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  • View in gallery

    (a) The regional geography of Australia [from Qi et al. (2000)] and (b) the terrain of SE Australia (filled contours).

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    Mean sea level pressure analysis from the Bureau of Meteorology, valid at (a) 1200 UTC 28 Oct, (b) 1200 UTC 29 Oct, and (c) 0600 UTC 30 Oct 2010. (d) The AWAP 48-h accumulated precipitation from 1200 UTC 28 Oct (filled, every 10 mm) for SE Australia [the region in the black dashed box in (a), see Fig. 8b for a detailed map of topography in the region]. Approximate position of the Alpine crest is shown in orange. Initials represent the following weather stations: B (Mount Buller), Ch (Charlton), D (Deniliquin), Gr (Griffith), Ho (Mount Hotham), Me (Melbourne), Mi (Mildura), Wa (Wagga Wagga), and Yw (Yarrawonga). Additional weather stations used in the analysis are represented by the gray asterisks.

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    The 1-min observations at (a) Mildura, (b) Charlton, (c) Deniliquin, and (d) Griffith. Observations at each AWS are of (top) 10-m wind direction (black, °) and 10-m wind speed (orange, m s−1); and (bottom) surface pressure (black, hPa), 2-m temperature (orange, °C), and precipitation 0.2 mm min−1 (asterisk) along the ordinate. Note the different abscissa scales: (a),(b) are for 24 h from 0000 UTC 29 Oct and (c),(d) are for 12 h from 1002 UTC 29 Oct.

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    Observations from the Yarrawonga radar of the undular bore, indicated by solid lines, propagating in the direction of the arrows. (a),(b) The Doppler wind field and (c) radar reflectivity.

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    Observations from the Wagga Wagga radar of radar reflectivity associated with the undular bore, indicated by solid lines, propagating in the direction of the arrows through Griffith.

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    Observations from the Yarrawonga radar of the two prefrontal precipitation events: PF-1 and PF-2. They show the radar reflectivity at (a) 2000 UTC 29 Oct and (c) 2330 UTC 29 Oct, and radar-derived rainfall accumulations (mm) during (b) 1800–2100 UTC 29 Oct and (d) 2100–0100 UTC 29–30 Oct. The dashed arrows indicate the direction of system propagation. The black circles show in (a),(c) the 128-km range radius of the radar. Terrain contours in (b),(d) every 200 m.

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    Observations from the Wagga Wagga radar of radar reflectivity showing the development of the second prefrontal precipitation event, PF-2.

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    (a) Nest 1 (with 30-km grid spacing) with boxes marking locations of nest 2 (10 km), nest 3 (3.33 km), and nest 4 (1.11 km). (b) Topography of the Australian Alps in Control (km) and (c) modified topography in No-Plateau. The dashed box in (b),(c) shows nest 4, and the solid box in (c) indicates where the topography has been modified for No-Plateau, as described in the appendix.

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    Results from the Control simulation at (a) 1000 UTC 29 Oct, (b) 1500 UTC 29 Oct, and (c) 2000 UTC 29 Oct of surface flow convergence (red indicates flow convergence 0.001 m s−1); surface wind vectors; and accumulated precipitation from the preceding hour (filled in mm). (d) Accumulated precipitation (mm) from Control for 1800–0100 UTC 29–30 Oct on the same projection as Fig. 6d. Terrain contours are every 200 m.

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    Vertical cross section of the undular bore in Control along A–B in Fig. 9a at (a) 1300 UTC 29 Oct, (b) 1500 UTC 29 Oct, and (c) 1700 UTC 29 Oct. Each figure shows the southerly component of wind velocity (filled contours, every 2 m s−1), potential temperature (solid black lines, every 1 K), rainwater mixing ratio (dotted line at [0, 0.01, 0.1, 0.5, and 1.0] g kg−1), graupel mixing ratio (thin dashed lines at [0, 0.01, 0.1, 0.5, and 1.0] g kg−1), and freezing level (thick dashed line). The “D” and “Y” represent the approximate location of Deniliquin to the north and Yarrawonga to the south, respectively.

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    Vertical profile from Control of (solid line, 10−6 s−1) measured at the asterisk in Fig. 10a at 1300 UTC 29 Oct with c = 10 m s−1, the wind velocity in the positive x direction of the cross section (dashed line, m s−1), and (dashed–dotted–dashed line, 10−4 s−1).

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    As in Fig. 10, but along C–D in Fig. 9a at (a) 1200 UTC 29 Oct, (b) 1400 UTC 29 Oct, and (c) 1600 UTC 29 Oct.

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    Vertical cross section from Control along C–D in Fig. 9a at 1400 UTC 29 Oct of water vapor mixing ratio (filled, every 0.5 g kg−1 from 4 g kg−1), potential temperature (solid black lines, every 1 K), and freezing level (thick dashed line).

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    Model output (domain 3) from Control at (a),(b) 1200 UTC 29 Oct; (c),(d) 1600 UTC 29 Oct; and (e),(f) 2000 UTC 29 Oct. (left) The 800-hPa mixing ratio (filled, every 1 g kg−1 from 4 g kg−1) and 800-hPa horizontal wind vectors, and (right) 800-hPa CAPE (filled, every 20 J kg−1) and the black dashed line indicates the location of the leading edge of the prefrontal trough. All figures show surface flow convergence (red regions indicate flow convergence 0.001 m s−1) and terrain contours every 200 m.

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    Vertical cross section from Control (domain 4) along E–F in Fig. 14f at 2000 UTC 29 Oct. The figure shows the westerly component of wind velocity (filled contours, every 2 m s−1), potential temperature (thin black lines, every 1 K), total cloud (cloud water plus all ice types), mixing ratio (thick black line at 0.1 g kg−1), and freezing level (dotted line). “PF” and “UB” indicate the easternmost extent of the prefrontal trough and undular bore, respectively. Note that the undulations in the potential temperature contours around x = 200 km are generated by storm outflow and not the undular bore, which decayed several hours earlier.

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    As in Fig. 9d, but for No-Plateau.

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    The location of the leading edge of the prefrontal trough/bore at 1200 UTC 29 Oct for the Control simulation (black with white stripe) and the 30 ensemble members (colored). The leading edge of the prefrontal trough/bore is approximated from surface flow convergence 0.001 m s−1. Model output from domain 3. Terrain contours are every 200 m. The asterisk indicates the location of the profiles shown in Fig. 18.

