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Michael J. Reeder and Daniel Keyser

Abstract

The dynamics of frontogenesis at upper levels are investigated using a hierarchy of three numerical models. They are, in order of decreasing sophistication, the anelastic (AN), the geostrophic momentum (GM), and the quasi-geostrophic (QG) approximations to the full equations of motion. Each model is two-dimensional and assumes the same basic-state, which incorporates the frontogenetical mechanisms of confluence and horizontal shear. The dependence of the numerical solutions on the initial vertical shear of the cross-front component of the geostrophic wind, λ, and its associated along-front temperature gradient is examined in detail. For the values of λ chosen, the along-front temperature gradient is either zero (λ = 0) or such that cold air is advected along the upper front (λ < 0).

Intercomparison of the broad-scale structure of the upper-level jet–fronts as described by the AN and GM models shows close agreement. For zero or weak shears (λ = 0 s−1 or λ = −2 × 10−3 s−1), the solutions are essentially identical. Vertical shear in the cross-front geostrophic wind serves to increase the amplitude of the cross-front circulation and displace the subsiding branch toward the warmer air. In the cases of weak or zero shear, the dominant mechanism for generating vertical vorticity at upper levels is the stretching of preexisting vertical vorticity, whereas for stronger shear (λ = −5.741 × 10−3 s−1) the key process becomes the tilting of horizontal vorticity into the vertical by differential vertical motion. In contrast, the QG model exhibits marked differences with its AN and GM counterparts, which become even more pronounced as |λ| is increased. These differences are related largely to the neglect of vortex tilting in generating vertical vorticity in the OG model.

The GM and QG models assume cross-front thermal wind balance at all time. A posteriors examination of the numerical solutions shows this to be an excellent approximation when the vertical shear in the cross-front geostrophic wind is weak. For strong vertical shear of the cross-front geostrophic wind, the unbalanced along-front ageostrophic wind is proportional to the vertical advection of the cross-front velocity. Diagnoses of these simulations reveal thermal wind balance to be less well satisfied. It is shown that in contrast to the GM and QG models, wherein the along-front ageostrophic velocity is passive and thus cannot contribute to the evolution of the jet–front system, the unbalanced along-front flow contributes significantly to the dynamics as described by the AN model.

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Gareth Berry and Michael J. Reeder

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An objective method for the identification of the intertropical convergence zone (ITCZ) in gridded numerical weather prediction datasets is presented. This technique uses layer- and time-averaged winds in the lower troposphere to automatically detect the location of the ITCZ and is designed for use with datasets including operational forecasts and climate model output. The method is used to create a climatology of ITCZ properties from the Interim ECMWF Re-Analysis (ERA-Interim) dataset for the period 1979–2009 to serve as an indicator of the technique's ability and a benchmark for future comparisons. The automatically generated objective climatology closely matches the results from subjective studies, showing a seasonal cycle in which the oceanic ITCZ migrates meridionally and the land-based ITCZ features are predominantly summertime phenomena. Composites based on the phase of the El Niño–Southern Oscillation index show a major shift in the mean position and changes in intensity of the ITCZ in all ocean basins as the index varies. Under La Niña conditions, the ITCZ intensifies over the Maritime Continent and eastern Pacific, where the ITCZ weakens over the central and equatorial eastern Pacific. An analysis of changes in the ITCZ and its divergence during the period 1979–2009 indicates that the mean position of the ITCZ shifts southward in the western Pacific and a broad global intensification of the convergence into ITCZ regions. The relationship between tropical cyclogenesis and the ITCZ is also examined, finding that more than 50% of all tropical cyclones form within 600 km of the ITCZ.

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Gareth J. Berry and Michael J. Reeder

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The wet season of the Australian monsoon is characterized by subseasonal periods of excessively wet or dry conditions, commonly known as monsoon bursts and breaks. This study is concerned with the synoptic evolution prior to monsoon bursts, which are defined here by abrupt transitions of the area-averaged rainfall over the tropical parts of the Australian continent.

There is large variability in the number of monsoon bursts from year to year and in the time interval between consecutive monsoon bursts. Reanalysis data are used to construct a lag composite of the sequence of events prior to a monsoon burst. It is determined that a burst in the Australian monsoon is preceded by the development of a well-defined extratropical wave packet in the Indian Ocean, which propagates toward the Australian continent in the few days leading up to the onset of heavy rainfall in the tropics. As in previous studies on the monsoon onset, the extratropical disturbances propagate equatorward over the Australian continent. These extratropical systems are accompanied by lower-tropospheric airmass boundaries, which also propagate into low latitudes. Ahead of these boundaries, relatively warm moist air is advected from the surrounding oceans, locally increasing the convective available potential energy.

Commonly employed climate indices show that monsoon bursts are more likely to occur when the active phase of the Madden–Julian oscillation is in the vicinity of Australia. Neither El Niño–Southern Oscillation nor the southern annular mode has a significant impact on the occurrence of monsoon bursts.

