The Australian Summertime Cool Change. Part III: Subsynoptic and Mesoscale Model

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  • 1 Cloud Physics Laboratory, Division of Atmospheric Research, CSIRO, Sydney, Australia
  • | 2 Bureau of Meteorology, Melbourne, Victoria 3001
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Abstract

Observations of the Australian summertime cool change have ranged over both the subsynoptic scale (200–2000 km) and the mesoscale (20–200 km). In this paper, the final in a series of three, a conceptual subsynoptic and mesoscale model is developed. The model suggests that such features as

(i) the speed of movement of the surface cold front,

(ii) the inflow of moisture into the frontal transition zone, and

(iii) the midtropospheric wind field associated with the frontal transition zone

are determined largely by the synoptic scale flow. Mesoscale controlled processes occur within the frontal transition zone.

The model is consistent with models developed for the Northern Hemisphere. The significant difference between the subsynoptic and mesoscale models appropriate to the United Kingdom and the Pacific Northwest of the United States and the summertime cool change is the lack of low-level moisture in the Australian situation. The model highlights the influence of the hot, dry Australian continent on the development of fronts.

The application of the model to forecasting in the Australian region is explored. In particular, it provides a systematic framework for analyzing the weather lines associated with the passage of the frontal transition zone. Further, the conceptual model suggests that the speed of the frontal transition zone should be able to be forecast with some skill by current operational numerical models. Finally, the model predicts the type of surface weather associated with the passage of the frontal transition zone.

Abstract

Observations of the Australian summertime cool change have ranged over both the subsynoptic scale (200–2000 km) and the mesoscale (20–200 km). In this paper, the final in a series of three, a conceptual subsynoptic and mesoscale model is developed. The model suggests that such features as

(i) the speed of movement of the surface cold front,

(ii) the inflow of moisture into the frontal transition zone, and

(iii) the midtropospheric wind field associated with the frontal transition zone

are determined largely by the synoptic scale flow. Mesoscale controlled processes occur within the frontal transition zone.

The model is consistent with models developed for the Northern Hemisphere. The significant difference between the subsynoptic and mesoscale models appropriate to the United Kingdom and the Pacific Northwest of the United States and the summertime cool change is the lack of low-level moisture in the Australian situation. The model highlights the influence of the hot, dry Australian continent on the development of fronts.

The application of the model to forecasting in the Australian region is explored. In particular, it provides a systematic framework for analyzing the weather lines associated with the passage of the frontal transition zone. Further, the conceptual model suggests that the speed of the frontal transition zone should be able to be forecast with some skill by current operational numerical models. Finally, the model predicts the type of surface weather associated with the passage of the frontal transition zone.

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