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  • View in gallery

    Color-enhanced MTSAT-IR image sequence of the Duck at (a) 2300 UTC 4 Mar, (b) 2000 UTC 7 Mar, (c) 1100 UTC 8 Mar, and (d) 1900 UTC 8 Mar 2001. Images have been enhanced using a colored Dvorak scale (°C). The locations of Australia’s major capital cities on the eastern coast, and the approximate position of the Duck (given by L), are indicated in yellow. (Source: New Zealand Meteorological Service)

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    The 0.125° MESO_LAPS 500-hPa air temperature (°C) and wind vectors (m s−1) at (a) 1100 UTC 5 Mar and (b) 1100 UTC 8 Mar 2001. Black line represents the LAPS cyclone track (commencing at 0000 UTC 4 Mar, 6-h time step) and white line represents the MESO_LAPS cyclone track (commencing at 2300 UTC 3 Mar, 12-h time step).

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    (a) Average SST anomaly over the period 25 Feb–2 Mar. (b) Average difference between SST and 1000-hPa air temperature; (c) sensible heat fluxes, 2-m air temperature, and 10-m winds; and (d) latent heat fluxes and 10-m winds averaged over the period 3–8 Mar 2001 using 0.25° Reynolds SSTs, 0.125° MESO_LAPS, and ERA-40 surface temperatures and winds. Contours of specific humidity are also included in (d) for 0000 UTC 6 Mar. Solid black line represents the MESO_LAPS track of Duck. BI and ZAI denote blocking and zonal index regions in (b). Units are: temperature °C; fluxes, W m−2; near-surface winds, m s−1; and 2-m specific humidity, g kg−1.

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    Average 0.125° MESO_LAPS (shaded) and vector EVWS anomaly (m s−1) during 4–8 Mar 2001. MESO_LAPS track is also included in white.

  • View in gallery

    (a) Three-dimensional air parcel back-trajectory calculations for the air parcels arriving at a point 2° to the south of the center of Duck and (b) the respective vertical profile of each trajectory identified by numbers, using the Melbourne University back-trajectory program (Lagrangian Advective Trajectory Software). Backward trajectories are calculated commencing at 0000 UTC 8 Mar through 0000 UTC 1 Mar 2001, with small dots every 12 h. The exact coordinate used was 30°S, 156°E, which is denoted by a solid black star corresponding to 8 Mar 2001. ERA-Interim 1.5° resolution data are used.

  • View in gallery

    The 0.125° MESO_LAPS west-to-east vertical cross section of the relative vorticity (s−1 × 10−6) centered on the surface low at (a) 1100 UTC 5 Mar (latitude averaged over 29° to 27°S) and (b) 1100 UTC 8 Mar 2001 (latitude averaged over 29.75° to 27.75°S). Horizontal wind barbs (m s−1) shown for each analyzed level. Here, L denotes the position of the surface low.

  • View in gallery

    The 700-hPa BI (red bars, gpm) and ZAI (blue line series, m s−1) during February–March 2001. Duck life cycle period is indicated by light blue shading.

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    ERA-40 average MSLP (hPa) and 700 hPa winds (m s−1) over the period 1–8 Mar 2001.

  • View in gallery

    Simulated LAPS time–height cross section of omega vertical motion (shading, Pa s−1): (a) relative vorticity (s−1 × 10−6) and (b) equivalent potential temperature (K), averaged over a 300-km-radius circle centered on the cyclone vortex. Here, L denotes the maximum relative vorticity and W indicates warm troughs. Vertical dashed line indicates the simulated landfall time (0000 UTC 8 Mar).

  • View in gallery

    Simulated LAPS PV (10−6 K m2 s−1 kg−1) on the 300-K isentropic surface and streamlines: 0000 UTC (a) 5 and (b) 8 March. The PV on the 320-K isentropic surface and the streamlines: (c) 1100 UTC 5 March and (d) 0000 UTC 8 March. Red cross denotes the location of the cyclone as tracked by the LAPS simulation. Approximate pressure levels of key regions are also noted to the right of the image. Gray tones denote areas in which PV is undefined as the surface descends underground.

  • View in gallery

    Simulated LAPS 10-m wind vectors and shaded isotachs (m s−1) for 0000 UTC (a) 5, (b) 6, and (c) 8 March (simulated landfall) 2001.

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Tropical Transition of the 2001 Australian Duck

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  • 1 School of Earth Sciences, The University of Melbourne, Melbourne, Victoria, Australia
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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 (0.15°) 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.

Corresponding author address: Luke A. Garde, School of Earth Sciences, The University of Melbourne, 3010 VIC, Australia. Email: l.garde@pgrad.unimelb.edu.au

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 (0.15°) 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.

Corresponding author address: Luke A. Garde, School of Earth Sciences, The University of Melbourne, 3010 VIC, Australia. Email: l.garde@pgrad.unimelb.edu.au

1. Introduction

The Australian coast is often battered by severe cyclonic systems that result in billions of dollars in damage every decade. These include the severe tropical cyclones (TCs) that primarily affect the tropical regions of Australia and the extratropical cyclones (ECs), or simply lows, that affect the southern reaches of the continent. The most intense of these systems pose a hazard to infrastructure and the general population and there are many recent examples of severe losses associated with extreme events. While a certain amount of loss is inevitable, improving forecasts and warnings has the potential to minimize losses and the subsequent dangers to the general public.

For a cyclone to be deemed tropical it must consist of a warm-core cyclonic circulation throughout the troposphere (Frank 1977). As an indicator of convective activity, a warm core can be developed via the wind induced surface heat exchange (WISHE) mechanism. According to the WISHE view, the potential energy for TCs arises from the disequilibrium between the atmosphere and the underlying ocean (Emanuel 1986; Craig and Gray 1996). Conversely, ECs grow mainly by extracting available potential energy stored in regions of strong baroclinicity (Bjerknes and Solberg 1922; Reale and Atlas 2001).

Over the last century, TCs and ECs have been studied as two mutually exclusive groups (Hart and Evans 2001; Hart 2003). Toward the beginning of the twenty-first century, interest turned to objectively quantifying the gray areas and the transitional processes between TCs, ECs, and hybrid cyclones (HCs; Beven 1997; Klein et al. 2000; Roth 2002; Davis and Bosart 2003; Hart 2003; Jones et al. 2003; Davis and Bosart 2004; Guishard et al. 2007, and others). Beven (1997) describes HCs as systems whose energy sources are driven by multiple processes. These cyclones often show characteristics in environments where they are not expected. Beven proposes an expanded classification system that utilizes core temperature and frontal characteristics, acknowledging the existence of hybrid systems. Beven’s scheme poses no sharp boundaries between classification groups but, rather, a continuous spectrum. More recently, Guishard et al. (2007) defined HCs as subtropical storms with cold upper- and warm lower-cyclonic components, spawned as baroclinic developments in the presence of cyclonic low-level vorticity over relatively warm sea surface temperatures (SSTs).

