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K. J. Wilson and H. Stern

Abstract

In this, the first of a series of three papers on the structure of be summertime cool change of southeastern Australia, the prevailing climatological conditions are established, a background given to each of the cool changes studied and an assessment made of the representativeness of the systems sampled.

A conceptual model of the kinematics of the cool change is presented and is similar to that proposed previously for cold fronts in the Northern Hemisphere. The model includes prefrontal warm and cool ascending “conveyor belts” and subsiding postfrontal streams. The warm conveyor belt was found to have a relatively low moisture content, a consequence of continental effects, and significant recirculation of conveyor belt air into the postfrontal region was observed. Typical vorticity distributions are presented and the applicability of quasi-geostrophic diagnostic techniques to the cool change examined. Geostrophically forced frontogenesis was found to be organized on the subsynoptic scale, was often confined to shallow near-surface layers, and some shallow cool changes appeared only weakly or indirectly coupled to upper tropospheric troughs.

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B. F. Ryan and K. J. Wilson

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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B. F. Ryan, K. J. Wilson, and E. J. Zipser

Abstract

The evolution of an oceanic prefrontal subcloud layer of continental origin was examined by analyzing data gathered on the subsynoptic and mesoscale during Phase III of the Australian Cold Fronts Research Program. Characteristics of the atmosphere ahead of the surface cold front included deep ascent through the troposphere and horizontal low-level warm air advection. Temporal and spatial variation in the thermodynamic structure of the 3.5-km thick prefrontal subcloud layer confirmed that significant local cooling was occurring despite the presence of horizontal warm air advection. Over a 24-hour period this cooling created a new low-level baroclinic zone several hundred kilometers ahead of the front.

Observations from surface stations, rawinsodes, radars, satellites, and research aircraft were combined to demonstrate that the observed local cooling in the prefrontal air mass was most plausibly due to evaporation of precipitation. The region in question contained mesocale bands of precipitation, including convective showers that generated strong downdrafts. The observed mesoscale features are consistent with those obtained in a numerical modelling study of the generation of similar prefrontal rainbands, and appear to account for the observed modification of the baroclinic structure of the frontal zone.

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J. F. Turner, J. C. Iliffe, M. K. Ziebart, C. Wilson, and K. J. Horsburgh

Abstract

As part of the U.K. Hydrographic Office (UKHO)-sponsored Vertical Offshore Reference Frames (VORF) project, a high-resolution model of lowest astronomical tide (LAT) with respect to mean sea level has been developed for U.K.–Irish waters. In offshore areas the model relies on data from satellite altimetry, while in coastal areas data from a 3.5-km-resolution hydrodynamic tide-surge model and tide gauges have been included. To provide for a smooth surface and predict tidal levels in unobserved areas, the data have been merged and interpolated using the thin plate spline method, which has been appropriately tuned by an empirical prediction test whereby observed values at tide gauges were removed from the solution space and surrounding data used to predict its behavior. To allow for the complex coastal morphology, a sea distance function has been implemented within the data weighting, which is shown to significantly enhance the solution. The tuning process allows for independent validation giving a standard error of the resulting surface of 0.2 m for areas with no tidal observations.

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K. J. Wilson, S. L. Barrell, and R. Del Beato

Abstract

An approach to verifying position forecasts of line phenomena was developed. The technique, which tests for occurrence/nonoccurrence and the time of arrival of line phenomena, was applied to frontal position prognoses over southeastern Australia. It was found that operational manual prognoses exhibited no skill in cases of frontogenesis or total frontolysis. In other cases, accuracy varied seasonally and geographically, with summer apparently being the most difficult period for frontal prognosis. Frontal position forecasts were successful for 70 percent of frontal crossings in the 27-month trial period, but two of every five forecast events were false alarms. The Hanssen and Kuipers skill discriminant for a 24-hour Yes/No forecast of a frontal crossing was new 0.6.

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J. D. Wilson, T. K. Flesch, and P. Bourdin

Abstract

In reference to previously observed concentrations of methane released from a source enclosed by a windbreak, this paper examines a refined “inverse dispersion” approach for estimating the rate of emission Q from a small ground-level source, when the surface-layer winds near that source are highly disturbed. The inverse dispersion method under investigation is based on simulation of turbulent trajectories between sources and detectors, using a Lagrangian stochastic (LS) model. At issue is whether it is advantageous to recognize the flow as being disturbed and use a computed approximation to that disturbed flow to drive a fully three-dimensional LS model (3D-LS), or whether it suffices to ignore flow disturbance and adopt an LS model attuned to the horizontally homogeneous upwind flow (MO-LS, as Monin–Obukhov similarity theory describes the vertical inhomogeneity). It is demonstrated that both approaches estimate the source strength to within a factor of 2 of the true value, irrespectively of the location of the concentration measurement, and moreover that both approaches estimate the source strength correctly (to within the experimental uncertainty), when based on concentrations measured far away from the immediate influence of obstacles in the flow. However, if the concentration detector is positioned close to the flow-disturbing obstacles, then inverse dispersion based on 3D-LS provides a better estimate of source strength than does MO-LS.

