Three‐dimensional baroclinic instability and summertime frontogenesis in the Australian region

Abstract

The structure of a mature three-dimensional baroclinic wave and its attendant cold front is examined using an idealized numerical model with a simple parametrization of dry buoyant convection resulting from diurnal heating. Relative-flow isentropic analyses of the numerical solution and of an observed Australian summertime frontal system are compared. It is shown that the model well represents the typical synoptic environment accompanying frontogenesis in the Australian region. A diagnosis of the three-dimensional vertical circulation is obtained using a technique involving a vector streamfunction. The circulation in a cross-sectional plane parallel with the low-level temperature gradient is found to be highly two-dimensional, with geostrophic confluence constituting the principal frontogenetical forcing at low levels, while at upper levels the frontogentical forcing is dominated by geostrophic horizontal shear. In a second cross-section, taken zonally, the frontogenetic forcing is found to consist almost solely of geostrophic horizontal shear throughout the entire troposphere. However, in this case the circulation in the plane of cross-section is found to be only qualitatively twodimensional, as a significant proportion of the low-level postfrontal subsidence is associated with circulation normal to the cross-section. It is shown that the frontal structure analysed is sensitive to the location of the chosen cross-sectional plane, and that this in turn provides an explanation for the observed relative shallowness of Australian summertime fronts. The effects of diurnal heating are important but confined to the surfacebased mixed layer. With the addition of diurnal heating, the prefrontal updraught amplifies almost threefold and the frontal temperature contrast over the heated region approximately doubles. The diabatic forcing term in the well-known Sawyer-Eliassen equation is shown to be negligible. This suggests that the increased lowlevel convergence is a reflection of the model's enhanced response to frontogenetic forcing in the presence of low static stability in the surface-based well-mixed layer. Finally, the numerical model solution is compared with a simpler two-dimensional model specifically developed to study Australian summertime fronts.

The intensity and motion of hybrid cyclones in the Australian region in a composite potential vorticity framework

“…It may be that the consistency between this study and previous idealized studies concerning the cyclone motion is specific to the baroclinic‐channel‐like flow conditions in the Southern Hemisphere. For example, it has been shown by Reeder et al () that baroclinic instability and frontogenesis described by a channel model agree remarkably well in detail with observations taken in the Australian region. Hence, an intriguing research opportunity may be to extend the analysis to the global scale and to elaborate regional differences concerning the dynamics and motion of HCs.…”

The meteorology of Black Saturday

“…The key ingredients are a surface low and the two adjacent broad anticyclones (Figure 4(a)), which together drive a low-level flow field over the southern part of the continent similar to the classic hyperbolic deformation field used in many of the theoretical descriptions of frontogenesis (e.g. see Reeder et al, 1991;Reeder and Smith, 1992;Muir and Reeder, 2010). As the cyclone approaches the heated continent, this pattern of deformation strengthens the temperature contrast between the continent and the ocean, culminating in a strong cold front.…”

Extratropical–Tropical Interaction during Onset of the Australian Monsoon: Reanalysis Diagnostics and Idealized Dry Simulations

“…The flow configuration used in this study is an extension of the classic three-dimensional channel model often used for idealized baroclinic instability and frontogenesis studies, the details of which are described in detail by Reeder et al (1991) and Tory (1997). The basic state follows that of Keyser et al (1989) with minor changes to take into account the Southern Hemisphere, spherical configuration using a latitudelongitude grid, a -coordinate system in the vertical, and an extension into the Tropics with the options of trade easterly flow, monsoon westerly flow, and a subtropical jet in the basic state.…”