Tropical Cyclone Maintenance and Intensification Event (TCMI)

Description

Tropical cyclones are known to be powered by large heat fluxes from the underlying ocean and when they make landfall, they are commonly expected to lose energy. However, some systems maintain strength or even intensify over land surfaces, and are known as tropical cyclone or intensification (TSMI) events. One possible cause is the release of sensible latent heat derived from wet soils. Such systems can occur in the United States and China, but the most conducive environment is Australia.

Although the underlying soil in Northern Australia is desert sand, some tropical cyclones have been observed to reintensify when they make landfall, often reacquiring classical inner-core structure, including eyes. Such redeveloped warm-core cyclones are commonplace in the remote desert areas of northern Australia, where they are called “agukabams” from the aboriginal word roots “agu,” meaning land, and “kabam,” meaning storm.

In terms of causes, Australia seems to be unique because this occurs over desert regions. It’s believed that the sensible heat from the hot soil is the major energy source with tropical cyclone rain water improving the heat conductivity of the soil. This allows heat stored below to be transferred more readily to the surface.

Causal Factors

Simulations carried out by researchers at the Massachusetts Institute of Technology (MIT) and the Brisbane, Australia-based Bureau of Meteorology, suggest that warm-core cyclones, moving inland over northern Australia, can indeed intensify when the underlying soil is sufficiently warm and wet and are maintained by heat transfer from the soil. The simulations also suggest that when the storms are sufficiently isolated from their oceanic source of moisture, the rainfall they produce is insufficient to keep the soil wet enough to transfer significant quantities of heat, and the storms then decay rapidly.

In order for the warm core cyclone effect to take place, three land conditions must be met:

  • the ground must be very warm and the atmosphere must have minimal temperature variation.
  • The ground must be very wet or saturated and so this land 'mimics' the sea - and hence the name 'brown ocean effect.' The high amount of moisture in the soil then allows a high rate of evaporation, which then acts as a source of heat energy for the storm or cyclone, technically known as latent heat - similar to the process over the sea.
  • The amount of this latent heat must be at a certain level - at least 70 watts per square metre. Over the sea it would normally be around 200 watts per square metre. Studies have found that the latent heat released must measure at least 70 watts averaged per square meter and concluded that latent surface heat flux from land surfaces actually have the potential to be larger than from the ocean, albeit for brief periods only.

These conditions were met in Texas and Oklahoma in June 2015. In early summer 2015, the ground had warmed up enough, and it was saturated following a record wet May. The latent heat had also reached or exceeded the required level.

Tropical Storm Bill (2015)

The second named storm of the season, Tropical Storm Bill developed from a broad area of low pressure over the northwestern Gulf of Mexico on 16 June. Initially continuing northwestward, Bill re-curved west-northwestward later on 16 June and peaked with maximum sustained winds of 60 mph (95 km/h). Just under five hours later, Bill made landfall along the Texas coast on Matagorda Island at the same intensity. The cyclone weakened to a tropical depression and turned northward early on 17 June. However, due to brown ocean effect, Bill remained a tropical cyclone until late on 18 June, when it degenerated into a remnant low. The brown ocean effect enabled Bill to dump large amounts of rain onto Texas and Oklahoma, much more than would normally be expected from this kind of tropical storm over land.

Tropical Cyclone Kelvin (2018)

Shortly after making landfall over Western Australia, Kelvin developed a clear eye and continued strengthening despite moving over the Great Sandy Desert. The system was first identified as a weak tropical low on the morning of 11 February over the Northern Territory's Tiwi Islands by the Bureau of Meteorology. The low moved southwestwards over land and emerged over the Indian Ocean near Broome on 16 February. The storm intensified into a Category 1 tropical cyclone on the following day, and subsequently moved slowly eastwards and proceeded to rapidly intensify in the hours prior to landfall. The system crossed the coast as a high-end Category 2 tropical cyclone on the Australian scale, and a high-end Category 1 hurricane-equivalent cyclone on the Saffir-Simpson scale. Kelvin weakened at a very slow rate over the next few days, and was downgraded to a tropical low on 19 February.


Tropical Cyclone Kelvin brought widespread heavy rainfall to the Kimberley region which had already been saturated by other tropical cyclone systems. As a result, significant flooding occurred in parts of the Kimberley, including in the towns of Broome and Bidyadanga. Broome Airport received 376.8 mm (14.8 in) of precipitation in the 24 hours prior to 9:00 am on 17 February.

Tropical Storm Alberto (2018)

Tropical Storm Alberto of 2018 is another example of the brown ocean/agukabams effect. The first storm of the 2018 Atlantic hurricane season, Alberto developed on May 25 near the Yucatán Peninsula as a subtropical cyclone. As it entered the Gulf of Mexico, Alberto intensified and transitioned into a tropical cyclone. Early on May 28, Alberto reached its peak intensity, with maximum sustained winds of 65 mph (100 km/h) and a minimum pressure of 990 mbar (hPa; 29.23 inHg). The storm sustained it's strength as a Tropical Depression after landfall, lasting for an additional three days after its landfall. Alberto became one of only eleven cyclones to reach Lake Huron as a tropical depression.

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