Eastward-Propagating Undular Bores over Cape York Peninsula

The existence of eastward-propagating bores over Cape York Peninsula is shown from data obtained during the Gulf Lines Experiment (GLEX) conducted during September and October 2002 and from numerical modeling experiments. The disturbances were detected regularly at two stations, observable on 24 days during the 40-day experiment. The passage of a typical disturbance exhibits a sudden increase in pressure of around 1 hPa, often accompanied by undulations, and a change in the wind speed and direction from an easterly to a westerly flow. Disturbances were not observed during days of strong easterly flow. A two-dimensional nonhydrostatic mesoscale model is used to examine the formation of these disturbances. It is shown that the west coast sea breeze is shallow and does not penetrate far inland because of the opposing low-level easterly flow. In contrast, the east coast sea breeze is deeper and is less stable because it is modified by daytime convective mixing as it crosses the peninsula. As the east coast sea breeze overrides the west coast sea breeze, the west coast sea breeze produces an eastward-propagating bore on the stable layer laid down by the east coast sea breeze. About 2 h after generation, the bore becomes undular. These eastward-propagating disturbances are shown to be associated with the westward-propagating north Australian cloud line and the northeasterly morning glory. In addition, it is shown here that an undular bore can be formed when cold-air downdrafts from afternoon deep convection enter the stable layer created by the east coast sea breeze. Four events from GLEX are believed to have been formed in this way. The eastward-propagating disturbances produced in this way are accompanied neither by the north Australian cloud line nor by the northeasterly morning glory.