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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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R. A. Maddox, D. J. Perkey, and J. M. Fritsch

Abstract

An intense mesoscale convective complex developed over the central Mississippi Valley during the night and early morning hours of 24 and 25 April 1975. Analyses of upper tropospheric features during this period indicate strong changes in temperature, wind and pressure-surface heights occurred over the convective system in a period of only 6 h. It is hypothesized that the convective system is responsible for these changes. The question of whether the diagnosed changes reflect a natural evolution of large-scale meteorological fields or are a result of widespread deep convection is considered utilizing two separate numerical forecasts produced by the Drexel-NCAR mesoscale primitive equation model. A “dry” forecast, in which no convective clouds are permitted, is considered representative of the evolution of the large-scale environment. This forecast is contrasted with a “moist” forecast which, through the use of a one-dimensional, sequential plume cumulus model, includes the effects of deep convection. Differences between the forecasts are substantial and the perturbations produced by the convection are quite similar to diagnosed features. The numerical results support the contention that mososcale, convectively driven circulations associated with large thunderstorm complexes can significantly alter upper tropospheric environmental conditions.

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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. 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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