mexico. This difference was caused by the deletion of one wind sounding located at the base of the trough from the initial observations used by the analysis scheme. Numerical results show profound impact of such changes on the 24 h simulations. Detailed comparisons between the experiments based on the simulation motion fields are made to provide physical understanding of the impact.

The numerical experiments underline the seriousness of uncertainty in wind analyses. In particular, due to missing or inadequate observations, the consequential errors in portraying the jet streak can result in the generation of spurious vorticity in the upper troposphere. This spurious vorticity, as it moves toward warm, moist air over the Gulf States, can cause erroneous prediction of the circulation and of the organized convection. This case shows clearly that sensitivity to sparse data is strong function of the gradients that exist at observation time and other factors such as the associated topographic features and moisture patterns.

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C. B. Chang, D. J. Pepkey, and C. W. Kreitzberg

Abstract

Using real-data numerical simulation experiments, latent heat induced energy transformations during the development of the wave cyclone of 20 May 1977 are investigated. During a 24 h period over 5 cm of precipitation fell despite baroclinically inactive synoptic conditions. The numerical experiments which were conducted included two 24 h fine-mesh forecasts, one with and the other without latent heating.

The following conclusions resulted from kinetic energy budget calculations performed on isobaric surfaces at 100 mb increments from 900 mb to 100 mb.

1) Heating enhanced the generation of kinetic energy at all levels, slightly weakened its dissipation (to sub-grid scales) in the lower troposphere and increased this dissipation in the upper troposphere.

2) Because of the rapid increase of kinetic energy with height the latent heat's contribution to the kinetic energy balance was, in a relative sense, most significant in the lower troposphere.

It is shown that while the maximum latent heating rates occurred in the middle to upper troposphere, the most significant response to the warming appeared in the lower troposphere. The enhancement of ageostrophic generation of kinetic energy and the reduction of sub-grid scale dissipation provides an important source of energy for the maintenance of the lower tropospheric circulation.

From potential energy calculations it was found that although the heating rates within the simulation domain were quite large, the condensation processes were not efficient in increasing the total potential energy of the model atmosphere. The contribution of heating to generation of total potential energy was 60 × 1055 J m−2 while the actual increase of total potential energy from the dry simulation to the wet simulation was 5 × 105 J m−2. The bulk of the discrepancy between the generation and the net gain was due to the changes in the boundary flux in the simulation's upper troposphere as a result of beating. The growth of this midlatitude cyclone did not depend on the short-term generation of potential energy by condensation processes to provide a source of energy. Rather, latent heat acted as a catalyst to enhance the conversion of potential to kinetic energy within the cyclone. The induced upper-level kinetic energy then was very effective at increasing the export of potential energy from the cyclone to its large-scale environment.

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C. B. Chang, D. J. Perkey, and C. W. Kreitzberg

Abstract

The effects of latent heating on the development of a wave cyclone are investigated using a multi-level primitive equation model to simulate the cyclone system with (wet) and without (dry) latent heating. While the dry simulation failed to properly predict either the formation of the closed circulation which developed throughout the depth of the troposphere or the pronounced northwest-to-southeast horizontal tilt of the upper-level trough shown by observations, the wet simulation successfully reproduced both these features.

The mechanisms for the generation of the regional-scale closed system are examined and the influence of latent heating on large-scale dynamics and energetics is discussed. Results indicate that latent heat release stabilized the troposphere and reduced the large-scale horizontal temperature gradient. Also, through the enhancement of ageostrophic flow, the addition of latent heat generated kinetic energy in both the lower and upper troposphere at the expense of the available potential energy.

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J. B. Tupaz, R. T. Williams, and C-P. Chang

Abstract

The structure and behavior of barotropically unstable and stable waves in the vicinity of a zonally varying easterly jet are studied numerically with a linearized barotropic vorticity equation on a β plane. The easterly jet is approximated by a Bickley jet with a slow zonal variation. The numerical results are also compared with a simple mechanistic analytical model using the local phase speed and growth rate concepts. In several aspects the results are grossly similar to that expected from the parallel flow theory of barotropic instability. However, in the unstable region the resultant structure of the waves causes a spatial growth rate greater than predicted by the local growth rates computed with a parallel flow model. In the stable region, the structure leads to a strong dynamic damping. When a uniform advective velocity is added to a variable mean flow, the difference between the magnitude of the growth rate of the computed waves and that implied by the parallel flow theory is somewhat reduced. However, in this case a stronger zonal asymmetry in the spatial growth rate curve with respect to the jet maximum occurs as a result of slower adjustment of the wave structure to the local stability conditions.

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Melinda S. Peng, B-F. Jeng, and C-P. Chang

Abstract

A limited-area numerical model designed specifically for forecasting typhoon tracks has been operational at the Central Weather Bureau (CWB) in Taipei, Taiwan, since January 1990. It is a primitive equation model with nine σ levels and a grid size of 70 km. The model domain of 8500 km × 6000 km is centered near Taiwan, and covers the western part of the Pacific Ocean and southeast China. A model-balanced vortex is bogussed into the analysis to initialize the forecast. To ensure the maintenance of the vortex circulation throughout the forecast period, artificial heating options are incorporated to supplement the Kuo-type cumulus parameterization in the model.

