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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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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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