Modeling of Summertime Flow and Dispersion in the Coastal Terrain of Southeastern Australia

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.