A Study of Frontal Dynamics with Application to the Australian Summertime “Cool Change”

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  • 1 Geophysical Fluid Dynamics Laboratory, Monash University, Clayton, Victoria, Australia 3168
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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.

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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