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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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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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Gerald L. Thomsen
and
Roger K. Smith

Abstract

The importance of the boundary layer parameterization in the numerical prediction of low-level convergence lines over northeastern Australia is investigated. High-resolution simulations of convergence lines observed in one event during the 2002 Gulf Lines Experiment are carried out using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Calculations using five different parameterizations are compared with observations to determine the optimum scheme for capturing these lines. The schemes that give the best agreement with the observations are the three that include a representation of countergradient fluxes and a surface layer scheme based on Monin–Obukhov theory. One of these, the Medium-Range Forecast scheme, is slightly better than the other two, based on its ability to predict the surface pressure distribution. The findings are important for the design of mesoscale forecasting systems for the arid regions of Australia and elsewhere.

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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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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
Michael T. Montgomery

Abstract

The authors review an emerging paradigm of tropical cyclone intensification in the context of the prototype intensification problem, which relates to the spinup of a preexisting vortex near tropical storm strength in a quiescent environment. In addition, the authors review briefly what is known about tropical cyclone intensification in the presence of vertical wind shear. The authors go on to examine two recent lines of research that seem to offer very different views to understanding the intensification problem. The first of these proposes a mechanism to explain rapid intensification in terms of surface pressure falls in association with upper-level warming accompanying outbreaks of deep convection. The second line of research explores the relationship between the contraction of the radius of maximum tangential wind and intensification in the classical axisymmetric convective ring model, albeit in an unbalanced framework. The authors challenge a finding of the second line of research that appears to cast doubt on a recently suggested mechanism for the spinup of maximum tangential wind speed in the boundary layer—a feature seen in observations. In doing so, the authors recommend some minimum requirements for a satisfactory explanation of tropical cyclone intensification.

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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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Michael J. Reeder
and
Roger K. Smith

Abstract

The dynamics of frontal evolution is investigated in the context of the Australian summertime cool change using a two-dimensional numerical model. The model is essentially the same as that used by Reeder and Smith, but with different initial conditions and with an open (rather than periodic) flow domain. The initial conditions are an idealization of cross-sections through a typical preexisting (finite amplitude) disturbance prior to its amplification to an intense front-trough system. Basically, they consist of a warm prefrontal northerly airstream embedded in a zonal shear flow in thermal wind balance.

The model develops a quasi-steady surface cold front during the 24 hours (real time) of integration and this front is shown to have many features in common with a kinematic model of the Australian summertime cool change. The latter model was synthesized from observational data on surface cold fronts obtained during the Phase I and II field experiments of the Australian Cold Fronts Research Programme. Significant features of the model simulation are the development of a postfrontal low-level southerly airstream and the fad that low-level winds normal to the front are everywhere slower than the speed of the front: i.e., there is no region of “advected relative-flow” towards the front. Good agreement is found, including these features, between the simulated low-level wind and thermal fields and those of the kinematic model. Thus, our study provides a dynamics foundation for the kinematic model.

The model simulation and kinematic model are compared also with a 24 hour prediction of the “Ash Wednesday” cold front of 16 February 1983 using the ANMRC three-dimensional nested-grid model. This front was a classic example of a summertime cool change in southeastern Australia. Broad agreement is found between the models, provided the comparison is made south of Tasmania where the Ash Wednesday front appears to be more nearly two-dimensional.

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Julie A. Noonan
and
Roger K. Smith

Abstract

A numerical study of sea-breeze circulations over Cape York Peninsula in northern Australia is presented. Simulations using a two-dimensional version of the University of Virginia mesoscale model provide insight into the thermally induced circulations over the peninsula during the “dry” season under typical conditions of a prevailing easterly geostrophic flow. Calculations are performed for a wide southern cross section of the peninsula (400 km of land) and a narrow northern section (200 km of land), with terrain appropriate to these cross sections included. A southern section simulation without topography is discussed for comparison.

Particular interest is focused on the collision of the east and west coast sea breezes. The possibility is explored that the double sea-breeze convergence may act as a trigger, not only for morning glory cloud line disturbances in the southern Gulf, but also for the so-called North Australian Cloud Line (NACL). The latter is a line of convective cloud which forms frequently in the early evening on the western side of the peninsula in the north and subsequently moves westwards across the Gulf, and sometimes even across the “Top-End” of Australia, retaining its identity for up to two days. It is an important feature of the weather in the north Australian tropics. The model calculations show that with a typical easterly geostrophic flow of 5 m s−1, the location and timing of the collision of the sea breezes in the northern section correspond well with those for the onset of the NACL. However, in the absence of moist processes, the disturbance arising out of the collision decays within a few hours. In the southern section, collision occurs in the late evening and leads to the formation of a westward traveling bore wave in the lower atmosphere. The bore continues to propagate throughout the night with little diminution in amplitude and is believed to be the model-analogue of the morning glory disturbance. However, if the easterly geostrophic flow is increased to 8 m s−1, collision of the sea breezes occurs in the early evening and again the resulting disturbance decays within a few hours. The significance of these results in regard to morning glory formation is discussed.

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