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Deborah J. Abbs

Abstract

Observations and numerical modeling of the bay and ocean breezes of Port Phillip Bay show that the interaction of these two breezes produces features undocumented in previous sea-breeze studies. The first of these is the formation of a mesoscale cyclonic eddy over the northern half of Port Phillip Bay. This eddy is due to enhanced convergence resulting-from the interaction of the bay and ocean breezes. The occurrence and position of this eddy is dependent on the strength and direction of the prevailing synoptic flow. The modeling studies show that the surrounding orography contributed to this enhanced convergence by channeling the low-level flow. It has also been found that at times the bay and ocean breeze coexist over the northern half of Port Phillip Bay. Under these conditions, the warmer, land-modified ocean breeze overlies the cool, shallow bay breeze. This is due to ground-based warming of the ocean breeze as it penetrates inland. The numerical results were in close agreement with the observations.

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Deborah J. Abbs and Jørgen B. Jensen

Abstract

A nonhydrostatic mesoscale model is used to simulate the dynamics and microphysics of postfrontal flow in the mountainous region of southeastern Australia. The aim of the paper is to determine if it is possible to use 2D models to simulate the characteristics of the liquid water field upstream from Baw Baw Plateau under postfrontal conditions. Results from both 2D and 3D simulations are compared with aircraft and surface observations taken during the Australian Winter Storms Experiment I, conducted during July and August 1988. The observations and both the 2D and 3D simulations show that under postfrontal conditions, the main feature of the flow is a series of standing lee waves downstream from Baw Baw Plateau. The microphysical fields are characterized by a cap cloud over Baw Baw Plateau and a region of high liquid water content extending at least 50 km upstream from the plateau. Convective elements form upstream from the plateau and are subsequently advected to the northeast. As the convective elements cross Baw Baw Plateau, they precipitate and subsequently evaporate in the drier subsidence region to the lee of the plateau. The main features of the airflow and cloud fields are well simulated by the 2D model runs; however, the 2D runs overestimate the precipitation amounts as compared with the surface observations and the 3D model results.

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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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Deborah J. Abbs and Roger A. Pielke

Abstract

Numerical model simulations have been performed with the Colorado State University mesoscale model to determine the regions of most likely occurrence of first cumulonimbus activity. It is shown that during the day, convergence along the eastern slopes of the Continental Divide and along the Cheyenne Ridge and Palmer Lake Divide coincides with the regions of most moist and unstable air. During the evening the flow reverses and the main convergence is located in the Platte River Valley. The interaction of these features with the nocturnal jet and Denver convergence/vorticity zone is also discussed.

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