Mesoscale Modeling of a Cold Front and Its Interaction with a Diurnally Heated Land Mass

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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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