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Sarah J. Arnup and Michael J. Reeder

Abstract

The diurnal and seasonal variations of the northern Australian dryline are examined by constructing climatologies of low-level dynamic and thermodynamic variables taken from the high-resolution Australian Bureau of Meteorology’s Limited Area Prediction Scheme (LAPS) forecasts from 2000 to 2003. The development of the dryline is analyzed within the framework of the frontogenesis function applied to the mixing ratio and the airstream diagnostics of Cohen and Schultz. A case study of 12–13 October 2002 illustrating the airmass boundaries over the Australian region is also examined. Daytime surface heating produces sea-breeze circulations around the coast and a large inland heat trough that extends east–west along northern Australia. At night, air parcels accelerate toward low pressure, increasing convergence and deformation within the heat trough. This sharpens the moisture gradient across the tropical and continental airmass boundary into a dryline. This is different than the dryline of the Great Plains in the United States, which generally weakens overnight. The Australian dryline is strongest in spring just poleward of the Gulf of Carpentaria, where the moisture gradient across the heat trough is enhanced by the coast, and the axis of dilatation is closely aligned with mixing ratio isopleths. The dryline is weakest in winter, when the heat trough is weak. The LAPS 3-h forecasts are in good agreement with observations obtained from the Automatic Weather Station network. The 3-h forecasts capture the observed diurnal and seasonal cycle of the airmass boundaries. However, the sea-breeze circulation and ageostrophic flow into the surface heat trough is limited by the model resolution. The LAPS 3-h forecasts may therefore underestimate the nocturnal intensification of the dryline, especially since the inland moisture content is overestimated.

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Daniel Keyser, Michael J. Reeder, and Richard J. Reed

Abstract

Petterssen' frontogenesis equation relates the Lagrangian rate of change of the magnitude of the horizontal potential temperature gradient, referred to as the frontogenesis function, to invariant kinematic properties of the horizontal velocity field. It is not uncommon in synoptic practice to infer the presence of vertical circulations in frontal regions from the spatial distribution of the scalar frontogenesis function. On the other hand, Hoskins and collaborators have introduced a form of the quasi-geostrophic omega equation in which the dynamical and forcing is proportional to the horizontal divergence of the so-called Q vector. The Q vector is defined as the Lagrangian rate of change following the geostrophic flow of the vector horizontal potential temperature gradient. The Q-vector formalism motivates us to generalize the Petterssen frontogenesis function to apply to the vector horizontal potential temperature gradient. This generalization, referred to as the vector frontogenesis function, consists of introducing an expression for the Lagrangian rate of change of direction of the horizontal potential temperature gradient.

In order to investigate quantitatively the relative importance of the magnitude and direction contributions to the vector frontogenesis function, we consider three analytical examples. These examples describe the evolution of a potential temperature field represented initially by a linear band of isentropes situated within specified horizontal wind fields that are nondivergent and steady state. The wind fields respectively are a hyperbolic streamline pattern characterized by pure deformation, a meridional wind field varying only in the zonal direction, and an axisymmetric vortex. In each of these examples, it is found that the Lagrangian rates of change of the magnitude and direction of the potential temperature gradient are comparable. In order to explore the dynamical implications of this finding, we separate the Q-vector forcing into contributions consisting of the magnitude and direction components of the vector frontogenesis function. The outcome of this partitioning suggests a possible dynamical basis for isolating vertical circulations associated with frontal zones in three-dimensional baroclinic disturbances: the frontal circulation is related to the magnitude component of the Q vector, whereas the background circulation (that associated with the baroclinic disturbance) is related to the direction component. Consequently, the proposed partitioning of the Q vector appears to lend dynamical support to adopting the scalar frontogenesis function as a qualitative indicator of frontal circulations, provided that these circulations are understood to constitute only a component of the total vertical motion field.

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Roger K. Smith and Michael J. Reeder

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This paper presents a review of theoretical and observational studies relating to the low-level structure of cold fronts and explores the factors that are pertinent to frontal motion.

Observational studies have shown that, in some cases, surface cold fronts move at speeds faster than the normal component of the wind at all levels in the lower troposphere and therefore propagate. Other case studies have shown that the low-level flow immediately behind the front and normal to it is faster than the front and that the front has the local structure of a gravity current, its speed of movement being well determined by the gravity current speed equation. Them different types of behavior are related to results of recent theoretical studies, and the mechanism by which fronts can propagate is elucidated. It is shown that a necessary requirement for propagation is the existence of an alongfront temperature gradient.

We question the relevance of the gravity current speed equation in general, despite its apparent accuracy in some observed fronts, and note that it cannot be applied to the cold fronts simulated in simple frontogenesis models. The applicability of other simple frontal models providing estimates for the frontal speed is critically reviewed also.

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Mick Pope, Christian Jakob, and Michael J. Reeder

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A cluster analysis is applied to the mesoscale convective systems (MCSs) that developed in northern Australia and the surrounding oceans during six wet seasons (September–April) from 1995/96 to 2000/01. During this period, 13 585 MCSs were identified and tracked using an infrared channel (IR1) on the Japanese Meteorological Agency Geostationary Meteorological Satellite 5 (GMS5). Based on the lifetimes of the MCSs, the area covered by cloud, the expansion rate of the cloud, the minimum cloud-top temperature, and their zonal direction of propagation, the MCSs are grouped objectively into four classes. One of the strengths of the analysis is that it objectively condenses a large dataset into a small number of classes, each with its own physical characteristics.

MCSs in class 1 (short) are relatively short lived, with 95% having lifetimes less than 5 h, and they are found most frequently over the oceans during the early and late parts of the wet season. MCSs in classes 2 and 3 [long and intermediate west (Int-West)] are longer lived and propagate to the west, developing over continental northwest Australia in deep easterly flow during breaks in the monsoon. These two classes are distinguished principally by their lifetime, with 95% of MCSs in the long class having lifetimes exceeding 4 h. Class 4 (Int-East) comprises MCSs that form over the subtropical latitudes of eastern Australia and in the deep westerly flow over northern parts of the continent during the monsoon and active phases of the MJO.

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Michael J. Reeder and Roger K. Smith

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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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Mick Pope, Christian Jakob, and Michael J. Reeder
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