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    Vertical profile of (10−6 s−1) measured at the black asterisk in Fig. 17 at 1200 UTC 29 Oct for the Control simulation (black with white stripe) and the 30 ensemble members. Model output from domain 3.

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    (a) The 2-mm precipitation exceedance occurrence from the 30 member ensemble, where the filled contours indicate the number of ensemble members that accumulated at least 2 mm of precipitation during the accumulation period of PF-2 (1600–0100 UTC 29–30 Oct). (b) The 2-mm precipitation exceedance threshold area derived from Yarrawonga radar reflectivities (hatched) from 2100 to 0100 UTC 29–30 Oct. Model output in (a) is from domain 3. Terrain contours are every 200 m. Note that the band of yellow and orange on the left side of (a) is related to the approaching cold front, the location of which exhibited significant spread among ensemble members.

  • View in gallery

    At 1200 UTC 29 Oct the (a) 800-hPa mixing ratio ensemble mean (filled, every 1 g kg−1 from 4 g kg−1); (b) skew T–logp measured at the white asterisk in (a) for the Control simulation (black with white stripe) and the 30 ensemble members (gray); and the difference between the ensemble mean 800-hPa mixing ratio and the mean for (c) the successful six measured and (d) the failed eight at 1200 UTC 29 Oct (filled, every 0.5 g kg−1). The black dashed line in (a) indicates the average location of the leading edge of the prefrontal trough. The white box in (a) represents the area in which the southward WV flux is calculated (see text for details). Terrain contours are every 200 m. Model output is from domain 3.

  • View in gallery

    A summary of two environmental attributes important for PF-2 for each member of Ensemble where the prefrontal trough became stationary with respect to Control and the southward WVF (1012 kg s−1). The x axis is categorical (i.e., not continuous) and shows the approximate location of the prefrontal trough with respect to Control. The y axis is continuous and shows the southward WVF for each member calculated within the white box in Fig. 20a at 1200 UTC 29 Oct. The dashed horizontal line shows the WVF of Control for reference. Blue numbers indicate members of the successful six and orange numbers indicate members of the failed eight. See text for further details.

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A Case of an Undular Bore and Prefrontal Precipitation in the Australian Alps

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  • 1 IBM Research, T. J. Watson Research Center, Yorktown Heights, New York
  • 2 School of Earth Sciences, and ARC Centre of Excellence for Climate System Sciences, University of Melbourne, Melbourne, Victoria, Australia
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Abstract

This study explores the mesoscale processes that led to the development of two prefrontal precipitation events in the Australian Alps on 29–30 October 2010. The synoptic setting was characterized by the passage of an interacting front and prefrontal trough across southern Australia. Observations and model simulations revealed that when the prefrontal trough entered southeast Australia it resembled a density current advancing into a stable nocturnal layer, forming a bore at its leading edge. The bore detached from and propagated ahead of the prefrontal trough and became undular, supported by a wave-ducting mechanism. The undular bore was observed in the Doppler wind field of a radar, parts of which were collocated with bands of reflectivity. Strong winds coincident with this band of reflectivity suggest the undular bore triggered convection that eventually led to the bore’s demise. An ensemble of high-resolution model simulations (with perturbed initial and boundary conditions) was used to understand the key processes affecting the undular bore and two prefrontal precipitation events. While no member of the ensemble reproduced the first prefrontal precipitation event, at least six members (20%) reproduced parts of the second prefrontal precipitation event. Despite the low precipitation predictability, analysis of the ensemble suggests the undular bore was both a predictable phenomenon and integral to the initiation and/or evolution of the two prefrontal precipitation events.

Corresponding author address: Campbell D. Watson, IBM Research, T. J. Watson Research Center, 1101 Kitchawan Rd., Yorktown Heights, NY 10598. E-mail: cwatson@us.ibm.com

Abstract

This study explores the mesoscale processes that led to the development of two prefrontal precipitation events in the Australian Alps on 29–30 October 2010. The synoptic setting was characterized by the passage of an interacting front and prefrontal trough across southern Australia. Observations and model simulations revealed that when the prefrontal trough entered southeast Australia it resembled a density current advancing into a stable nocturnal layer, forming a bore at its leading edge. The bore detached from and propagated ahead of the prefrontal trough and became undular, supported by a wave-ducting mechanism. The undular bore was observed in the Doppler wind field of a radar, parts of which were collocated with bands of reflectivity. Strong winds coincident with this band of reflectivity suggest the undular bore triggered convection that eventually led to the bore’s demise. An ensemble of high-resolution model simulations (with perturbed initial and boundary conditions) was used to understand the key processes affecting the undular bore and two prefrontal precipitation events. While no member of the ensemble reproduced the first prefrontal precipitation event, at least six members (20%) reproduced parts of the second prefrontal precipitation event. Despite the low precipitation predictability, analysis of the ensemble suggests the undular bore was both a predictable phenomenon and integral to the initiation and/or evolution of the two prefrontal precipitation events.

Corresponding author address: Campbell D. Watson, IBM Research, T. J. Watson Research Center, 1101 Kitchawan Rd., Yorktown Heights, NY 10598. E-mail: cwatson@us.ibm.com

1. Introduction

The Australian Alps are the highest section of Australia’s Great Dividing Range. They span over 500 km across southeast (SE) Australia and comprise the only peaks on the continent that exceed 2 km (see Fig. 1 for a regional map). Precipitation in the Alps (and SE Australia in general) is largely controlled by the passage of synoptic weather systems from the west, especially cold fronts. The conceptual model of a cold front advancing eastward through SE Australia shows moisture in the prefrontal air mass, sourced from the Coral Sea off the northeast coast of Australia, advected southward over the Alps where part of it falls as precipitation [e.g., see Fig. 21 in Wilson and Stern (1985)]. Community understanding of cold fronts in Australia escalated from 1979 when the Australian Cold Fronts Research Program was initiated and Reeder and Smith (1992) provide a review of results from this program.

Fig. 1.
Fig. 1.

(a) The regional geography of Australia [from Qi et al. (2000)] and (b) the terrain of SE Australia (filled contours).