Hart (2003) also proposed an objectively defined continuum for evaluating a cyclone’s phase and structure. Utilizing phase diagrams calculated throughout the cyclone life cycle, Hart’s method draws on measures of storm-motion-relative 900–600-hPa thickness asymmetry (frontal nature) and tropospheric wind (cold–warm-core structure) evaluated between two layers of equal mass: 900–600 and 600–300 hPa. The phase diagrams also illustrate the transition between warm- and cold-core systems, effectively highlighting extratropical transitions (ETs), tropical transitions (TTs), and the development of HCs. The transition idea appreciates that there are no physical constraints in the free atmosphere to prevent one given type of cyclone from transforming into a different type or from having hybrid characteristics.

In March 2001, a system was detected that drew the attention of the research community due to its unusual observational characteristics. The name Duck was coined from the phrase “if it quacks like a duck, looks like a duck, and waddles like a duck, it’s usually a duck,” implying that observed characteristics could have been a justification in classifying this system as a TC. Interest in this event was further reinforced as research revealed apparent similarities with the first South Atlantic Hurricane Catarina phenomenon of March 2004, which has been researched extensively (i.e., Pezza and Simmonds 2005; McTaggart-Cowan et al. 2006; Veiga et al. 2008, and others).

This paper’s aim is to examine the large-scale circulation and thermodynamics associated with the genesis of this unusual system “Duck,” with emphasis on the dynamic processes happening as a result of a tropical–extratropical interaction. This interaction is discussed as a physical mechanism contributing to the apparent propagation of wind shear anomalies from higher latitudes. This is evident in the backward-trajectory calculations performed with a three-dimensional scheme available at the University of Melbourne. These results add to previous developments in the literature (Callaghan 2001; McCrone 2002; Buckley et al. 2003; Qi et al. 2006), by addressing the Duck’s dynamics and thermodynamics in detail. The data and methods used in this research are shown in section 2. Background, cyclone tracks, and the Duck’s thermodynamic and dynamic signatures are discussed in section 3. Simulation results with the Australian high-resolution limited-area model are discussed in section 4, where it is shown that recent improvements in the model allowed for satisfactorily simulating the Duck’s entire track and landfall. Finally, a comparison with the first South Atlantic hurricane is discussed in section 5.

2. Data and methods

This study used analysis data provided by the Australian Bureau of Meteorology (BoM). The nested high-resolution 0.125° Mesoscale Limited Area Prediction System (MESO_LAPS), with a twice-daily output (Puri et al. 1998), covering a regional domain of 55°S–5°N, 95°–170°E, was used. For all calculations involving climatology, the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis dataset (Uppala et al. 2005) was used. Three-dimensional backward air parcel trajectories are computed using 1.5° ERA-Interim reanalysis data. The SST dataset used in this study is the National Oceanic and Atmospheric Administration/National Climatic Data Center’s optimum interpolation 0.25° daily SST analysis (Reynolds et al. 2007). Approximate calculations of the sensible (QS) and latent (QL) heat fluxes are computed using the following “bulk formulas” (1) and (2), as used by Reale and Atlas (2001):
i1520-0493-138-6-2038-e1
i1520-0493-138-6-2038-e2
where ρ is the dry air density, CD is the drag coefficient (1.5 × 10−3), cp is the specific heat at constant pressure, Lν is the latent heat of vaporization, U is the 10-m wind speed, Ta is the 2-m air temperature, qs is the saturated specific humidity calculated from the SST, and qa is the 2-m air specific humidity.
For this study, the environmental vertical wind shear (EVWS) is defined as the magnitude of the difference between the 250- and 850-hPa wind vectors (m s−1). The EVWS anomaly is calculated throughout the Duck’s life cycle, based on a 45-yr ERA-40 climatology as (3), bounded by 55°S–5°N, 95°–170°E:
i1520-0493-138-6-2038-e3

An atmospheric blocking index (BI) is defined as the average 700-hPa geopotential anomaly bounded by the region 35°–50°S, 145°–170°E, following the same physical principle of Pezza and Simmonds (2005) for the South Atlantic. These coordinates have been specifically chosen to consider an area large enough to capture the variability associated with the ridge component of the blocking system observed during the Duck’s life cycle. Sensitivity tests showed that smaller BI regions depicted similar variability. A zonal shear anomaly index (ZAI) is defined as the average zonal shear anomaly bounded by the region 26°–32°S, 154°–161°E calculated based on gridded data. This domain was chosen to enclose the cyclone development region while not being contaminated by the jet or the Australian coastline. BI and ZAI values convey information as to how anomalous the large-scale environment was, highlighting the link between blocking activity and the initiation of transition.

A three-dimensional air parcel “back trajectory” scheme (the Lagrangian Advective Trajectory Software) was applied to study further details of the tropical–extratropical interaction observed during Duck’s life cycle. Parcel trajectories are often calculated to obtain an appreciation of the history of an air mass (e.g., Fuelburg et al. 1996). From a specified parcel location in the atmosphere, xn at time n, a finite integral is solved to advect the parcel and generate the trajectory path. Given the three-dimensional wind v(xn), the governing prognostic equation for the trajectory path over a short time interval Δt is xn+1 = xn + vΔt. The wind at a given point is found by cubically interpolating from a spatial grid then linearly in time. For back trajectories the wind direction is reversed. The finite integral is solved using a fourth-order Runge–Kutta scheme to obtain an estimate of the wind. This method is considerably more accurate for trajectory calculations than is a simple first-order approach. For the air parcel analysis the ERA-Interim reanalysis at 1.5° × 1.5° resolution has been used. This new dataset offers higher resolution as compared to the ERA-40 reanalysis, thus being more suitable for the Lagrangian calculations. For further details regarding the three-dimensional algorithm, see Noone and Simmonds (1999).

The BoM Limited Area Prediction System (LAPS) model (Puri et al. 1998) was used to simulate Duck. LAPS was run with a horizontal grid resolution of 0.15° and 29 vertical levels. Refer to Table 1 for a summary of the LAPS simulation details and Table 2 for a brief list of improvements incorporated into LAPS since 2002. The LAPS model was initialized with ERA-40 reanalysis data commencing on 4 March, with simulation intervals of 6 h and running for 120 h. The 300- and 320-K isentropic surfaces were used to investigate both the low- and middle-tropospheric levels during the Duck’s evolution based on Ertel potential vorticity (PV) analysis (Hoskins et al. 1985).

