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Tropical Cyclone Oswald (2013) is considered to be one of the highest-impact storms to make landfall in northern Australia even though it only reached a maximum category 1 intensity on the Australian category scale. After making landfall on the west coast of Cape York Peninsula, Oswald turned southward, and persisted for more than 7 days moving parallel to the coastline as far south as 308S. As one of the wettest tropical cyclones (TCs) in Australian history, the favorable configurations of a lower-latitude active monsoon trough and two consecutive midlatitude trough–jet systems generally contributed to the maintenance of the Oswald circulation over land and prolonged rainfall. As a result, Oswald produced widespread heavy rainfall along the east coast with three maximum centers near Weipa, Townsville, and Rockhampton, respectively. Using high-resolution WRF simulations, the mechanisms associated with TC Oswald’s rainfall are analyzed. The results show that the rainfall involved different rainfall mechanisms at each stage. The land–sea surface friction contrast, the vertical wind shear, and monsoon trough were mostly responsible for the intensity and location for the first heavy rainfall center on the Cape York Peninsula. The second torrential rainfall near Townsville was primarily a result of the local topography and land–sea frictional convergence in a conditionally unstable monsoonal environment with frictional convergence due to TC motion modulating some offshore rainfall. The third rainfall area was largely dominated by persistent high vertical wind shear forcing, favorable large-scale quasigeostrophic dynamic lifting from two midlatitude trough–jet systems, and mesoscale frontogenesis lifting. [ABSTRACT FROM AUTHOR]

Six weather types (WTs) are computed for tropical Australia during the wet season (November–March 1979–2015) using cluster analysis of 6-hourly low-level winds at 850 hPa. The WTs may be interpreted as a varying combination of at least five distinct phenomena operating at different time scales: the diurnal cycle, fast and recurrent atmospheric phenomena such as transient low pressure, the intraseasonal Madden–Julian oscillation, the annual cycle, and interannual variations mostly associated with El Niño–Southern Oscillation. The WTs are also strongly phase-locked onto the break/active phases of the monsoon; two WTs characterize mostly the trade-wind regime prevalent either at the start and the end of the monsoon or during its breaks, while three monsoonal WTs occur mostly during its core and active phases. The WT influence is strongest for the frequency of wet spells, while the influence on intensity varies according to the temporal aggregation of the rainfall. At hourly time scale, the climatological mean wet intensity tends to be near-constant in space and not systematically larger for the monsoonal WTs compared to other WTs. Nevertheless, one transitional WT, most prevalent around late November and characterized by weak synoptic forcings and overall drier conditions than the monsoonal WTs, is associated with an increased number of high hourly rainfall intensities for some stations, including for the interior of the Cape York Peninsula. When the temporal aggregation exceeds 6–12 h, the mean intensity tends to be larger for some of the monsoonal WTs, in association with more frequent and also slightly longer wet spells. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A high-resolution cloud model is used to explore in detail the generation of the morning glory, a low-level nonlinear atmospheric internal wave observed on the southwestern side of Cape York Peninsula (Australia). The model is two-dimensional and nonhydrostatic and simulates an east–west cross section of the southern part of Cape York Peninsula at a horizontal resolution of 200 m. Most of the numerical experiments are initialized at sunrise with a 5 m s-1 easterly flow and a sounding taken upstream from the peninsula. The sea breezes that develop over Cape York Peninsula are highly asymmetric with the east-coast sea breeze being both deeper and warmer than its western counterpart. When the sea breezes meet, the east-coast sea breeze rides over that from the west coast and in the process produces a series of waves that propagate on the west-coast sea breeze. The model calculations show that when the phase speed of these waves matches the westward propagation speed of the east-coast sea breeze, the waves grow to large amplitude, thus forming the morning glory. When the east-coast sea breeze propagates too fast relative to the waves, the waves do not amplify. In this sense the morning glory is generated by a resonant coupling between the east-coast sea breeze and the disturbances that propagate on the shallow stable layer produced by the west-coast sea breeze. The number of waves produced depends on the stability of the west-coast sea breeze and the strength of the east-coast sea breeze. These numerical experiments have for the first time explicitly modeled the generation of morning glory waves through the interaction of two sea breezes. The inclusion of orography representative of Cape York Peninsula does not change the overall result with a morning glory forming in much the same way as in the case without orography. The main difference is that the sea breezes meet earlier when orography is included. [ABSTRACT FROM AUTHOR]