Tropical Transition of the 2001 Australian Duck

Abstract

In March 2001, a hybrid low pressure system, unofficially referred to as Donald (or the Duck), developed in the Tasman Sea under tropical-extratropical influence, making landfall on the southeastern Australian coast. Here, it is shown that atmospheric blocking in the Tasman Sea produced a split in the subtropical jet, allowing persistent weak vertical wind shear to manifest in the vicinity of the developing low. It is hypothesized that this occurred through sustained injections of potential vorticity originating from higher latitudes. Hours before landfall near Byron Bay, the system developed an eye with a short-lived warm core at 500 hPa. Cyclone tracking revealed an erratic track before the system decayed and produced heavy rains and flash flooding.A three-dimensional air parcel backward-trajectory scheme showed that the air parcels arriving in the vicinity of the mature cyclone originated from tropical sources at lower levels and from the far extratropics at higher levels, confirming the hybrid characteristics of this cyclone. A high-resolution (0.158) nested simulation showed that recent improvements in the assimilation scheme used by the Australian models allowed for accurately simulating the system's trajectory and landfall, which was not possible at the time of the event. Compared to the first South Atlantic hurricane of March 2004, the large-scale precursors were similar; however, the Duck was exposed to injections of upper-level potential vorticity and favorable surface heat fluxes for a shorter period of time, resulting in it achieving partial tropical transition only hours prior to landfall.

Subtropical cyclones over the oceanic basins: a review

“…Australian east coast cyclones, also known as east coast lows (ECLs), are any surface cyclone type (independently of the formation processes) occurring near subtropical east coast of Australia, affecting the coastal zone with intense winds and heavy precipitation . To date, there is no a pure subtropical cyclone climatology near eastern Australia.…”

“…Until 2002, previous studies did not use the name subtropical or hybrid for the ECLs, and this denomination was first used by McCrone and after by Garde et al ., who studied a hybrid system, called Duck, occurring in the Tasman Sea in March 2001, under a mix of extratropical–tropical influences. As discussed in Holland et al ., and Hopkins and Holland, the cyclone Duck developed in the presence of an atmospheric blocking, which imposed a weak vertical wind shear (about 5 ms –1 ) in the cyclogenesis region . This shear was lower than the climatological value of approximately 25 ms –1 .…”

Large-Scale Influences on the Evolution of Winter Subtropical Maritime Cyclones Affecting Australia’s East Coast

“…Figure 3 illustrates the mean evolution of ETL events; there are many individual exceptions. For example, partial tropical transition has been observed as late as March (Qi et al 2006;Garde et al 2010). There are also numerous examples of ETL events making a full extratropical transition to the midlatitudes.…”

“…ECC have previously been investigated in the context of individual extreme events (Bridgman 1985;Holland et al 1987;Garde et al 2010;Qi et al 2006;McInnes et al 1992;Mills et al 2010), heavy rain occurrences (Hopkins and Holland 1997), extreme wave producing systems (Allen and Callaghan 2001;Shand et al 2011), and elevated ocean levels (Public Works Department 1985, 1986. ECC have previously been investigated in the context of individual extreme events (Bridgman 1985;Holland et al 1987;Garde et al 2010;Qi et al 2006;McInnes et al 1992;Mills et al 2010), heavy rain occurrences (Hopkins and Holland 1997), extreme wave producing systems (Allen and Callaghan 2001;Shand et al 2011), and elevated ocean levels (Public Works Department 1985, 1986.…”

Simulation of medicanes over the Mediterranean Sea in a regional climate model ensemble: impact of ocean–atmosphere coupling and increased resolution

“…For example, subtropical cyclones have been observed in the Atlantic Ocean (Guishard et al 2009;Evans and Braun 2012;González-Alemán et al 2015) and in the Pacific Ocean (Garde et al 2010). There is no clear-cut separation between tropical and extratropical cyclones.…”