3. Analysis of the 2001 Australian Duck

a. Background

The Australian Duck was not recorded in the official Australian TC database. Given its hybrid origins, it was classified as a subtropical low or eastern Australian low. McCrone (2002) suggests that this system originated as a subtropical cyclone with dynamics similar to the Kona low as described by Morrison and Businger (2000). The Duck is clearly unusual, as it initially resembled an EC, intensifying outside the climatological TC genesis region. Dare and Davidson (2004) showed that Australian TCs tend to originate equatorward of 20°S. In 40 yr of satellite-supported best-track data, only two previous cases of TC genesis were noted to have occurred near the Duck development region.

Callaghan (2001) reports that the Australian regional LAPS model forecast a weakening low to move southward, clearly underestimating the threat posed by this system. Callaghan (2001) also states that analysis of satellite imagery at 0030 UTC 7 March 2001 (image not shown), using the Dvorak (1984) TC intensity technique, yielded Dvorak T numbers between T3.5 and T4, corresponding to a mean wind speed of between 28 and 33 m s−1. These results would have traditionally led to Duck being classified as a category-2 TC according to the BoM cyclone severity category (BureauofMeteorology 2002) and marginally below a category-1 TC under the Saffir–Simpson hurricane scale (SSHS; Simpson 1974).

Figure 1 represents the evolution of the convection as captured by the Japan Meteorological Agency’s Multifunctional Transport Satellite (MTSAT). The approximate locations of Brisbane, Sydney, Melbourne, and Hobart—Australia’s major capital cities on the Eastern coast—are indicated in yellow. The approximate position of the Duck’s core is given by the “L” (note that for visualization purposes L has been slightly displaced from the cyclone’s center). Figure 1a shows the preliminary developments associated with a tropical cloud band receiving upper-level moisture inflow from a decaying TC to the west (TC Abigail). At this time most of the convective activity was seen to the south of the cyclone core, while the tropical band was seen linked to the north. Figure 1b shows the maturing stage when the cyclone separated from the initial tropical cloud band. At this stage colder convective cells are spread throughout the developing area (shown in blue-green tones). Prior to landfall on 8 March, the Duck appeared to undergo TT as an eyelike feature began to develop on its northeast flank (Fig. 1c). The Dvorak color enhancement highlights a modest upper-level warm-core signature, which clearly contrasts against the much colder surrounding cells as it crossed the Australian coast. This feature symbolizes the Duck reaching maturity, and achieving partial TT (see thermodynamic analysis), which was also present in the high-resolution MESO_LAPS analysis data available for this event, which will be discussed later. Radar observations recorded by the Grafton radar (from 0400 UTC 8 March) clearly showed a circular rain-free region associated with a major rainband to the Duck’s southwest (image not shown). As the Duck moved onto the coast near Byron Bay (28.64°S, 153.64°E), a mean sea level pressure (MSLP) of 991.6 hPa was recorded at 0700 UTC 8 March. Consequently, severe weather and gale warnings were issued for the southeast Queensland coast where large swells, winds, and tides caused beach erosion and saltwater inundation (Callaghan 2001). At landfall, sustained 10-min-average winds of 28 m s−1 were measured by the automatic weather station (AWS) at Evans Head (50 km south of Byron Bay), with peak wind gusts measuring 39 m s−1 (Callaghan 2001; Buckley et al. 2003).

Postlandfall, the system decayed quickly, losing its eye characteristics but still conserving a significant coverage of cirrus to the north and an extensive area of convection to the south (Fig. 1d). A line of thunderstorms became quasi-stationary over southeast Queensland, producing locally heavy rainfalls associated with flash flooding (Muller and Malone 2001; Padgett 2001). Rainfall statistics illustrate that rainfall amounts were significant and, in several cases, exceeded once-per-century figures.

b. Thermodynamic analysis

1) Core temperature analysis

Core temperature is defined as the temperature measured over the cyclone, as opposed to the larger-scale environmental temperature. The MESO_LAPS analysis with resolution of 0.125° has enough resolution to discriminate between the vortex and the environment. Figure 2 shows the 500-hPa air temperature and wind vectors for 1100 UTC 5 March (panel a) and 8 March (panel b) 2001 (beginning of maturity and landfall). Figure 2 also indicates the MESO_LAPS-analyzed track and the 0.15° LAPS-simulated track for the whole life cycle. The MESO_LAPS track is plotted every 12 h, while the LAPS simulation is plotted every 6 h. From these tracks we observe close agreement between the analyzed and simulated tracks, placing the landfall around 29°S. This latitude is also in agreement with surface station observations and with satellite data. More details regarding the simulated characteristics of this system are discussed in section 4.

Initially, a surface trough developed (3 March) along a SW–NE orientation with a pressure of 1005 hPa, before a weak warm core began to develop at 500 hPa, almost aligned with the surface low on 5 March (Fig. 2a). This was the beginning of the maturation of the cutoff low, which was starting to undergo a TT process. This gradual structural change was associated with an increase in convective activity as the cyclone became more organized (Fig. 1). A stronger warming reappears just before landfall when the cyclone traversed a tongue of warmer SSTs. The vertically aligned warming (not shown) in both the middle and upper troposphere suggests that the Duck underwent at least partial TT at landfall, as shown in Fig. 2b for 500 hPa. Note that the warming observed at landfall was at a maximum compared with the whole life cycle, with a difference of approximately 2° compared to the initial stages (cf. Figs. 2a and 2b).

2) Air–sea interaction analysis

Figure 3 gives a general overview of the average SST anomaly and air–sea fluxes calculated over the Duck’s life cycle, using the average SST for the week prior to Duck’s development to avoid contamination by a possible storm-induced SST cooling. Figure 3 is computed based on daily fields. Figures 3a and 3b illustrate the average SST anomaly and the differences between the SST and 1000-hPa temperature for 3–8 March, respectively. Figure 3a also includes the position of the conventional hurricane threshold of the 26.5°C isotherm (Palmén 1948; Gray 1968, DeMaria et al. 2001). In addition, the MESO_LAPS trajectory has been superimposed onto all of the maps in Fig. 3.