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

There are a number of synoptic types responsible for precipitation in the Alps. Chubb et al. (2011) found cutoff lows [cold-cored lows cut off from the midlatitude westerly airstream; Pook et al. (2006)] and embedded lows (low pressure systems centered south of 50°S within the midlatitude westerly airstream that create frontal bands extending over 1000 km into southern Australia) are the primary contributors to Alpine precipitation in the cooler months. Cold fronts that interact with cloud masses of low-latitude origin—so-called interacting fronts—are also significant contributors [especially in the warmer months; Landvogt et al. (2008)]. However, the synoptic systems responsible for large precipitation accumulations in the Alps are commonly of mixed types. For example, the existence of a cutoff low does not preclude the existence of a cold front (Chubb et al. 2011).

Prefrontal troughs are also sometimes observed migrating ahead of cold fronts in southern Australia, and when frontogenesis occurs within the prefrontal trough (generally at the expense of the original cold front), precipitation and a significant cool change may occur well before the arrival of the cold front (Hanstrum et al. 1990b; Reeder and Smith 1992; Schultz 2005). The accurate prediction of these complex systems are of high importance for precipitation forecasts, although there is often substantial uncertainty in the forecasted timing and impact of such systems, especially at the mesoscale (e.g., Reeder and Smith 1992; Engel et al. 2013). They can also cause significant societal impacts, demonstrated during the devastating Ash Wednesday bushfires in 1983 (Hanstrum et al. 1990a) and Black Saturday bushfires in 2009 (Engel et al. 2013) when the accurate forecast of an intensifying prefrontal trough was critical to firefighting efforts.

One aim of the present study is to explore the mesoscale processes leading to multiple prefrontal wind shifts and the generation of prefrontal precipitation in the Australian Alps. Despite their importance for forecasting during periods of fire weather and heavy rainfall, such processes have received limited attention at these scales in Australia. Unlike other parts of the world (e.g., the United States), Australia has nonoverlapping radar coverage and a limited network of surface and upper-air observations, which makes case studies challenging. Here, we use available observations and model simulations to investigate the development of two prefrontal precipitation events in the Australian Alps on 29–30 October 2010. The synoptic setting was characterized by the passage of an interacting front and prefrontal trough across southern Australia. Local observations showed an undular bore propagated ahead of the prefrontal trough along a nocturnal stable layer; the bore appeared to trigger convection and contribute to two distinct prefrontal precipitation events.

An atmospheric bore is a type of gravity wave disturbance that forms at the interface between two fluids—in this case, the cold front was acting as a gravity current advancing into a surface-based stable layer (the nocturnal inversion). A bore can bring about a sharp rise in pressure, a strong but in some cases temporary wind shift or surge, and frequently an abrupt rise in temperature through the downward mixing of potentially warm air by turbulence within the bore (e.g., Koch et al. 1991; Reeder and Smith 1992; Koch and Clark 1999). If the stable layer is sufficiently deep, the bore may propagate ahead of the cold front and become undular, whereby the bore is accompanied by a group of waves that radiate energy downstream (Clarke 1986; Haase and Smith 1989; Hartung et al. 2010). Such an occurrence implies the bore has been generated from a subcritical flow as the bore does not remain attached to its source (see, e.g., Engel et al. 2013).

In some cases, near-surface bores can trigger convection some distance from their original source, making their occurrence important to predict during periods of severe weather. This is more common at night when bores can propagate along a nocturnal inversion. For this reason, the development and evolution of nocturnal bores were a focus of the 2015 Plains Elevated Convection at Night field experiment in the central United States [see Geerts (2013)].

Perhaps the most well-known undular bore is the “Morning Glory” that forms over Cape York Peninsula and the Gulf of Carpentaria in northeastern Australia (see Smith 1988; Christie 1992; Smith and Reeder 2014). Although less frequent, undular bores have also been observed in southern Australia propagating ahead of cold fronts and prefrontal troughs using satellite (e.g., Clarke 1986; Reeder and Smith 1992) and surface-based weather stations (L. Davies 2015, personal communication). Schmidt and Goler (2010) successfully simulated several undular bores off the south coast of Australia using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) with a horizontal grid spacing of 1 km. Engel et al. (2013) successfully simulated two observed nocturnal bores generated by a cold front in SE Australia during the Black Saturday bushfires using the Met Office Unified Model with a horizontal grid spacing of down to 444 m. We find in the present study the bore is successfully simulated with a horizontal grid spacing of 3.33 km; however, a grid resolution of 1.11 km is required to resolve the observed undulations. [Note that the simulated bore by Engel et al. (2013) was solitary despite the observed undular structure; hence, horizontal grid spacing is not the solitary cause of a failure to resolve undulations.]

In northeastern Australia, the Morning Glory undular bore is said to form with some predictability and regularity (Smith 1988). Elsewhere in Australia, the predictability of undular bores is unknown. Given the nonlinear dynamics of a bore, we would expect it to exhibit intrinsic low predictability; however, in the presence of a predictable and supportive wave-ducting mechanism (e.g., a nocturnal stable layer), the predictability of an undular bore may be tied to that of the synoptic-scale disturbance from which it is generated (e.g., a prefrontal trough). A goal of this study is to obtain some insight into the forecast sensitivity of an undular bore along with the prefrontal wind shifts they induce.

Toward this end, we use an ensemble of model simulations to account for our uncertainty in initial conditions. Even with a perfect model and perfect observations, the atmosphere retains a predictability limit of just a few weeks due to its dynamic, chaotic nature (Lorenz 1965; Zhang et al. 2003; Rotunno and Snyder 2008; Zhang et al. 2009). Ensemble prediction systems have been designed to combat this loss of predictability by sampling the set of likely atmospheric states and integrating a set of simulations with perturbed initial conditions (or different/perturbed model physics) that represent the uncertainty in an analysis (e.g., Zhang et al. 2003; Rotunno and Snyder 2008; Zhang et al. 2009). Such a strategy accomplishes two things: it provides an ensemble average forecast that normally has better skill than a single (deterministic) forecast; and the ensemble spread provides valuable information about the uncertainty of a particular forecast (Kalnay 2003). A number of studies have demonstrated the benefit of applying some form of ensemble prediction system to cases where low predictability is an issue, both on the synoptic scale (e.g., Molteni et al. 1996; Houtekamer et al. 1996; Toth and Kalnay 1997) and at finer scales (e.g., Walser et al. 2004; Argence et al. 2008; Hohenegger et al. 2008; Leoncini et al. 2013). While such strategies have also been applied to study the physical mechanisms behind high-impact weather and its sensitivities to the larger-scale flow (e.g., Reinecke and Durran 2009; Hanley et al. 2011; Barrett et al. 2015), there have been limited studies of this type on precipitating systems in SE Australia.

