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A. J. Troup and N. A. Streten

Abstract

A classification scheme for cloud vortices recorded on hemispheric digital mosaics southward of 20S during a period of some 550 days is used to construct models based on observations of associated atmospheric structure. Such models for various vortex types are expressed in terms of anomaly patterns of surface pressure, and 500- and 300-mb geopotential. Hopefully such models may find application in the development of analysis techniques over the data-void regions of the hemisphere.

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C. H. B. Priestley and A. J. Troup

Abstract

No abstract available.

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J. R. Garratt, W. L. Physick, R. K. Smith, and A. J. Troup

Abstract

Observations of four cold-frontal systems traversing the coastal region of southeast Australia in late spring and early summer are described in terms of process occurring on the mesoscale. A conceptual model is presented which summarizes the main results of the data analysis. Features found in common with other studies of cold fronts include:

(i) the multiple-line nature of the frontal transition zone (FTZ);

(ii) concentration of cyclonic relative vorticity at a height z≈1 to 1.5 km in the rear of the FTZ; and

(iii) the existence of a prefrontal jet at z≈1.5 km, northerly in our case, southerly in the Northern Hemisphere.

The change lines within the FTZ (and at the leading edge if there is no sea breeze) are most probably convective instability lines whose alignment and movement depend on the large-scale, cloud-layer winds. The lines are evident as mesoscale cloud bands from satellite imagery and as rainbands from radar. At least one of these develops into a vigorous squall line whose cold outflow produces a pressure jump, and related wind-shift line. Movement of the pressure-jump line depends both on the gravity-current nature of the cold outflow and the environmental wind field. The squall line and pressure-jump line are associated with mesoscale high and low pressure features to which the boundary-layer wind field responds.

The structure of the FTZ up to z=2 km appears to be dominated by the presence of the squall line, with upwards motion ahead and downwards behind. On a horizontal scale of 100 km, cyclonic vorticity reaches twice the Coriolis parameter f in the vicinity of the squall line. Frontogenesis occurs largely within the FTZ with horizontal convergence and deformation processes being of comparable importance.

The prefrontal jet is broadly in thermal wind balance with the horizontal temperature gradient which is, itself, determined by the fact that prefrontal air closest to the FTZ originates farther to the north and is therefore hotter than prefrontal air more distant from the zone.

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W. L. Physick, W. K. Downey, A. J. Troup, B. F. Ryan, and P. J. Meighen

Abstract

Phase I of the Cold Fronts Research Programme was carried out during November/December 1980 in south-eastern Australia. Data from a frontal event on 27 November are analyzed, with particular emphasis on a squall line that formed over the Southern Ocean and moved across the predominantly land-based network.

Potential instability between 750 and 550 mb, arising from the subsynoptic scale circulation associated with the frontal system, existed ahead of the squall fine. Boundary layer forcing resulting from convergence of the cold outflow and presquall air was responsible for release of this instability as the line passed through the observing network.

The lowest 200 m of the nocturnal boundary layer were undisturbed by the cold outflow, as this radiatively cooled layer was potentially colder than downdraft air. A surface pressure jump of 3 mb was recorded and the low-level wind rotated through 360° in two hours in response to the associated mesohigh.

Two-dimensional (xz) cross sections of wind and thermodynamic variables composited from aircraft, radiosonde and pibal data reveal centre of upward motion at 800 mb near the leading edge of the outflow and 50 km behind at 500 mb, with the latter being the stronger of the two. Cooling extends to 620 mb and the important role played by relatively dry air entering the system from the rear at middle levels is clearly shown.

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R. K. Smith, B. F. Ryan, A. J. Troup, and K. J. Wilson

Much of the significant weather of southeastern Australia is associated with the passage of cold fronts. In summer, such passages are often accompanied by rapid and extreme temperature falls, as hot continental northerly winds are replaced with much colder southwesterlies from the Southern Ocean; for this reason, they are popularly and aptly known as “cool changes.” These summertime fronts, which normally form part of a front-trough complex sandwiched between two anticyclones, are ill-understood and lead to many forecasting problems. In early 1979, a Cold Fronts Research Programme was established as a coordinated long-term project to study front-trough systems affecting this region of Australia. The program, which involves all of the major Australian meteorological centers, has been designed to include three observational phases over five years, with emphasis being placed on summertime frontal systems. Each phase of intensive observations is of four weeks duration, and Phases I and II have now been completed. This article summarizes the philosophy behind the program, outlines its scientific objectives, and describes the observational networks employed. A brief review of the results of Phases I and II and an outline of future activities also is presented.

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