The general feature of the SST distribution in the Tasman Sea is poleward advection of relatively warm water adjacent to the coast by the East Australian Current (EAC; Tomczak and Godfrey 1994) with a region of cooler water east of approximately 158°E and south of the Tropic of Capricorn (Fig. 3a). Although Duck traversed a region of negative SST anomalies near the genesis area (Fig. 3a), the overall SST pattern appears to be suited for TT where the system originates east and moves zonally toward a warm coast, as occurred due to the blocking high. It is observed that the SST was below the hurricane threshold during the Duck’s life cycle, with the system originating over SSTs of 24°C. As the system veered toward the southwest, it traversed a region along the eastern Australian coast with SSTs in excess of 26°C due to local features associated with the EAC. Holland et al. (1987) showed that the poleward incursion of warm water is maintained by transient warm eddies, which move down the eastern Australian coast.

Figure 3b shows the mean difference between the SST and the 1000-hPa temperature and, in addition, the BI and ZAI regions are also illustrated, which will be discussed within the dynamic analysis (section 3c). Most areas east of Australia present positive values (above +2°C), illustrating that the SST during the week prior to genesis was warmer than the 1000-hPa temperature over the Duck’s life cycle. The strong positive values recorded in the southern Tasman Sea reflect a combination of anomalously warmer water (Fig. 3a) and the advection of colder air associated with the blocking high. As the Duck encountered SSTs above 26.5°C near the coast, the resulting difference between the SST and 1000-hPa temperature was at a maximum, approximately 3.5°C. This finding indicates an environment that would reinforce WISHE, helping to explain the achievement of partial TT prior to landfall as indicated in Fig. 2b. Most of the BI region relevant for the maturation of the blocking structure appears largely positive. This area will be discussed in detail in section 3c.

Using bulk Eqs. (1) and (2), Figs. 3c and 3d show the estimated sensible and latent heat surface energy fluxes averaged over the Duck’s life cycle. Referring to Fig. 3c, it can be seen that the maximum positive areas of sensible heat flux (60–80 W m−2) are observed to the southwest of the Duck’s trajectory as well as over the area where the partial transition occurred close the coast. Those regions correspond to areas of positive difference between the SST and 1000-hPa air temperature (cf. with Fig. 3b). In addition, Fig. 3c also includes the average 2-m air temperature contours and 10-m winds, demonstrating onshore winds of colder air onto the Australian coast. A surface warm trough is seen where landfall occurred. Figure 3d shows that the latent heat flux has an even more pronounced positive pattern spreading over most of the Duck’s trajectory, with values reaching 350–370 W m−2. It is likely that the enhanced fluxes influenced the Duck’s maturation toward a partial TT. We also note that this pattern is different from a classic extratropical cyclone where the winds are normally offshore. In our case, it is evident that the Duck extracts at least part of its energy from the fluxes as they will increase the temperature near where landfall occurred (where a trough of warm temperature was already present as seen in Fig. 3c), positively reinforcing the cyclone’s growth. Moreover, Fig. 3d shows 2-m specific humidity contours for 0000 UTC 6 March. This time is relevant as the moisture field appears to be enhanced in the area where the cyclone intensified, acquiring a modest warm core (cf. with Fig. 2a). Daily analyses of 2-m specific humidity and near-surface winds (figure not shown) suggest that the enhancement of specific humidity above was partially driven locally and by large-scale advection from the northeasterly winds near the genesis area. As discussed later, this feature was also evident via a backward air parcel trajectory calculation, indicating parcels from the northeast were arriving at the cyclone at low levels.

Using these bulk surface flux values, a total surface heat flux is estimated at 420–440 W m−2 averaged over the life cycle of the Duck (image not shown). Total flux values of this magnitude are lower than the values expected in a tropical storm environment (1000 W m−2) as noted by Reale and Atlas (2001). The location of strong surface fluxes along the eastern Australian coast in both Figs. 3c and 3d is reflecting the presence of the EAC and the near-surface wind distribution, which is discussed in section 4 in addition to diagnostic simulated flux values.

c. Dynamic analysis

Daily analysis of the EVWS conditions during the Duck’s life cycle shows areas of weak vertical shear (5 m s−1) extending eastward in a belt between 35° and 40°S. Climatologically, this region is associated with EVWS values as high as 25 m s−1, which defines the subtropical jet. EVWS values less than 8 m s−1 offer the ideal conditions for TC development (Montgomery and Farrell 1993; Davis and Bosart 2003). The presence of atmospheric blocking (AB) conditions in the Tasman Sea produced a split in the subtropical jet (omega blocking), resulting in two discrete branches. As a result, regions of weak shear developed and appeared to propagate northward along the eastern Australian coast (see the following back-trajectory analysis). The Duck’s development then took place, as weak EVWS zones “sheltered” the cyclone from the climatological strong shear environment. Figure 4 highlights the split in the subtropical jet together with the average wind shear anomaly over the Duck’s life cycle during 4–8 March. EVWS anomaly values for the Duck’s genesis region were on the order of −15 m s−1, corresponding to shear values on the order of 10 m s−1, which are close to ideal TC development conditions. It must be noted that this sheltered region is much larger than the cyclone vortex, so very little contamination from the vortex is apparent in the environmental field.

Figure 5 shows (a) the three-dimensional air parcel back trajectory for the air parcels arriving at a point 2° south of Duck’s center (30°S, 156°E, denoted by black star) and (b) the respective vertical profile of each trajectory identified by numbers 1–13. Backward trajectories were calculated commencing at 0000 UTC 8 March through 0000 UTC 1 March 2001. Several tests were performed for points encompassing the vicinity of the cyclone and the results are robust. From Fig. 5a it is clear that the low-level air parcels reaching the cyclone originated in the far southeast and then circulated through the subtropical region before reaching the Duck. As discussed in the next paragraph, this is best exemplified by trajectory 3, supporting the physical hybrid nature hypothesis associated with this cyclone formation.

The back-trajectory calculations strengthen the view that the shear anomalies important in Duck’s formation originated at higher latitudes. Trajectory 12 in Fig. 5 tracks the behavior of the 200-hPa air parcel. From Fig. 5 it is clear that this trajectory followed a long path over the blocking area, where the negative shear anomalies were observed (cf. with Fig. 4). It would be reasonable to assume that the upper-level wind would be the largest contributor toward the shear anomalies given that climatologically the wind speed is greater in the upper levels. Hence, it can be argued that at least parts of the blocking–shear signal responsible for Duck’s intensification originated at higher latitudes. Analysis of back trajectories for air parcels arriving in the region of the blocking (not shown) confirmed the robustness of the blocking pattern with mid- and upper-level air originating from the areas of low shear south of Australia (40°S, 130°E) and descending around the high.