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B. N. Hanstrum, K. J. Wilson, and S. L. Barrell

Abstract

A climatology of the prefrontal westerly trough of southern Australia is presented, based on data for the 10-yr period 1976–85. Trough formation was confined to the longitudes of the Australian continent in the warmer months of the year. An average of approximately 15 troughs/yr were observed, evenly distributed each month from September to April, with appreciable interannual variability in incidence. Troughs formed in advance of a major Southern Ocean cold front in a region of differential thermal advection. Genesis occurred primarily over the west of the continent in spring and autumn, but over southeastern Australia in summer.

Variability in the location of formation at different times of year was related to the seasonal shift of the subtropical ridge over the continent, the shape of the southern continental coastline, and periods of low zonal-index flow. A class of troughs prone to rapid intensification after formation was also identified.

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B. N. Hanstrum, K. J. Wilson, and S. L. Barrell

Abstract

A case study of frontogenesis within a surface prefrontal trough over southern Australia is presented. The trough developed ahead of a surface cold front and, over a period of approximately 24 h, intensified into a mature summertime frontal system while the original front underwent total frontolysis. Two-hourly rawinsonde ascents at three locations were used to examine the structure of the trough both before and after frontogenesis. Diagnosis of the frontogenetic tendencies showed that horizontal deformation of the potential temperature field was the main forcing mechanism.

The process studied occurs frequently in the warmer months of the year, and a conceptual model is proposed for the process of frontogenesis within the trough and frontolysis of the preexisting midlatitude front. Important components include differential thermal advection, the action of Coriolis turning in association with ageostrophic vertical circulations, and horizontal deformation of the thermal field.

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T. K. Flesch, J. D. Wilson, and L. A. Harper

Abstract

Inverse-dispersion techniques allow inference of a gas emission rate Q from measured air concentration. In “ideal surface layer problems,” where Monin–Obukhov similarity theory (MOST) describes the winds transporting the gas, the application of the technique can be straightforward. This study examines the accuracy of an ideal MOST-based inference, but in a nonideal setting. From a 6 m × 6 m synthetic area source surrounded by a 20 m × 20 m square border of a windbreak fence (1.25 m tall), Q is estimated. Open-path lasers gave line-averaged concentration CL at positions downwind of the source, and an idealized backward Lagrangian stochastic (bLS) dispersion model was used to infer Q bLS. Despite the disturbance of the mean wind and turbulence caused by the fence, the Q bLS estimates were accurate when ambient winds (measured upwind of the plot) were assumed in the bLS model. In the worst cases, with CL measured adjacent to a plot fence, Q bLS overestimated Q by an average of 50%. However, if these near-fence locations are eliminated, Q bLS averaged within 2% of the true Q over 61 fifteen-minute observations (with a standard deviation σ Q/Q = 0.20). Poorer accuracy occurred when in-plot wind measurements were used in the bLS model. The results show that when an inverse-dispersion technique is applied to disturbed flows without accounting for the disturbance, the outcome may still be of acceptable accuracy if judgment is applied in the placement of the concentration detector.

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B. F. Ryan, K. J. Wilson, J. R. Garratt, and R. K. Smith

Following the analysis of data collected during Phases I and II of the Cold Fronts Research Programme (CFRP) a conceptual model for the Australian summertime “cool change” has been proposed. The model provides a focus and a framework for the design of Phase III.

The model is based on data gathered from a mesoscale network centered on Mount Gambier, South Australia, and includes the coastal waters to the west and relatively flat terrain to the east. The first objective of Phase III is to generalize the model so that it is applicable to the ocean waters to the far west of Mount Gambier and to the more rugged terrain farther to the east in the vicinity of Melbourne, Victoria. The remaining objectives concentrate on resolving unsatisfactory aspects of the model such as the evolution of convective lines and the relationship between the surface cold front and the upper-tropospheric cold pool and its associated jet stream.

The integrated nature of the Cold Fronts Research Programme has meant that it has stimulated a wide range of research activities that extend beyond the field observations. The associated investigations include climatological, theoretical, and numerical modeling studies.

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