The statistics of track errors for all forecast cases conducted during the development and operational checkout period (before December 1989) and during 1990, the first year of real-time operation, are reported. During the operational checkout period, 12 typhoons were forecasted, with an average 48-h track error of 415 km (62 forecast cases). For the 1990 season, there were 11 typhoons, with an average 48-h error of 392 km (63 forecast cases). The errors are compared with the One-Way Interactive Tropical Cyclone Model (OTCM), which is considered as the best long-term operational numerical track model for the western Pacific, using a homogeneous sample. The results indicate that the two models have similar average errors. The model had larger errors than the climatology and persistence (CLIPER) method. However, for all three typhoons with erratic movements, the model outperformed the CLIPER.

The model was modified in several ways prior to the beginning of the 1990 season. The most beneficial modification appears to have been the enlargement of the forecast domain. However, the domain was still not large enough to cover important synoptic fields for Typhoon Marian, which was the westernmost typhoon forecasted by the model. Postoperational experiments were conducted and the forecast track of Typhoon Marian improved when the model domain was expanded to the west. Examination of the synoptic patterns indicates that the track forecast depends closely on the forecast of the subtropical high circulation.

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C-B. Chang, D. J. Perkey, and W-D. Chen

Abstract

A major extratropical cyclone developed over the East China Sea during the initial phase of the AMTEX'75. Using cross-section analyses the dynamic structure of the cyclone is investigated with an emphasis on obtaining a better understanding of the physical mechanisms controlling the development and maintenance of this intense oceanic system. The dynamic and kinematic variables examined include ageostrophic acceleration, potential and absolute vorticity, and vertical and horizontal motions.

During the 24-h period following 1200 UTC 14 February, the surface cyclone moved eastward more than 22° longitude, while the upper-level trough propagated only about 15°. In the same period, the central pressure at the surface dropped more than 20 mb while the areas extent of the closed circulation showed little change. Vertical decoupling of the wave-cyclone system is suggested by this discrepancy in phase speed, and by the temporal evolution of the trough axis and the height fields during the lust 12 h.

Latent heat release, which seemed to contribute to the vertical decoupling and the decrease in zonal baroclinicity, tended to strengthen the ascent aloft and convergence below in the vicinity of the cyclone. Strong ascent enhanced the thermally direct east–west circulation across the upper trough, while convergence resulted in further latent heating and consequently larger ageostrophic components and convergence near the surface. Thus, kinetic energy was generated to support the system's development despite the decoupling in the vertical and the weakening of the horizontal baroclinicity.

The roles of conservation of potential vorticity and ageostrophic acceleration in the dynamics of this system were strongly influenced by latent heating. Analyses suggested that the observed explosive oceanic cyclogenesis was a result of latent heat enhanced regional baroclinic processes.

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Melinda S. Peng, Der-Song Chen, Simon W. Chang, C-P. Chang, and B-F. Jeng

Abstract

In an effort to improve the tropical cyclone track forecast, two preprocessing procedures are applied to an operational baroclinic forecast system at the Central Weather Bureau (CWB) in Taipei. The first replaces the environmental wind field near the storm by the previous 6-h.movement vector of the storm. The second incorporates a wavenumber-1 asymmetry constructed by matching the flow at the center of the asymmetry with the previous 6-h storm movement. Applying both processes to the 32 typhoon casts archived at the CWB in 1990 reduces the averaged 48-h forecast distance error from 474 to 351 km.

Multiexisting typhoons may have interactions among themselves that depend on relative intensity. Proper representation of the intensities in the initial bogus is important for the track forecast. Experiments with different initial bogus intensities are conducted on a case of dual typhoons-Nat and Mireille in 1991. The forecast using different bogus vortices according to the estimated intensities of each typhoon gives substantially smaller errors than that using identical bogus vortices. The impact of initial bogus vortex intensity on the track forecast for single typhoon cases is also illustrated.

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Changhyun Yoo, Nathaniel C. Johnson, Chueh-Hsin Chang, Steven B. Feldstein, and Young-Ha Kim

Abstract

A composite-based statistical model utilizing Northern Hemisphere teleconnection patterns is developed to predict East Asian wintertime surface air temperature for lead times out to 6 weeks. The level of prediction is determined by using the Heidke skill score. The prediction skill of the statistical model is compared with that of hindcast simulations by a climate model, Global Seasonal Forecast System, version 5. When employed individually, three teleconnections (i.e., the east Atlantic/western Russian, Scandinavian, and polar/Eurasian teleconnection patterns) are found to provide skillful predictions for lead times beyond 4–5 weeks. When information from the teleconnections and the long-term linear trend are combined, the statistical model outperforms the climate model for lead times beyond 3 weeks, especially during those times when the teleconnections are in their active phases.

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