Trajectories 2 and 3 highlight the tropical nature of the low-level air parcel movement (Figs. 5a and 5b). Although initiated to the south of the cyclone, these air parcels traveled significantly to the north of Duck before descending and then ascending into the vicinity of the vortex (Fig. 5b). Trajectory 3, for instance, is seen to traverse northeast, nearing New Caledonia. The existence of tropical trajectories can be further observed for choices of destination points slightly to the NE of the cyclone center. Trajectories were mapped circulating near 10°S before reaching the Duck in the scenario above (image not shown), but Fig. 5a offers a good opportunity to appreciate the hybrid nature of the storm as it also incorporates trajectories 12 and 13, which moved through the blocking area (this being one of the main points of our discussion). During cyclogenesis (4 March), Fig. 5b clearly shows a vertical motion change in most of the air parcels from descending to ascending. This motion is potentially responding to a decrease in environmental static stability over the area in which Duck formed. We also note that trajectories 12 and 13 can be followed from outside the domain area but for visualization purposes this is not shown.

To develop an understanding of the Duck’s vertical structure, daily west-to-east relative vorticity and wind cross sections through the cyclone center were explored. From these cross sections it can be seen in Fig. 6a (5 March) that as the Duck matured, the atmosphere began to show vertical alignment about the surface low. Also evident is the location of the maximum winds, appearing in the lower troposphere on the western side of the cyclone center. It is proposed that this alignment in vertical structure is associated with the beginning of the TT, when the warm core started to develop (cf. with Fig. 2a). Figure 6b shows that a well-pronounced vortex from the surface to approximately 300 hPa is apparent that coincides with the Duck’s landfall and development of a stronger 500-hPa warm core. This vortex has a clear warm core structure as discussed earlier (Fig. 2b). Although a slight tilt to the west is observed, this near vertical structure indicates predominately barotropic-like conditions, although they are embedded in a large-scale baroclinic atmosphere (see Fig. 2b).

Figure 7 illustrates both the BI and ZAI as a time series using MESO_LAPS data for February and March 2001. The strength of the BI (shown in red) and the magnitude of the ZAI (shown in green) are given in geopotential meters (gpm) and meters per second, respectively. The results showed that over the 2-month period (1 February–31 March) the longest AB period was during the life cycle of the Duck, commencing 5 days before cyclogenesis, with a maximum of approximately 100 m at the time of landfall. This AB period reflects weaker westerly circulation in the defined region illustrated in Fig. 3b (BI), resulting in an atmosphere associated with less large-scale baroclinicity. However as we emphasize throughout the analyses, both the baroclinic and barotropic processes were important for Duck’s development.

During this time, coinciding with the AB episode, negative ZAI values can also be identified, reaching a minimum of −20 m s−1 (actual shear of 5 m s−1), further evident that the Duck’s genesis corresponded with a rapid decline in zonal shear.

Figure 7 plots other pronounced negative zonal shear and positive blocking anomalies occurring in early February, suggesting that the observed anomalies during 3–8 March were not unusual (Fig. 3b); therefore, it can be concluded that the dynamic interplay between AB and low EVWS conditions is necessary but not exclusively responsible for establishing the conditions required for this hybrid development.

To understand the physical processes that influenced AB during the period 1–8 March, an analysis of the MSLP and 700-hPa wind conditions for the Southern Hemisphere (SH) was conducted using the ERA-40 reanalysis. Figure 8 shows averaged MSLP and 700-hPa winds during this time, highlighting the presence of an elongated AB structure in the Tasman Sea as part of a wavenumber 4. This pattern is important because an increase in the amplitude of this hemispheric wave train will be reflected as a strengthening of the blocking over the Tasman Sea (indicated by H). These findings draw attention to the spatial distribution of anticyclonic vorticity south of Australia. As discussed earlier, the air parcel trajectories over the blocking area shown in Fig. 8 confirm that there is subsidence of locally originated air contributing to the strengthening of the blocking pattern (figure not shown). Figure 8 also draws a visual link with the Southern Annular Mode (SAM; Gong and Wang 1999). Although in principle the AB would reinforce the positive polarity of the SAM, in the Duck’s case the high pressure system was seen to the south of the traditional latitude upon which the SAM is normally defined. In section 5 a comparison between Duck and the first South Atlantic hurricane is offered, including some perspective on the polarity of the SAM.

4. LAPS simulation of the Duck

At the time of the event, the Duck’s path and intensity were poorly predicted by the Australian forecasts (Callaghan 2001). However, the LAPS simulation used in this research accurately represented the cyclone trajectory. As pointed out by Qi et al. (2006), the inclusion of scatterometer winds provided verification of near-surface winds, which is one of the fundamental factors behind the model skill increase. Additionally, the simulated LAPS results could also be responding to improved resolution, which is an implication of a bulk explicit microphysics scheme and/or the ERA-40 initial conditions [as opposed to Global Analysis and Prediction (GASP), which was used to initialize the operational forecast back in 2001], which are based on a more comprehensive observation dataset and sophisticated data assimilation techniques. Table 2 briefly summarizes the details of most of the assimilation improvements incorporated into the LAPS model since 2002.

To understand how the Duck’s vortex evolved, time–height radially averaged cross sections were developed (Fig. 9). Defined over a 300-km radius from the cyclone center, Fig. 9 shows how the vertical velocity (Pa s−1) field evolved with respect to (a) relative vorticity (s−1 × 10−6) and (b) equivalent potential temperature (K), respectively. Radii sensitivity tests were also performed at 100-, 500-, and 1000-km intervals to see if there was any detectable gain in physical detail made for the average sample. These results were similar, with comparable tropospheric patterns observed at all levels, while the magnitude of these features was the only variation. As it was considered that a 300-km radius was a better representation of the actual cyclone vortex radius, other radii cases are not shown here. Two key periods are observed in the vertical velocity field. The times of 1800 UTC 4 March and 0600 UTC 7 March denote time steps in which the rate of ascending motion increased (green tones, indicated by L). The maximum simulated ascent occurred prior to landfall, in agreement with our fluxes discussion.

Figure 9a reveals that as the cyclone developed (1800 UTC 4 March), a sudden increase in cyclonic relative vorticity and ascent was observed throughout the lower and middle troposphere, with a maximum around 850 hPa. Further analysis showed that as the cyclone veered toward the north before heading southwest, the relative vorticity field weakened before reintensifying once more. These findings are in agreement with the development of a warm core due to the partial transition (Fig. 2), as a maximum center of relative vorticity is observed 12 h prior to the simulation landfall in the lower 850-hPa level. Referring to Fig. 9b, a surface warming (and moistening) is present from 0600 UTC 5 March, with a maximum around 7 March, coinciding with the increase in surface fluxes described earlier (the warm troughs are indicated by W). Finally, postlandfall identified a cooling phase.