Thus, the aims of this study are to explore the dynamics and key processes underlying a precipitation event in SE Australia, including the roles of an undular bore and the terrain. The dynamics and predictability are explored using a set of ensemble mesoscale simulations.

The remainder of the manuscript is organized as follows. Available observations are used to reconstruct the case study, providing evidence of an undular bore propagating ahead of a prefrontal trough and triggering convection. Details of two distinct prefrontal precipitation events are also presented. Results from a series of NWP model simulations are then presented to provide insight into the formation and propagation characteristics of the undular bore and its possible role in triggering and supporting the two prefrontal precipitation events. The model simulations reveal a complex series of processes at play, elucidated by an ensemble of model simulations that also provide insight into the predictability of the important mesoscale constituents.

2. Case overview

a. Synoptic evolution of the event

Figures 2a–c show the mean sea level pressure analysis during 28–30 October 2010 from the Australian Bureau of Meteorology (BOM). At 1200 UTC 28 October (all times reported in UTC = local daylight savings time − 11 h). A low pressure system centered around 60°S (hereafter called the parent low) produced an eastward-moving cold front that reached the southern coast of Western Australia (WA; Fig. 2a). Similar to the case study presented by Hanstrum et al. (1990b), the accompanying flow appeared to distort a stationary surface heat trough embedded in weak low-level flow over WA, and the heat trough was subsequently aligned as a prefrontal trough migrating ahead of the cold front. The cold front and prefrontal trough created an uninterrupted band of cloud between the midlatitudes and the tropics (not shown), which at face value suggests interaction between the frontal cloud band and cloud mass of tropical origin [i.e., an interacting front as defined by Wright (1989)].

Fig. 2.
Fig. 2.

Mean sea level pressure analysis from the Bureau of Meteorology, valid at (a) 1200 UTC 28 Oct, (b) 1200 UTC 29 Oct, and (c) 0600 UTC 30 Oct 2010. (d) The AWAP 48-h accumulated precipitation from 1200 UTC 28 Oct (filled, every 10 mm) for SE Australia [the region in the black dashed box in (a), see Fig. 8b for a detailed map of topography in the region]. Approximate position of the Alpine crest is shown in orange. Initials represent the following weather stations: B (Mount Buller), Ch (Charlton), D (Deniliquin), Gr (Griffith), Ho (Mount Hotham), Me (Melbourne), Mi (Mildura), Wa (Wagga Wagga), and Yw (Yarrawonga). Additional weather stations used in the analysis are represented by the gray asterisks.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

Around 0000 UTC 29 October, an anticyclone south of New Zealand became quasi stationary, creating a blocking scenario (see Coughlan 1983). The BOM analysis suggests local cyclogenesis subsequently occurred southwest of Victoria, Australia (VIC), which produced a cutoff low that became progressively more detached from the westerly airstream across the Southern Ocean, fracturing the cold front (Fig. 2b). Over the following 24 h, the blocking high appeared to slow the movement of the cutoff low and cold front across SE Australia (Fig. 2c), eventually forcing the cutoff low to change direction and move south over Tasmania (TAS). The cold front weakened to a surface trough and passed over the Alps around 0000 UTC 31 October.

The observed 48-h accumulated precipitation during the event is shown in Fig. 2d, courtesy of the Australian Water Availability Project (AWAP) dataset (Jones et al. 2009). Precipitation accumulated predominantly on the northwestern slopes of the Australian Alps (the orange line approximates the Alpine crest). Almost 150 mm of precipitation was observed at Mt. Hotham and over 50 mm on the flat plains northwest of the Alps. Several moderate flood warnings were declared in the ensuing days for rivers in the Alpine region and the city of Melbourne, Australia.

b. Observational data

The BOM operate a number of automatic weather stations (AWS) across SE Australia (see Fig. 2d for station locations). They provide surface-based observations at 1-, 30-, or 60-min resolution of 2-m pressure, temperature, and humidity; 10-m wind direction and wind speed; and accumulated precipitation. A WSR 81C C-band radar is located at Yarrawonga, Australia, around 150 km northwest of the VIC Alps, and an older WF 100 C-band radar is located at Wagga Wagga, Australia. These radars provide spatial and temporal information of radar reflectivity and, at Yarrawonga, Doppler wind fields. The algorithm described in Chumchean et al. (2006) is used to estimate rainfall from the Yarrawonga radar reflectivity measurements. Radiosonde soundings are recorded daily (0000 UTC) at Wagga Wagga. Note that the focus of this case study falls between 1500 and 0100 UTC 29–30 October and the undular bore was observed near Yarrawonga around 1500 UTC. The Wagga Wagga sounding is, therefore, of limited use in this study.

c. Mesoscale evolution of the prefrontal environment

1) Development of the undular bore

The nocturnal passage of the prefrontal trough across SE Australia was first observed in Mildura (0800 UTC) followed by Charlton (1100 UTC) and Deniliquin (1530 UTC; Fig. 3). The Mildura AWS in western VIC observed a sharp 5°C drop in temperature, a pressure jump of nearly 2 hPa (with a further 3-hPa rise over the following 3 h) and a wind shift from north to southwest (Fig. 3a). There was also a small amount of precipitation (the asterisks in Fig. 3 indicate precipitation 0.2 mm min−1), which likely contributed to the sharp temperature drop in Mildura through evaporative cooling. The Charlton AWS (Fig. 3b) observed the prefrontal trough 3 hours later with a southwest wind shift and a temperature drop of 4°C. In Deniliquin (Fig. 3c), the observed surface changes were more subtle when the prefrontal trough arrived (the wind shifted west and the temperature drop was more gradual). The notable difference in the structure of the prefrontal change is related to the sunset at 0900 UTC and the growth of the nocturnal stable layer, and the synoptic blocking pattern that likely weakened the prefrontal trough as it moved across SE Australia.

