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Roger K. Smith

Abstract

The problem of explaining the surface pressure rise in simple balanced models of fronts, discussed at length by Sutcliffe, is reexamined. It is shown that air mass models for steadily translating fronts (including the Margules' front) are dynamically consistent, except along a vertical line above the surface front, only if there is vertical motion (subsidence for a cold front, ascent for a warm front) in the warm air that overlies the cold air. In this case, the local post-frontal pressure rise in a model cold front and the pre-frontal pressure fall in a model warm front can be attributed to advection. However, the presence of the vertical motion is a limiting factor in the applicability of such models.

The analysis resolves an apparent inconsistency between the surface pressure changes computed in Boussinesq models and the prediction of a theorem of Brunt.

Irrespective of the Boussinesq approximation, it is shown that, in the model, the surface pressure change at any fixed location bears no relation to the variation of surface pressure normal to the front at any given instant. This would imply that it is inappropriate to infer space cross-sections of pressure from observed time series at a single station, even for a steadily translating front. The result highlights a further limitation of balanced air mass models when applied to fronts in the atmosphere.

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Roger K. Smith

Abstract

The cyclostrophic and hydrostatic adjustment of simple one-layer and multilayer vortex flows to the local removal and/or redistribution of mass and angular momentum are studied, and a detailed physical interpretation of the dynamics of adjustment is given for the one-layer model. The calculations provide insight into possible responses of tropical cyclones to modification by cloud seeding and facilitate an appraisal of the Simpson-Malkus modification hypothesis.

Calculations for two- and three-layer models show that the maximum tangential velocity is increased whether or not mass transfer takes place predominantly inside or outside the radius at which the maximum occurs, and the central surface pressure decreases due to subsidence at one or both interface levels. However, the magnitude of these effects are comparatively small in relation to the strengths of the induced meridional circulation and corresponding changes in tangential wind speed outside the core, at, or beyond, the radii at which mass transfer occurs. Moreover, the estimated maximum change in tangential wind speed that might be produced in a tropical cyclone by following the seeding procedure suggested by Simpson and Malkus is small compared with observed natural variations.

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Hongyan Zhu and Roger K. Smith

Abstract

The minimal three-dimensional tropical cyclone model developed by Zhu et al. is used to explore the role of shallow convection, precipitation-cooled downdrafts, and the vertical transport of momentum by deep convection on the dynamics of tropical cyclone intensification. The model is formulated in σ coordinates and has three vertical levels, one characterizing a shallow boundary layer, and the other two representing the upper and lower troposphere, respectively. It has three options for treating cumulus convection on the subgrid scale and a simple scheme for the explicit release of latent heat on the grid scale.

In the model, as in reality, shallow convection transports air with low moist static energy from the lower troposphere to the boundary layer, stabilizing the atmosphere not only to itself, but also to deep convection. Also it moistens and cools the lower troposphere. For realistic parameter values, the stabilization in the vortex core region is the primary effect: it reduces the deep convective mass flux and therefore the rate of heating and drying in the troposphere. This reduced heating, together with the direct cooling of the lower troposphere by shallow convection, diminishes the buoyancy in the vortex core and thereby the vortex intensification rate.

The effects of precipitation-cooled downdrafts depend on the closure scheme chosen for deep convection. In the two closures in which the deep cloud mass flux depends on the degree of convective instability, the downdrafts do not change the total mass flux of air that subsides into the boundary layer, but they carry air with a lower moist static energy into this layer than does subsidence outside downdrafts. As a result they decrease the rate of intensification during the early development stage. Nevertheless, by reducing the deep convective mass flux and the drying effect of compensating subsidence, they enable grid scale saturation, and therefore rapid intensification, to occur earlier than in calculations where they are excluded. In the closure in which the deep cloud mass flux depends on the mass convergence in the boundary layer, downdrafts reduce the gestation period and increase the intensification rate.

Convective momentum transport as represented in the model weakens both the primary and secondary circulations of the vortex. However, it does not significantly reduce the maximum intensity attained after the period of rapid development. The weakening of the secondary circulation impedes vortex development and significantly prolongs the gestation period.

Where possible the results are compared with those found in other studies.