To gain dynamical insight into the environmental circulation surrounding the development of the Duck, PV on the 300- and 320-K isentropic surfaces for 5 and 8 March were produced (Fig. 10), with standard PV units equal to 10−6 K m2 kg−1 s−1. Streamlines on the 300-K isentropic surface imply a northeast tropical moisture source east of the cyclone in both the 1100 UTC 5 March (Fig. 10a) and 0000 UTC 8 March (Fig. 10b) time steps, promoting “up gliding” convection as warm moist air is advected to the development region along the isentropic surface. This surface also shows the location of the elongated blocking high to the south of the cyclone in both time steps. Streamlines indicating equatorward or “down gliding” movement suggest the subsidence of dry, higher-latitude upper-tropospheric air. It is hypothesized that the continuous mechanism of PV injections from higher latitudes played a significant role in maintaining the observed elongated blocking structure, and this is supported by the air parcel trajectories discussed earlier. This process, as discussed in section 5, appears to have significant similarities with the overall dynamics during the development of the first South Atlantic hurricane. PV values on the 300- and 320-K surfaces register less than −1.5, indicating likely tropospheric air masses (Bluestein 1993). Regions shaded in gray indicate areas in which the PV is undefined as the surface descends underground.

Figure 10c shows the PV on the 320-K isentropic surface at 1200 UTC 5 March. Figure 10c highlights the connection between the cyclone and higher latitudes, thus strengthening the hypothesis that Duck’s development was a combination of tropical and extratropical conditions as supported by the air parcel trajectory analysis. In Fig. 5, the trajectories follow the more negative isentropic surfaces, connecting Duck with higher latitudes (Fig. 10c). The isentropes (Figs. 10c and 10d) about 40°S have a pressure level of approximately 500 hPa.

It is interesting to compare Figs. 10a and 10c (i.e., 5 March) with the results discussed earlier for the Lagrangian trajectories (Fig. 5). It is evident that trajectories 2, 3, and 9 (Fig. 5b) describe a path similar to the extratropical PV injection implied in Figs. 10a and 10c for adiabatic motion. The approximate equivalent pressure levels are indicated in Fig. 10 on the right-hand side for the key areas discussed above. Note that in Fig. 5b the pressure level for which the trajectories are initiated can also be seen in the diagram, clearly showing that the trajectories discussed above underwent subsidence following the isentropes from the extratropics toward Duck, before being enveloped by the vortex and acquiring ascending motion. In this sense the Lagrangian trajectories validate our PV analysis, showing a connection between Duck and the extratropics. Finally, Fig. 10d shows that for the time of landfall over the 320-K surface the cyclone appears completely cut off from any source of extratropical PV injection, suggesting that once maturity was reached the Duck’s development and decay were predominantly given by local effects.

The simulated near-surface winds (10 m) are shown at three times in Fig. 11. When studying the surface winds generated by the Duck, it is seen that the wind field is complex, evolving from an elliptical distribution to a more circular circulation. During the initial stages on 5 March (Fig. 11a), the surface wind distribution shows the strongest winds well away from the Duck’s core, with winds approaching 20 m s−1 in the western and southern quadrants. As the Duck continued toward the eastern Australian coast, the wind field appears to become more symmetrical (6 March); however, the wind magnitude is greater to the south of the cyclone where the translational speed adds to the wind speed as is typically observed in TCs. In addition, two distinct bands are observed with one surrounding the cyclone and the other farther removed toward the south (Fig. 11b). This secondary band of southeasterly winds to the south is a result of the very intense pressure gradient between the Duck and the anticyclonic component of the blocking dipole in which Duck was embedded. Approaching landfall (Fig. 11c), the two bands rejoin as the surface winds strengthened, during what was identified earlier as the Duck’s partial TT. At this time (8 March), a confluent wind pattern is generated between the Duck and the blocking high to the south, producing peak winds at outer radii. The 0.5° Quick Scatterometer (QuikSCAT) daily mean wind measurements for the same period agree remarkably well with the simulated wind pattern at all times (not shown), suggesting the incorporation of QuikSCAT winds into LAPS had a large contribution in defining the Duck’s final intensity, in agreement with Qi et al. (2006). However, as discussed earlier in relation to Table 2, this is only one of the many improvements that have been incorporated into the LAPS model since 2002.

LAPS diagnostic surface sensible and latent heat fluxes were also investigated and compared to the bulk flux approximations from section 3b. The overall spatial distribution of the simulated fluxes (figure not shown) agrees well with the bulk approximation. Average LAPS sensible heat fluxes attained values of 60 W m−2 over the life cycle of the Duck. The magnitude of the average latent heat flux is slightly higher (450 W m−2) than the values shown in Fig. 3d, perhaps due to the greater resolution and computation method, as the bulk approximation assumes a fixed drag coefficient. However, these values agree well with findings by Reale and Atlas (2001), who calculated the typical fluxes associated with two HCs with tropical signatures occurring over the Mediterranean (“Mediterranean hurricane”), helping explain why the Duck underwent partial TT.

The LAPS simulation results are also in accord with AWS observations recorded along the Australian coast and the MESO_LAPS analysis, placing landfall near Byron Bay and areas of significant wind damage farther south near Evans Head (Fig. 2 cyclone tracks).

It must be noted that the main destructive wind region was well removed from the eye and is therefore quite different from a traditional TC; this is of significant importance in forecasting. This has implications in the warning of HCs, as a TC warning would not represent the location of the destructive wind zone. As commented upon by Callaghan (2001), in the event of a TC warning, communities under threat assume that the worst damage will occur near the center of the cyclone.

Having considered the wind profile of the Duck, identifying and understanding HCs is of significant importance. Consequently, a warning system upgrade is needed to be able to provide appropriate warnings for such systems to the communities located along threatened coastlines. In the years since the 2001 Australian Duck, ongoing improvements are being made to numerical weather prediction models, and the inclusion of scatterometer winds is one such example.

