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William L. Physick

Abstract

A mesoscale numerical model incorporating a detailed planetary boundary-layer scheme, including momentum, heat and moisture exchange with the lower boundary, is used to study the change in structure of a dry summertime front as it moves towards a coastline. Two-dimensional experiments show that the diurnal pressure fall over land causes a trough to form in the coastal region ahead of the approaching front. Frontogenesis at this mesoscale trough leads to the formation of a “new” front near the coast, giving the appearance of strong acceleration of the original frontal system in the offshore region. This process can occur over a wide range of times from early afternoon to late evening, but no such acceleration occurs for fronts crossing the coast in the early morning hours.

These results indicate a tendency for more fronts to cross the coastline in the afternoon-early evening period than at other times, a statistic which is observed in southeast Australia and Oregon, USA. They also imply fronts should align themselves more parallel to the coastline. This behavior is also found in Australia, and is confirmed here in a three-dimensional simulation.

The mechanisms controlling frontal movement are also investigated, and boundary-layer heating is identified as an important link between two types of fronts observed in the Australian region. When heated from below, a frontal system moving faster than the low-level winds behind it (termed a propagating system) develops local characteristics of an unsteady gravity current whereby the following winds over a limited region are faster than the frontal speed. In the context of the diurnal cycle, the front reverts to its propagating mode towards midnight, several hours after the cessation of convective heating around sunset.

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William L. Physick
and
Deborah J. Abbs

Abstract

The Latrobe Valley is situated in a coastal region of complex terrain in southeastern Australia. During typical summertime conditions of light synoptic winds and clear skies, the low-level regional wind field is dominated by sea-breeze and slope-wind circulations. Westerly winds at heights between 1500 and 3000 m are observed to move progressed down to the surface during the night and early morning hours. This unusual feature of the diurnal wind cycle occurs when the synoptic pressure gradient up to 700 hPa is weak, producing a 3–4 m s−1 geostrophic wind from the northeast.

A three-dimensional mesoscale model with detailed boundary-layer physics is employed to examine this behavior. Good agreement with observed winds can only be obtained when a negligible synoptic-scale wind is specified. Laboratory experiments and use of a nested model with a larger outer domain show that this occurs because blocking of the northeasterly synoptic flow by upstream orography produces a stagnation region over the model domain.

By analyzing the contribution of various forcing terms in the equations of motion, the study shows that the upper-level westerly winds are newly in geostrophic balance with mesoscale pressure gradients created in the return flow region of the previous day's sea-breeze and upslope circulations. The counterclockwise turning of lower-level winds through the night is by no means a purely inertial rotation with significant contributions coming from frictional, nonlinear, and pressure-gradient forces.

The role of the sea breeze in the dispersion of plumes from industrial sources in the Latrobe Valley is investigated using winds and turbulence from the mesoscale model as input for a Lagrangian particle model. As the sea breeze moves up the valley, it replaces polluted air with clean marine air until it reaches the sources well inland in late afternoon. By this time, the lower layers of the atmosphere are beginning to stabilize and plume emissions are no longer mixed to the ground.

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William L. Physick
and
Deborah J. Abbs

Abstract

An analysis is carried out of summertime surface and upper-air wind and temperature data from the Latrobe Valley in southeastern Australia. An easterly sea breeze is found to regularly penetrate over 100 km up the east-west-oriented valley, meeting a sea breeze from the south coat in late afternoon. The latter enters the valley over a saddle in the Strzelecki Ranges to the south. Over a 5-day period of steady synoptic flow, winds below 1500 m fluctuated between easterly and westerly with a diurnal period, while above this height up to 3000 m, the wind direction remained westerly. The westerly winds were particularly surprising, as the synoptic pressure charts showed a northeasterly pressure gradient over the period.

Power stations are located in the Latrobe Valley well inland from the coast, and findings from the wind-field analysis are used to examine the dispersion of plumes from these sources. It is concluded that the sea breezes replace polluted mixed-layer air with clean air as they penetrate up the valley, and that plume material is advected out of each end of the valley at upper levels overnight.

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