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Roger K. Smith and Wolfgang Ulrich

Abstract

An analytical theory is presented for the motion of an initially symmetric barotropic vortex on a beta-plane at rest, the prototype problem in the theory of tropical cyclone motion. In the case of vortices with parameter values appropriate to tropical cyclones, the theory shows excellent agreement with equivalent numerical model calculations for a period of between one and two days. In particular, the vortex track and the evolution of vortex asymmetries, the so-called beta gyres, are accurately predicted. The calculations provide further insight into dynamics of tropical cyclone motion in general and provide a firmer basis for interpreting the numerical solutions in particular. They are relevant also to the important problem of designing more appropriate “bogus” vortices for the initialization of dynamically based tropical cyclone forecast models.

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Sabine P. Haase and Roger K. Smith

Abstract

Early in the morning on 9 June 1982 a system of traveling wave cloud lines passed over 0klahoma, and in particular, over the relatively dense mesonetwork of surface stations, including the instrumented 444 m KTVY television tower, operated by the U.S. National Severe Storms Laboratory. An analysis of the network and other data presented herein shows that, in structure, the associated disturbance was an internal undular bore propagating on a low-level stable layer, similar to “morning glory”-type disturbances, which are common at certain times of the year in parts of northern Australia. Moreover, the speed of propagation of the component bore-waves is in broad agreement with theoretical calculations.

There is evidence that the disturbance emanated late the previous evening from an area of rapidly growing intense thunderstorms located more than 400 km north-northwest of the surface mesonetwork. Possible mechanisms for generation are discussed, but the data pertaining to genesis are insufficient to be conclusive.

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Roger K. Smith and Michael J. Reeder

Abstract

This paper presents a review of theoretical and observational studies relating to the low-level structure of cold fronts and explores the factors that are pertinent to frontal motion.

Observational studies have shown that, in some cases, surface cold fronts move at speeds faster than the normal component of the wind at all levels in the lower troposphere and therefore propagate. Other case studies have shown that the low-level flow immediately behind the front and normal to it is faster than the front and that the front has the local structure of a gravity current, its speed of movement being well determined by the gravity current speed equation. Them different types of behavior are related to results of recent theoretical studies, and the mechanism by which fronts can propagate is elucidated. It is shown that a necessary requirement for propagation is the existence of an alongfront temperature gradient.

We question the relevance of the gravity current speed equation in general, despite its apparent accuracy in some observed fronts, and note that it cannot be applied to the cold fronts simulated in simple frontogenesis models. The applicability of other simple frontal models providing estimates for the frontal speed is critically reviewed also.

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Michael J. Reeder and Roger K. Smith

Abstract

We examine air parcel trajectories in the two-dimensional model for a cold front by Reeder and Smith. These are found to be in close agreement with trajectories deduced from analyses of summertime “cool changes” in southeastern Australia, adding further support to the applicability of the numerical model to this kind of cold front. The favorable comparison points also to the dynamical consistency of the conceptual model for the cool change, which has evolved from the analysis of data from observational experiments.

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Roger K. Smith and Julie A. Noonan

Abstract

Thermally forced atmospheric circulations over the Gulf of Carpentaria region of northeastern Australia are investigated using a mesoscale numerical model. The region is renown for the common occurrence of long westward-moving convective- and wave-cloud lines, including the celebrated “morning glory” phenomenon. In the model, it is found that for uniform flows over the region ranging from northeasterly to southeasterly, westward-moving, low-level convergence lines develop over the gulf during the night and early morning. The authors suggest that similar convergence lines in the atmosphere are responsible for the initiation and maintenance of the observed cloud lines. For northeasterly and easterly flow, the convergence lines show little day-to-day variation, despite the relatively long inertial period in the region, which is nearly two days. The calculations, which extend an earlier study by the same authors, lead to a new hypothesis to account for the observed longevity of morning glory bore waves. They provide also an explanation for the marked diurnal oscillation in the low- level easterly flow observed at Weipa during a field experiment to investigate the so-called north Australian cloud line.

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Jeffrey D. Kepert and Roger K. Smith

Abstract

The Australian west coast trough forms near the west coast in the easterly flow over Australia in the warmer months of the year. Its development and movement is the major synoptic influence on the weather in those months, particularly in the production of extreme maximum temperatures and subsequent “cool changes.” This paper begins with a brief discussion of the climatology of the trough, followed by a case study. The main focus is on the development and interpretation of a simple dynamical model of the trough. The model is an adaptation of the linear diabatic equatorial β-plane models of Matsuno and of Gill for the particular situation of the trough. The study leads to a new hypothesis on the role of the sea breeze in the dynamics of the trough.

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