5. Comparisons with the first South Atlantic hurricane

Although the Duck cannot be regarded as a phenomenon as extreme as the first South Atlantic hurricane (Catarina, March 2004), many similarities can be observed, particularly in the large-scale connections relevant to the development of the TT. Catarina occurred in a region thought to be incapable of supporting TC development or transitioning systems, while the Duck formed in a region where TC occurrence is rare. The region in which Hurricane Catarina occurred is a local maximum for EC genesis (Pezza and Simmonds 2005; Pezza et al. 2009). Catarina drew much attention, as previously it had been accepted that hurricanes could not form over the South Atlantic Ocean due to the presence of climatologically strong EVWS and insufficiently warm SSTs (Pezza and Simmonds 2005; McTaggart-Cowan et al. 2006). Pezza and Simmonds (2005), McTaggart-Cowan et al. (2006), and Pezza et al. (2009) have shown that Catarina originated over relatively warm SSTs above 26.5°C as a classical EC embedded in a baroclinic wave.

Moving eastward away from the coast of Brazil, Catarina underwent a full TT under the weak extratropical cyclone paradigm of Davis and Bosart (2004) (McTaggart-Cowan et al. 2006), and as a result became cut off from the westerly circulation. Changing its trajectory by 180°, Catarina began to follow a track roughly parallel to its previous eastward course under the influence of an anomalous easterly steering flow (McTaggart-Cowan et al. 2006). This was induced by an AB structure, similar to the one observed for the Duck, with a broad region of reduced EVWS between the streams of a split flow. Pezza and Simmonds (2005), using independently modeled data obtained by the Department of Meteorology at The Pennsylvania State University (PSU), commented that during its mature phase Catarina had attained a minimum central pressure of 974 hPa with estimated sustained hurricane category 1 force winds of 33–42 m s−1, making it a much stronger system than the Duck.

It is apparent that highly anomalous large-scale conditions favored the development of TT for both the Duck and Catarina. Although the thermodynamics and local effects contributed differently to their life cycles, in both cases the precursors to the transition were similar. From an environmental perspective, both Catarina and the Duck presented a pronounced EVWS anomaly of −15 m s−1 to the south of the cyclone track, corresponding to shear values below the ideal hurricane threshold of 8 m s−1. Furthermore, in both cases there is a suggestion that the anomalous wind shear areas originated at higher latitudes via continuous injections of PV anomalies that reinforced the blocking system. In the case of the Duck, this was demonstrated via a three-dimensional air parcel trajectory scheme.

Table 3 facilitates a direct comparison of the Duck and Catarina; in particular, it highlights the similarities and the differences between the two systems. For example, as discussed in section 3, the Duck only acquired a 500-hPa warm core and thus only partially developed tropical characteristics, unlike Catarina, which developed a long-lived 300-hPa warm core. Additionally, although the Duck developed a vertically stacked barotropic-like vortex, it did not acquire an upper-level anticyclonic feature like Catarina. AB structures are detected in both cases; however, Pezza and Simmonds (2005) and Pezza et al. (2009) have shown that this combination of an Atlantic blocking and low-level shear was unprecedented and that the SAM was in a positive phase for Catarina. In the case of Duck, the blocking high was situated farther south, resulting in the SAM index being negative.

It is hypothesized that the Duck’s inability to “fully” undergo TT is the result of only being exposed to injections of upper-level PV and favorable surface fluxes for a shorter period, achieving partial TT hours prior to landfall as compared to the completed TT of Catarina. Had the Duck’s exposure to such dynamic and thermodynamic mechanisms been prolonged, the resultant cyclone could have been significantly stronger.

6. Conclusions

The degree of uncertainty that surrounds the classification of the 2001 Australian Duck was the motivation for this research, with the results further illustrating the complexity surrounding the development of the Duck. This system displayed hybrid properties resulting from a combination of both tropical and extratropical elements in a region typically not favorable for TC development. Detailed analyses and high-resolution simulations of this event have demonstrated that Duck was the result of numerous atmospheric and oceanic mechanisms. Satellite imagery captured the Duck originating as a cutoff low, which displayed a closed circulation at the surface and a deep trough at 500 hPa. Analysis of the large-scale environment has shown that during the Duck’s development, an AB episode was present in the Tasman Sea. By splitting and deflecting the subtropical jet northward, the AB structure enabled weak EVWS conditions to move from higher latitudes and self-sustain in the Tasman Sea. These conditions were reinforced by constant injections of anomalous PV originating from higher latitudes, which were also supported using a three-dimensional backward trajectory scheme. The Duck encountered a broad region of reduced EVWS between the streams of the split jet with an EVWS minimum of 10 m s−1, indicating a dynamic environment suitable for TC formation. Averaging EVWS anomalies over the period 4–8 March identified a pronounced region of negative EVWS of −15 m s−1 pertaining to the large-scale environment.

The analysis of SST conditions surrounding the Duck’s track showed predominately negative anomalies, but the cyclone intersected a tongue of sufficiently warm SSTs above 26.5°C near the coast, where the sensible and latent heat fluxes were favorable. Indicative of a partial TT, a warm core was identified at 500 hPa in combination with a well-pronounced barotropic cyclonic vortex extending from the surface to the upper levels. It was only during this phase that the cyclone began to resemble a TC, exhibiting an eyelike feature as the system made landfall near Byron Bay. This development made the system very rare when compared to a climatology of past-observed vortices over the eastern Australian coast.

The LAPS simulation has provided an additional perspective into the dynamics involved in the Duck’s partial TT. In agreement with our analysis, the cyclone track and landfall were simulated well. Insights into the evolution of PV on several isentropic surfaces have demonstrated the role that high-latitude injections of PV had in reinforcing the blocking conditions, and the negative shear anomalies, characterizing a clear extratropical influence in Duck’s development. As discussed earlier, these results were confirmed by the Lagrangian trajectory scheme.

Forecasts made at the time of the event underestimated the system’s intensity, because the assimilation of satellite data was insufficiently representative of the actual environmental processes.

When comparing this system to the first South Atlantic hurricane, Catarina phenomenon of March 2004, it is revealed that the precursors to transition were similar, with both systems originating in regions with a combination of baroclinic and barotropic growth processes. It is evident that large-scale blocking features influenced the formation of both cut-off systems, with the reduction in the EVWS conditions leading to the full TT of Hurricane Catarina and only partial TT of the Duck. It is also interesting that both Catarina and the Duck developed over a region with slightly negative SST anomalies. This apparent paradox highlights the participation of dynamic and thermodynamic combined growth in both cases, emphasizing the importance of the heat fluxes over the ocean arising from the air–sea temperature difference (Pereira Filho et al. 2009; Vianna et al. 2010).

The two systems discussed in this paper reinforce the importance of understanding the physical processes behind cyclone transition, particularly for the SH where many aspects of their formation are still obscure. Our overall understanding of TT in the SH would benefit from the development of a climatology that will include hybrid “ducks,” subtropical cyclones, and East Coast lows. As suggested by this work, large-scale processes do play a fundamental role in the dynamics leading to TT, in addition to the more traditional thermodynamic views, which help to explain the local intensification. Future work building on the considerations researched and analyzed above will bring a more integrated view into the relative participation of large-scale environmental and local conditions leading to cyclone transition.