Fig. 3.
Fig. 3.

The 1-min observations at (a) Mildura, (b) Charlton, (c) Deniliquin, and (d) Griffith. Observations at each AWS are of (top) 10-m wind direction (black, °) and 10-m wind speed (orange, m s−1); and (bottom) surface pressure (black, hPa), 2-m temperature (orange, °C), and precipitation 0.2 mm min−1 (asterisk) along the ordinate. Note the different abscissa scales: (a),(b) are for 24 h from 0000 UTC 29 Oct and (c),(d) are for 12 h from 1002 UTC 29 Oct.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

At 1010 UTC in Charlton, around 50 min before the prefrontal trough was observed at the same location, the AWS recorded a sharp jump in pressure (1 hPa) and temperature (1.5°C). The temperature rise persisted only until the prefrontal trough arrived, whereas the pressure jump was more permanent. This signal is typical of a bore [e.g., see Hartung et al. (2010), their Figs. 6 and 7; and Engel et al. (2013), their Fig. 3]. It will be shown with model simulations that the prefrontal trough advanced into a stable nocturnal inversion, producing a sudden and quasi-permanent increase in the depth of the stable layer (hence increased surface pressure) and mixing of potentially warmer air from above the inversion in the downdraft of the bore (hence a temporary increase in surface temperature).

At 1350 UTC in Deniliquin, around 2 h before the prefrontal trough was observed at the same location, the AWS observed a sharp 1.8-hPa pressure jump followed by several oscillations in the pressure and wind fields, and a temporary rise in temperature. This signal is typical of an undular bore (at least three undulations are evident in the pressure observations) propagating far ahead of the prefrontal trough.

Two important attributes of the upstream environment can be inferred from the development of the undular bore after sunset: 1) the stable layer is sufficiently deep to allow the bore to propagate ahead of the prefrontal trough, creating a double wind change; and 2) a wave ducting mechanism is present. These attributes will be explored later using model simulations.

The Yarrawonga radar observed the undular bore in the Doppler wind field, propagating at approximately 10 m s−1 in an east-southeast direction (between the black lines in Fig. 4a). A band of light-to-moderate radar reflectivity was collocated with parts of the undular bore (where moderate reflectivity is approaching 40 dBZ; Figs. 4b,c), implying the undular bore provided sufficient uplift to create cloud. Around 1540 UTC 30 October, the Doppler wind field showed strong, multidirectional winds north of Yarrawonga and within the next 20 min, the undular bore signature had vanished. It, therefore, appears the undular bore also triggered convection as the associated strong winds disrupted the structure of the undular bore, leading to its demise.

Fig. 4.
Fig. 4.

Observations from the Yarrawonga radar of the undular bore, indicated by solid lines, propagating in the direction of the arrows. (a),(b) The Doppler wind field and (c) radar reflectivity.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

There is further evidence the undular bore triggered convection provided by the Griffith, Australia, AWS and the Wagga Wagga radar. The Griffith AWS recorded a sudden change in surface conditions at 1455 UTC 29 October indicative of a gravity current generated by storm outflow (Fig. 3d) along with 2 mm of precipitation. Concurrently, the Wagga Wagga radar observed a distinct band of reflectivity moving over Griffith in an east-southeast direction (Fig. 5). This was around the same time the undular bore was observed in Yarrawonga, suggesting the undular bore had also triggered convection farther north. This is an important discovery as, to our knowledge, an undular bore triggering convection in SE Australia has never been documented in the literature.

Fig. 5.
Fig. 5.

Observations from the Wagga Wagga radar of radar reflectivity associated with the undular bore, indicated by solid lines, propagating in the direction of the arrows through Griffith.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

Of note, Griffith is nestled on the western edge of an elevated plateau (with a maximum height of 500 m) that extends north from the VIC Alps into northern New South Wales (see Fig. 1b). This topography may have also played a role in the development of convection and is considered in our model experiment design (see the next section).

Finally, the prefrontal trough halted its eastward advance shortly after passing through Deniliquin (at approximately 1630 UTC 29 October). Surface winds remained northeast at all stations east of Deniliquin (including Griffith) until the passage of the cold front more than 12 h later.

2) Two prefrontal precipitation events: PF-1 and PF-2

The Yarrawonga and Wagga Wagga radars observed two distinct precipitating systems develop within the prefrontal environment. The first event, hereafter called PF-1, formed 50 km north of Deniliquin around 1600 UTC 29 October during the passage of the undular bore. A region of light-to-moderate reflectivity moved southeast through Yarrawonga and toward the Alps, broadening and intensifying over time (e.g., at 2000 UTC 29 October; Fig. 6a). The radar-derived rainfall distribution from 1900 to 2100 UTC 29 October is shown in Fig. 6b. Accumulations were modest and Mt. Buller (elevation of 1707 m) only recorded 8 mm of precipitation during this period.

Fig. 6.
Fig. 6.

Observations from the Yarrawonga radar of the two prefrontal precipitation events: PF-1 and PF-2. They show the radar reflectivity at (a) 2000 UTC 29 Oct and (c) 2330 UTC 29 Oct, and radar-derived rainfall accumulations (mm) during (b) 1800–2100 UTC 29 Oct and (d) 2100–0100 UTC 29–30 Oct. The dashed arrows indicate the direction of system propagation. The black circles show in (a),(c) the 128-km range radius of the radar. Terrain contours in (b),(d) every 200 m.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

The origin of the second event, hereafter called PF-2, is less clear. From 1900 to 2330 UTC 29 October, the Yarrawonga and Wagga Wagga radars observed patchy regions of light reflectivity moving southward from Griffith toward the Alps (note that Griffith is on the cusp of the reliable range for both radars). From 2100 UTC 29 October, a region of reflectivity intensified northeast of Wagga Wagga (Fig. 7) and ultimately developed into a pronounced precipitating system west of Wagga Wagga (e.g., Fig. 6c). Unfortunately, the Wagga Wagga radar failed to archive reflectivity returns from 2220 UTC 29 October, though the Yarrawonga radar persisted and the radar-derived rainfall from 2100 to 0100 UTC 29 October (the duration of PF-2) is shown in Fig. 6c.

