Modeling study of tropical nonsquall and squall clusters
Chin, Hung-Neng Steve
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Permalink
https://hdl.handle.net/2142/20476
Description
Title
Modeling study of tropical nonsquall and squall clusters
Author(s)
Chin, Hung-Neng Steve
Issue Date
1990
Doctoral Committee Chair(s)
Wilhelmson, Robert B.
Department of Study
Atmospheric Science
Discipline
Atmospheric Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Atmospheric Science
Language
eng
Abstract
A numerical cloud model in both two-dimensional and three-dimensional form is used to investigate the dynamical differences between non-squall and squall line convection in the tropics. Two-dimensional simulations with cold-pool dynamics indicate that the simulated convective system propagates as a slow-moving multicellular squall line, in contrast to the nearly stationary observed non-squall convective line. Environmental winds are not included in the two-dimensional simulations. To assess their effect on the behavior and general structure of non-squall convection, three-dimensional simulations are performed. In addition, oceanic squall lines are also studied.
An observed non-squall and a squall line associated with the passage of a easterly wave are simulated to study their different dynamic behavior, and to identify the environmental parameters (e.g., wind and thermodynamical structure) favoring the development of one or the other. The same model external parameters (i.e., random temperature perturbations and large-scale forcing) are used for the control non-squall and squall simulations. The effect of large-scale low-level convergence on the organization of tropical mesoscale convective systems (MCSs) is also explored. The results indicate that the dynamic structure of tropical MCSs is strongly controlled by the environmental wind and moisture structure.
Model simulations show that the dominant easterly shear and weak convectively unstable environment favor the occurrence of non-squall line convection. Both the bell-shaped large-scale ascent and the environmental winds together play a crucial role in aligning the initially random distributed convective cells. Once the aligned structure forms, the dominant easterly shear further elongate the existing line structure. In contrast, the strong veering shear and more convectively unstable environment lead to the development of intense oceanic squall lines. Under the strong veering shear environment with a drier mid-level layer, a long-lasting develops and produces a strong updraft and precipitation to generate an intense low-level cold pool, which is advected southward by the environmental northerly wind, and trigger new successive cells to the south along the leading edge of the cold pool to form an arc-shaped squall line with a fast propagation speed.
The non-squall convection is aligned roughly parallel to the environmental wind shear and to the low-level convergence line, while the developing squall line is aligned approximately perpendicular to the environmental wind shear. Once the organized structure has developed, the large-scale forcing has no further effect on maintaining the dynamical organization of both non-squall and squall line systems. However, it does contribute positively to the total precipitation for the non-squall system, but negatively for the squall line system.
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