Another area of future research arising from our results would be the use of the Lorenz energetics to quantify the participation of the baroclinic and barotropic processes of the Duck’s development, similar to what was recently done for the first South Atlantic hurricane by Veiga et al. (2008). This is the subject of future research.

Acknowledgments

The authors would like to acknowledge the Australian Bureau of Meteorology for facilitating some of the data used in this research. First, we thank Drs. Andrew Watkins, Blair Trewin, and Terry Skinner (Australian Bureau of Meteorology) for drawing our attention to the rarity of this system and encouraging this publication. We thank Dr. Noel Davidson (Australian Bureau of Meteorology—Centre for Australian Weather and Climate Research) for his scientific contribution, encouragement, and suggestions regarding analysis of the LAPS simulation. Thanks are also extended to Mr. Kevin Keay (The University of Melbourne) for his assistance with the air parcel trajectory scheme. We also thank Prof. Ian Simmonds (The University of Melbourne) for his helpful comments, which led to improvements to a preliminary version of this manuscript, in addition to Dr. Todd Lane (The University of Melbourne), Dr. Graham Mills (Australian Bureau of Meteorology—Centre for Australian Weather and Climate Research) and Ms. Mai Nguyen (Monash University) for their helpful comments and suggestions. Appreciation is extended to Mr. Tristan Oakley (New Zealand Meteorological Service) for providing satellite images of the Duck. The authors are also very grateful for the insightful comments from three anonymous reviewers that greatly improved the work. Parts of this work were made possible with funding from the Australian Research Council to Ian Simmonds.

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Fig. 1.
Fig. 1.

Color-enhanced MTSAT-IR image sequence of the Duck at (a) 2300 UTC 4 Mar, (b) 2000 UTC 7 Mar, (c) 1100 UTC 8 Mar, and (d) 1900 UTC 8 Mar 2001. Images have been enhanced using a colored Dvorak scale (°C). The locations of Australia’s major capital cities on the eastern coast, and the approximate position of the Duck (given by L), are indicated in yellow. (Source: New Zealand Meteorological Service)

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 2.
Fig. 2.

The 0.125° MESO_LAPS 500-hPa air temperature (°C) and wind vectors (m s−1) at (a) 1100 UTC 5 Mar and (b) 1100 UTC 8 Mar 2001. Black line represents the LAPS cyclone track (commencing at 0000 UTC 4 Mar, 6-h time step) and white line represents the MESO_LAPS cyclone track (commencing at 2300 UTC 3 Mar, 12-h time step).

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 3.
Fig. 3.

(a) Average SST anomaly over the period 25 Feb–2 Mar. (b) Average difference between SST and 1000-hPa air temperature; (c) sensible heat fluxes, 2-m air temperature, and 10-m winds; and (d) latent heat fluxes and 10-m winds averaged over the period 3–8 Mar 2001 using 0.25° Reynolds SSTs, 0.125° MESO_LAPS, and ERA-40 surface temperatures and winds. Contours of specific humidity are also included in (d) for 0000 UTC 6 Mar. Solid black line represents the MESO_LAPS track of Duck. BI and ZAI denote blocking and zonal index regions in (b). Units are: temperature °C; fluxes, W m−2; near-surface winds, m s−1; and 2-m specific humidity, g kg−1.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 4.
Fig. 4.

Average 0.125° MESO_LAPS (shaded) and vector EVWS anomaly (m s−1) during 4–8 Mar 2001. MESO_LAPS track is also included in white.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 5.
Fig. 5.

(a) Three-dimensional air parcel back-trajectory calculations for the air parcels arriving at a point 2° to the south of the center of Duck and (b) the respective vertical profile of each trajectory identified by numbers, using the Melbourne University back-trajectory program (Lagrangian Advective Trajectory Software). Backward trajectories are calculated commencing at 0000 UTC 8 Mar through 0000 UTC 1 Mar 2001, with small dots every 12 h. The exact coordinate used was 30°S, 156°E, which is denoted by a solid black star corresponding to 8 Mar 2001. ERA-Interim 1.5° resolution data are used.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 6.
Fig. 6.

The 0.125° MESO_LAPS west-to-east vertical cross section of the relative vorticity (s−1 × 10−6) centered on the surface low at (a) 1100 UTC 5 Mar (latitude averaged over 29° to 27°S) and (b) 1100 UTC 8 Mar 2001 (latitude averaged over 29.75° to 27.75°S). Horizontal wind barbs (m s−1) shown for each analyzed level. Here, L denotes the position of the surface low.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 7.
Fig. 7.

The 700-hPa BI (red bars, gpm) and ZAI (blue line series, m s−1) during February–March 2001. Duck life cycle period is indicated by light blue shading.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 8.
Fig. 8.

ERA-40 average MSLP (hPa) and 700 hPa winds (m s−1) over the period 1–8 Mar 2001.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 9.
Fig. 9.

Simulated LAPS time–height cross section of omega vertical motion (shading, Pa s−1): (a) relative vorticity (s−1 × 10−6) and (b) equivalent potential temperature (K), averaged over a 300-km-radius circle centered on the cyclone vortex. Here, L denotes the maximum relative vorticity and W indicates warm troughs. Vertical dashed line indicates the simulated landfall time (0000 UTC 8 Mar).

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 10.
Fig. 10.

Simulated LAPS PV (10−6 K m2 s−1 kg−1) on the 300-K isentropic surface and streamlines: 0000 UTC (a) 5 and (b) 8 March. The PV on the 320-K isentropic surface and the streamlines: (c) 1100 UTC 5 March and (d) 0000 UTC 8 March. Red cross denotes the location of the cyclone as tracked by the LAPS simulation. Approximate pressure levels of key regions are also noted to the right of the image. Gray tones denote areas in which PV is undefined as the surface descends underground.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Fig. 11.
Fig. 11.

Simulated LAPS 10-m wind vectors and shaded isotachs (m s−1) for 0000 UTC (a) 5, (b) 6, and (c) 8 March (simulated landfall) 2001.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3220.1

Table 1.

LAPS simulation model configuration used in our Duck case study.

Table 1.
Table 2.

LAPS improvements since 2002 (Bureau of Meteorology 2007).

Table 2.
Table 3.

Comparisons of the 2001 Australian Duck and the first South Atlantic hurricane, Catarina.

Table 3.
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