Fig. 7.
Fig. 7.

Observations from the Wagga Wagga radar of radar reflectivity showing the development of the second prefrontal precipitation event, PF-2.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

While PF-1 appeared to be initiated by the bore around 1600 UTC 29 October near Deniliquin, the origin of PF-2 is less clear. Observations suggest PF-2 originated north of Griffith around the same time as PF-1, perhaps triggered by the undular bore, the prefrontal trough, orographic uplift provided by the elevated plateau, or a combination of these mechanisms. Overall, the radar-derived rainfall accumulations show ~15 mm of precipitation accumulated on the northwestern slopes of the Alps during PF-1 and PF-2 (Figs. 6b,d). For reference, the Mt. Buller AWS observed over 10 mm of precipitation during this period.

3. Model configuration

The observations provide strong evidence that an undular bore propagated ahead of a prefrontal trough and triggered convection. The development and evolution of two prefrontal precipitation events appears related to the undular bore, although many details of the relationship between the bore and the prefrontal precipitation are unknown. The successful simulation of undular bores in previous studies (e.g., Hartung et al. 2010; Engel et al. 2013) motivates the use of mesoscale model simulations to investigate the formation and propagation of the undular bore, the possible triggering mechanisms of PF-1 and PF-2, and the associated predictability.

a. The WRF Model

Version 3.3 of the Advanced Research version of the Weather Research and Forecasting (WRF) Model (ARW; Skamarock et al. 2008) is used to simulate this case. Two sets of simulations are presented in this study. The first set—Control and No-Plateau—use the four domains shown in Fig. 8a. The second set—Ensembles—use the three outer domains only. Control is used to examine the structure of the event and No-Plateau is used to determine the role of the terrain in the initiation of PF-2. The Ensembles are used to determine the predictability of the events and also identify the key role of moisture advection.

Fig. 8.
Fig. 8.

(a) Nest 1 (with 30-km grid spacing) with boxes marking locations of nest 2 (10 km), nest 3 (3.33 km), and nest 4 (1.11 km). (b) Topography of the Australian Alps in Control (km) and (c) modified topography in No-Plateau. The dashed box in (b),(c) shows nest 4, and the solid box in (c) indicates where the topography has been modified for No-Plateau, as described in the appendix.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

The horizontal grid spacing of the outer and three inner (one way) nests are 30, 10, 3.33, and 1.11 km. Each domain has 64 vertical levels, a 50-hPa model top, and a 5-km-deep upper-level absorbing layer. The vertical coordinate is terrain following with a stretched vertical grid spacing of 40 m near the surface and 400 m near the model top. Note that model output is from the innermost domain unless otherwise stated, and surface values are derived from the first σ level (approximately 20 m above ground level) assuming a logarithmic surface layer.

The WRF Model is nonhydrostatic, nonlinear, and fully compressible with a third-order Runge–Kutta scheme for time integration. The physics packages common to all simulations include the Rapid Radiative Transfer Model (RRTM; Mlawer et al. 1997), the Goddard shortwave radiation scheme (Chou and Suarez 1994), the Noah land surface model (Chen and Dudhia 2001), and the Mellor–Yamada–Janjić 2.5-level boundary layer scheme (Mellor and Yamada 1982; Janjić 2001). In domains 1 and 2, subgrid-scale cumulus convection is parameterized by the Betts–Miller–Janjić scheme (Betts and Miller 1986); convective processes are represented explicitly in domains 3 and 4. Microphysics is parameterized using the WRF single-moment 6-class bulk microphysics scheme (WSM6; Hong et al. 2004). For comparison, additional simulations were performed with the Purdue–Lin microphysics scheme (Lin et al. 1983; Rutledge and Hobbs 1984). Some differences were present between the simulations (not shown); however, they were not substantial enough to warrant further investigation. The outer domain of all simulations receives initial and boundary conditions from the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analysis data. The GFS model has a T574 resolution, which is roughly equivalent to a horizontal grid resolution of 25 km; the data were available on a 1.0° × 1.0° grid every 6 h and interpolated in time to provide the boundary conditions for the model. All simulations are initialized at 1200 UTC 27 October. This long lead time (~24 h) was chosen to allow sufficient time for the boundary layer to evolve and simulations with shorter lead time did not perform as well (see the next section).

b. Description of model experiments

Control and No-Plateau are identical to each other except for topography (Figs. 8b,c). No-Plateau removes the elevated plateau that extends north from the VIC Alps (see the appendix) to explore the role of terrain in the decay of the undular bore and the evolution of PF-2.

Ensemble comprises 30 members with initial and boundary conditions perturbed from those used in Control. An ensemble size of 30 is computationally affordable and considered reasonable to estimate the forecast spread (Houtekamer and Mitchell 2001; Snyder and Zhang 2003; Zhang et al. 2009). Following Zhang et al. (2009), the ensemble perturbations are generated using the cv3 background error covariance option in WRF’s three-dimensional variational data assimilation (3DVAR) system (Barker et al. 2004). To create the largely balanced perturbation, a set of random control vectors with normal distribution is created; the control increment vector is then transformed back to model space via an EOF transform, a recursive filter, and physical transformation via balanced equations. The only perturbed variables are pressure, horizontal winds, potential temperature, and water vapor mixing ratio, with standard deviations of approximately 1.0 hPa, 1.1 m s−1, 0.9 K, and 0.6 g kg−1, respectively. The boundary conditions are perturbed in the same manner. These values were guided by a time-lagged ensemble that was also examined in the early stages of this study.

Only three domains are used in Ensemble. While not shown here, a detailed comparison of the flow and precipitation fields in Control between domain 3 (dx = 3.33 km) and domain 4 (dx = 1.11 km) show only minor differences despite the absence of feedback from the fine to coarse domains. For example, both domain 3 and 4 simulate a bore and its propagation ahead of the prefrontal trough. The speed, amplitude, and location of the bore are similar in each domain; however, it only develops undulations in the finer resolution domain 4 (not shown).

