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J. M. Fritsch and C. F. Chappell

Abstract

A fine-mesh 20-level, primitive equation model is used as a tool for preliminary study of the potential for modification of mesoscale convective systems. The governing system of the model is hydrostatic with the non-hydrostatic convective components parametrically introduced through a convective cloud model subroutine. Two modification possibilities are tested: 1) dynamic seeding, and 2) alteration of the timing and location of initial convection.

Results of artificially changing the time and location of initial convection indicate that the evolution, structure, dynamics and precipitation of mesoscale systems are sensitive to the location where the initial convection happens to develop. Changing the time and location of initial convection may also substantially alter the location and significance of subsequent severe weather as well as potentially beneficial rainfall.

For idealized dynamic seeding (i.e., freezing occurs at −10°C in all cloud updrafts), model results suggest that seeding enhances convective precipitation and strengthens the dynamics of the mesoscale system. Although mesoscale convergence is increased by the additional latent heat release, the most promising link to additional precipitation seems to be through the enhancement of new growth by strengthening or accelerating moist downdrafts and their associated mesohigh outflow.

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J. M. Fritsch and C. F. Chappell

Abstract

A 20-level, three-dimensional, primitive equation model with 20 km horizontal resolution is used to predict the development of convectively driven mesoscale pressure systems. Systems produced by the model have life histories and structural characteristics similar to observed convectively driven meso-systems. Cooling by (parameterized) convective-scale moist downdrafts is largely responsible for meso-high formation, while warming by compensating subsidence strongly correlates with mesocyclogenesis.

An hypothesis for mesocyclogenesis associated with deep convective complexes is presented. The hypothesis recognizes that certain configurations of convective activity may produce focused areas of forced subsidence warming aloft. The warming in turn causes a thickness increase aloft which creates a hydrostatic circulation favorable for evacuating mass from the subsidence column. Consequently, pressure falls beneath the layer of high-level warming. Model results supporting this hypothesis are presented.

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J. M. Fritsch and C. F. Chappell

Abstract

A parameterization formulation for incorporating the effects of midlatitude deep convection into mesoscale-numerical models is presented. The formulation is based on the hypothesis that the buoyant energy available to a parcel, in combination with a prescribed period of time for the convection to remove that energy, can be used to regulate the amount of convection in a mesoscale numerical model grid element.

Individual clouds are represented as entraining moist updraft and downdraft plumes. The fraction of updraft condensate evaporated in moist downdrafts is determined from an empirical relationship between the vertical shear of the horizontal wind and precipitation efficiency. Vertical transports of horizontal momentum and warming by compensating subsidence are included in the parameterization. Since updraft and downdraft areas are sometimes a substantial fraction of mesoscale model grid-element areas, grid-point temperatures (adjusted for convection) are an area-weighted mean of updraft, downdraft and environmental temperatures.

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J. M. Fritsch, E. L. Magaziner, and C. F. Chappell

Abstract

A technique for generating analytical initial conditions for three-dimensional numerical models is presented. The technique combines trigonometric and other mathematical functions with meteorological constraints to construct an idealized atmosphere which exhibits commonly observed “real” atmosphere structural characteristics. For example, pressure and thermal waves which slope with height, tropopause, low-level moist tongue, phase differences in pressure and thermal waves, and a jet maximum at the tropopause level are all generated by the simple system of equations.

Examples of both mesoscale and synoptic-scale initial conditions are given, and results of integrating the mesoscale initial conditions in a three-dimensional model are shown. The initialization procedure is economical and flexible, and potential applications include testing weather modification sensitivity, finite-difference schemes, lateral boundary formulations, and various subgrid-scale parameterizations.

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R. A. Maddox, C. F. Chappell, and L. R. Hoxit

Meteorological conditions associated with more than 150 intense convective precipitation events have been examined. These heavy rainfalls caused flash floods and affected most geographic regions of the conterminous United States. Heavy rains associated with weather systems of tropical origin were not considered. Analyses of surface and standard level upper-air data were undertaken to identify and define important synoptic and mesoscale mechanisms that act to intensify and focus precipitation events over specific regions. These analyses indicated that three basic meteorological patterns were associated with flash flooding in the central and eastern United States. Heavy convective precipitation episodes that occurred in the West were considered as a separate category event. Climatological characteristics, composite analyses, and upper-air data are presented for these four classifications of events.

The large variability of associated meteorological patterns and parameters (especially winds aloft) makes identification of necessary conditions for flash flood-producing rainfall quite difficult; however, a number of features were common to many of the events. An advancing middle-level, short-wave trough often helped to trigger and focus thunderstorm activity. The storm areas were often located very near the mid-tropospheric, large-scale ridge position and occurred within normally benign surface pressure patterns. Many of the intense rainfalls occurred during nighttime hours. These elusive characteristics further complicate a difficult forecast problem.

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