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Bruce B. Ross

Abstract

The effects of mesoscale forcing and diabatic heating on the development of convective systems have been investigated using a simplified numerical model to simulate the squall line and the convective system preceding it that occurred over Texas and Oklahoma on 10– 11 April 1979. A simulation run without including latent but showed both systems to be initiated and maintained by convergence produced by larger-scale forcing. The first cloud system formed downwind of the convergence zone that was produced by the confluence of airstreams along a dryline. A cloud front approaching from the west then merged with this dryline, destroying its horizontal gradients through diffusive effects and replacing it with a frontal convergence line that was alinged with the low-level flow. This new configuration was then favorable for the formation of the squall line that developed in the simulation.

When latent heat was included the continuous cloud in the first convective system broke down into isolated cells which moved downstream from the convergence zone. In the non-latent heat case, the primary mechanism for providing moisture to this cloud was vertical diffusion from the moist surface layer. When latent heat was added, vertical advection within cell updraft provided a more efficient means to supply moisture to the convective system.

In the simulated squall line, latent heat release produced a deeper cloud system while intensifying and maintaining the low-level convergence. However, unlike the earlier system, the squall line did not break into convective cells when latent but was included in the simulation.

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Isidoro Orlanski
and
Bruce B. Ross

Abstract

A detailed analysis is made of a three-dimensional numerical simulation of the evolution of an observed moist frontal system over a 48 h period. The simulated front undergoes an initial period of frontogenetic growth, characterized by an alignment of vertical vorticity and horizontal convergence near the surface. The front then evolves to a mature, quasi-steady state as the line of maximum convergence moves ahead of the maximum vorticity. This phase shift is shown to be the result of a negative feedback mechanism which inhibits further vorticity growth while reducing the amount of viscous damping required to achieve a steady state. The influence of viscosity and surface drag upon this mechanism is also assessed.

When moisture is included in the numerical solution, the squall line which develops along the front exhibits a dual updraft structure with low-level convergence near the nose of the front and midlevel convergence located 100 km to the rear at a height of 3 km. This configuration is very similar to that found by Ogura and Liou in their analysis of an Oklahoma squall line not associated with a cold front.

Analysis of the equations of motion within the convective zone of the mature squall line shows the diabatic heating to be closely balanced by adiabatic cooling due to vertical temperature advection. As a result, the only net warming within this region occurs as adiabatic warming in the clear air outside of the cloud zone.

A linear, two-layer, dry model containing stable lower and unstable upper layers is shown to reproduce the dual updraft structure for certain low-level wind intensities without requiring microphysics. Also, for all wind conditions, this simple model produces strong convergence at the interface between the two layers. This suggests that the occurrence of a convergence maximum at the level of free convection should be a common feature of convectively unstable cloud systems.

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Isidoro Orlanski
and
Bruce B. Ross

Abstract

An investigation is made of the stability of a convectively unstable atmosphere in the presence of a stably stratified layer beneath, which is moving with a constant velocity relative to the upper air. This work is an extension of the linear model presented as part of the recent study of Orlanski and Ross in which they sought to explain the structure of their simulated squall line. A stability analysis shows that two modes are possible: 1) the gravitational or convective mode due to the unstable stratification in the upper layer which modifies the stable region below and 2) the classical Kelvin-Helmholtz mode due to shear across the interface. The Kelvin–Helmholtz mode is of limited physical interest in this case. On the other hand, the gravitational mode produces an updraft structure similar to updrafts in the stable lower layer of a convective system. Analysis of the horizontal displacement of the surface convergence for this mode relative to the convergence in the convective zone shows this displacement to depend primarily on the wind, stratification, and depth of the stable lower layer. The resulting relationship provides a method for determining whether a dual or single updraft will occur in a convective system.

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Bruce B. Ross
and
Isidoro Orlanski

Abstract

The 48 h evolution of an observed cold front is simulated by a three-dimensional mesoscale-numerical model for a typical springtime synoptic situation over the southeastern United States. The model used in this study employs anelastic equations of motion on a limited-area domain with locally determined inflow/outflow side boundaries.

Both the observed and simulated characteristics of the weather system indicate a mature front which intensifies and then weakens over the 48 h period. Moist convection occurs in the form of intermittent squall lines in the observed case; in the numerical simulation, convection develops above and somewhat ahead of the surface front after 24 h as in ensemble of convective cells.

An investigation is made of the mesoscale and subsynoptic-scale features of this solution to determine their sensitivity to the inclusion of moisture and to the magnitude of the eddy viscosity used in the numerical simulation. The primary effect of increased eddy viscosity is to reduce somewhat the propagation speed of the front. The major changes due to moisture inclusion occur when convection develops along the cold front; these convective effects, which are apparent in the subsynoptic as well as the mesoscale features of the solution, include increased low-level convergence, reduced surface pressure due to diabatic heating, and the deflection of winds due to upper-level divergence. In addition, small temperature changes occur in the middle troposphere between the jet stream and the surface front when either viscosity or moisture is varied; these disturbances are a clear manifestation of the effect which changes in the cross-stream circulation intensity have upon the frontal system.

A fundamental feature of the mesoscale structure of the front in all cases is the tendency of the line of maximum horizontal convergence at the surface to move ahead of the line of maximum vertical vorticity. This phase shift appears to be related to, the propagation characteristics of the frontal system. Also, the mesoscale moist convection develops a cellular structure throughout the convective zone in the low-viscosity solution; the use of higher viscosity tends to suppress these cells, particularly near the surface.

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Richard S. Hemler
,
Frank B. Lipps
, and
Bruce B. Ross

Abstract

A three-dimensional nonhydrostatic cloud model is used to simulate the squall line observed in central Texas on 11 April 1979. The cloud model covers an area 400 × 400 km2 with a 5-km horizontal resolution and is supplied initial and boundary conditions by a larger hydrostatic mesoscale model.

The model produces a back-building squall line ahead of the surface cold front, as would be expected based on an analysis of the pre-squall-line environment. A well-defined gust front and cold pool develop with the squall line. At the end of the 5-h simulation, deep convection is found along a line nearly 400 km long. The simulated squall line compares favorably both with observations and with a higher-resolution model simulation in an environment of similar shear, suggesting that the 5-km horizontal resolution is adequately representing the significant features of the squall line.

The major shortcoming of this study is the failure of the cloud model to produce the observed squall line at the proper time. Without the observed small-scale forcing, which was unresolved in the Severe Environmental Storms and Mesoscale Experiment (SESAME) dataset, the model is unable to generate the squall line until a larger-scale convergence area evolves, some 2–3 h after the appearance of the observed squall line.

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