Despite being initialized almost 36 h before the prefrontal trough entered SE Australia, the two sets of simulations examined in this study reproduced the analyzed synoptic-scale features soundly. An additional set of simulations were performed using model initialization times 12 and 24 h after Control (called T12 and T24, respectively). We note the prefrontal flow dynamics of T24 were distinctly different to Control and T12, with the location and strength of the prefrontal trough substantially influencing bore and prefrontal precipitation development. Based on available observations, Control reproduced the event with the highest accuracy. In consideration of length, these results are not included in this manuscript [see Watson (2013) for a detailed analysis].

4. Model results

The prefrontal environment—Control

1) Evolution of the prefrontal trough and undular bore

The simulated advance of the prefrontal trough across SE Australia in Control was generally consistent with surface observations, including the development of a bore at its leading edge before passing through Charlton. The simulated prefrontal trough arrived around 30 min earlier than observed in Mildura and Charlton, and 2 h earlier in Deniliquin. It became stationary west of Yarrawonga and Griffith, as observed.

At 1000 UTC 29 October (Fig. 9a), the prefrontal trough, identifiable by the region of westerly surface winds, developed a strong band of surface flow convergence at its leading edge (the red, north–south-oriented line). This flow convergence formed when the colder and denser prefrontal trough air advanced into the stable nocturnal layer over continental SE Australia, vertically displacing the stable air near the surface to create a large-amplitude wave characteristic of a bore. Over the next 5 h, the bore detached from and propagated ahead of the prefrontal trough and became undular. This is illustrated by the narrow bands of surface flow convergence detached from the upstream edge of the wind shift at 1500 UTC 29 October (Fig. 9b), and the undulations in the vertical cross section of potential temperature through the bore (Fig. 10b). Assuming the simulated bore is a realistic representation of the observed situation, it had a horizontal extent of at least 1000 km—extending northward from central VIC (based on flow fields from domain 3).

Fig. 9.
Fig. 9.

Results from the Control simulation at (a) 1000 UTC 29 Oct, (b) 1500 UTC 29 Oct, and (c) 2000 UTC 29 Oct of surface flow convergence (red indicates flow convergence 0.001 m s−1); surface wind vectors; and accumulated precipitation from the preceding hour (filled in mm). (d) Accumulated precipitation (mm) from Control for 1800–0100 UTC 29–30 Oct on the same projection as Fig. 6d. Terrain contours are every 200 m.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

Fig. 10.
Fig. 10.

Vertical cross section of the undular bore in Control along A–B in Fig. 9a at (a) 1300 UTC 29 Oct, (b) 1500 UTC 29 Oct, and (c) 1700 UTC 29 Oct. Each figure shows the southerly component of wind velocity (filled contours, every 2 m s−1), potential temperature (solid black lines, every 1 K), rainwater mixing ratio (dotted line at [0, 0.01, 0.1, 0.5, and 1.0] g kg−1), graupel mixing ratio (thin dashed lines at [0, 0.01, 0.1, 0.5, and 1.0] g kg−1), and freezing level (thick dashed line). The “D” and “Y” represent the approximate location of Deniliquin to the north and Yarrawonga to the south, respectively.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

Figure 10 shows three vertical cross sections of the undular bore near Yarrawonga (the line A–B in Fig. 9b). At 1300 UTC 29 October, the bore was located around 15 km upstream of the leading edge of the prefrontal trough (southerly flow is shown in green) and induced a pronounced 1.5-km vertical displacement of the low-level flow. Over the next 2 h, the bore propagated ahead of the prefrontal trough with an average speed of about c = 10 m s−1, developing three distinct undulations. This is remarkably consistent with observations (cf. Fig. 3b).

The presence of a wave-ducting mechanism is necessary for bore propagation over a significant distance (Crook 1988). Wave ducts may form below or between layers that can reflect incident, vertically propagating wave energy. The ground surface frequently acts as the bottom wave reflector; hence, only a top reflector is needed for ducting (as is the case here). A reflective layer is characterized by 0, where m is the vertical wavenumber, overlaying a region of 0 (Crook 1988). The Scorer parameter (Scorer 1949) is related to wave ducting by , where k is the horizontal wavenumber and here we use
e1
Here, is the Brunt–Väisälä frequency [as defined by Lalas and Einaudi (1974)], U is the ambient wind velocity in the direction of bore motion, and c is the bore speed. For the present study, the horizontal length scale of the examined bore wave is sufficiently large ( 10 km) to render negligible in the calculation of . Therefore, 0 is achieved when 0; this implies a layer in which the atmospheric stability is either small or negative and/or significant curvature in the wave-normal wind profile exists (e.g., a low-level jet that opposes the wave motion) may be sufficient to form a reflective layer.

The vertical profile of in the upstream environment (the asterisk in Fig. 10a) at 1300 UTC 29 October shows two layers of 0 around 0.8 and 1.7 km above ground level (AGL; Fig. 11). The vertical wind profile in the direction of bore propagation and the atmospheric stability () are also included. As gravity waves are evanescent (decay exponentially with height) within layers of 0, and wave energy incident on such layers is reflected, this indicates that a near-surface wave duct is present. Wave ducting exists in this environment because of three important attributes: 1) a nocturnal stable layer capped by a nearly neutral remnant mixed layer from the previous day, 2) a vertical wind profile characterized by a low-level jet that opposes the wave motion around 400 m AGL, and 3) negative wind curvature ( 0) above the jet. Above these evanescent layers, a wave critical level (U − c = 0) exists at around 3 km; this critical level is probably of secondary importance for the creation of the duct because of the underlying layers with 0.

Fig. 11.
Fig. 11.

Vertical profile from Control of (solid line, 10−6 s−1) measured at the asterisk in Fig. 10a at 1300 UTC 29 Oct with c = 10 m s−1, the wind velocity in the positive x direction of the cross section (dashed line, m s−1), and (dashed–dotted–dashed line, 10−4 s−1).

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

From 1600 to 1800 UTC 29 October, the simulated undular bore lost its coherent structure along its entire length (Fig. 9c). Its decay was not related to the removal of the nocturnal inversion by diurnal heating (sunrise was at 1900 UTC), nor was it related to storm outflow (convection only developed near Griffith). Rather, the simulated undular bore slowed as it approached the elevated plateau and gradually lost its coherent structure (Fig. 10c).

The decreased phase speed of the bore is consistent with hydraulic theory: the upward-sloping surface decreases the depth of the stable layer, which in turn decreases the height of the wave duct and therefore the bore speed. However, results from No-Plateau show the undular bore still decays across its entire length at a similar time and location when the elevated plateau is removed (not shown).

Given the two layers of 0 are relatively shallow, we believe the wave duct was probably leaky, allowing the amplitude of the undular bore to reduce over time. As the phase speed of the undular bore is directly related to its amplitude [i.e., it is a nonlinear wave, see Nappo (2002)], a leaky wave duct would cause the undular bore to progressively slow and ultimately dissipate. As discussed above, the reduction in amplitude and phase speed of the undular bore over time is seen in both Control (e.g., Fig. 10) and No-Plateau, and appears consistent with the Doppler wind observations south of Yarrawonga (cf. Fig. 4).

2) Lack of simulated PF-1

The Yarrawonga radar observed the first prefrontal precipitation event, PF-1, to be initiated north of Deniliquin coincident with the passage of the undular bore. This precipitation event was not reproduced by Control (or No-Plateau). In Control, the simulated bore passed through Deniliquin at 1200 UTC 29 October (around 1.5 h earlier than observed) and the prefrontal trough followed at 1320 UTC 29 October (around 2.5 h earlier than observed). No convection or rainwater was generated during this time in the model (Figs. 10a,b).

A vertical profile of the atmosphere near Yarrawonga in Control (not shown) reveals the simulated bore was propagating into a surface-based stable layer (nocturnal inversion) to 500 m, a conditionally unstable layer from 500 to 1500 m (with convective inhibition of 100–200 J kg−1), and stable air aloft. In the hours following the passage of the bore, the lower free troposphere progressively moistened where PF-1 was predicted to have been triggered, reducing the convective inhibition.

It is, therefore, possible that the failure of Control to reproduce PF-1 was related to the premature passage of the bore and prefrontal trough (by around 2 h) into an environment not supportive of convection. However, it is also possible Control failed to reproduce PF-1 because of one or more of the following: 1) Control had a negative moisture bias in the lower free troposphere, 2) Control had an overly stable temperature profile in the lower free troposphere, and 3) the 1.11-km grid spacing in Control did not properly represent the sharp vertical motion at the head of the bore required to trigger convection [as was the case in Warren et al. (2014), who needed a grid spacing of 500 m to properly represent the vertical forcing along an observed convergence line in their simulation of a quasi-stationary convective system]. Unfortunately, no profiler or sounding observations were available at the time and location of PF-1 to verify these results.

3) The initiation and evolution of PF-2

Features of the second prefrontal precipitation event, PF-2, were reproduced by Control. Convection was triggered 50–100 km north of Griffith around 1530 UTC 29 October and over the following hours, the convection developed into an organized system that moved parallel to the 700-hPa north-northwest flow toward the Alps. The accumulated precipitation from the simulated PF-2 (Fig. 9d) resembles that derived from the Yarrawonga radar (Fig. 6d).

Figure 12 presents three vertical cross-sections along the line C–D in Fig. 9a to illustrate the initiation of PF-2 in Control. The bore propagates ahead of the prefrontal trough at about c = 10 m s−1 into an environment featuring a shallow wave duct from 0.6 km AGL (not shown) and becomes undular. The first wave of the undular bore lifts low-level air around 500 m, and further displacements by successive waves through the duct create a total vertical displacement of around 1 km. The undular bore also transports moisture upward through its entire depth (Fig. 13) as in Hartung et al. (2010), which we will argue is important for supporting the convective organization critical to PF-2.

Fig. 12.
Fig. 12.

As in Fig. 10, but along C–D in Fig. 9a at (a) 1200 UTC 29 Oct, (b) 1400 UTC 29 Oct, and (c) 1600 UTC 29 Oct.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

Fig. 13.
Fig. 13.

Vertical cross section from Control along C–D in Fig. 9a at 1400 UTC 29 Oct of water vapor mixing ratio (filled, every 0.5 g kg−1 from 4 g kg−1), potential temperature (solid black lines, every 1 K), and freezing level (thick dashed line).

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

Figure 14 presents the evolution of PF-2 using the 800-hPa mixing ratio and convective available potential energy for a parcel lifted from 800 hPa (CAPE800 hPa). We focus on 800-hPa fields because the nocturnal surface inversion east of the prefrontal trough ensures any convection triggered would be elevated (see, e.g., Parker 2008). Figure 14 illustrates that PF-2 was initiated north of Griffith when the undular bore propagated into a moist air mass (Fig. 12c). This southward moving moist air had a higher CAPE800 hPa than its surrounds, highlighting the strong connection between the moist air mass and the potential strength of convection. Convection was initially weak and shallow and less than 1 mm of precipitation accumulated. However, as the moist air mass moved south toward the Alps, convection intensified and it developed into an organized system, eventually generating over 12 mm of precipitation on the windward slopes of the Alps.

Fig. 14.
Fig. 14.

Model output (domain 3) from Control at (a),(b) 1200 UTC 29 Oct; (c),(d) 1600 UTC 29 Oct; and (e),(f) 2000 UTC 29 Oct. (left) The 800-hPa mixing ratio (filled, every 1 g kg−1 from 4 g kg−1) and 800-hPa horizontal wind vectors, and (right) 800-hPa CAPE (filled, every 20 J kg−1) and the black dashed line indicates the location of the leading edge of the prefrontal trough. All figures show surface flow convergence (red regions indicate flow convergence 0.001 m s−1) and terrain contours every 200 m.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

A vertical cross section through the convective system at 2000 UTC 29 October reveals its organized structure (Fig. 15). The easterly wind anomalies also highlight that the inflow to the storm originates at about 2 km AGL, confirming its elevated nature. The location of the leading edge of the prefrontal trough (PF), which had become stationary several hours earlier, and the maximum eastward extent of the undular bore (UB) are indicated on Fig. 15. This highlights the location of convective organization: east of the now-stationary prefrontal trough and within the environment modified by the undular bore. It also emphasizes two possibly important aspects of the event: 1) the role of the undular bore in preconditioning the environment south of the moist air mass to support convective organization (such occurrences have been previously detailed by Koch et al. 2008, among others), and 2) the negative influence of the more stable prefrontal trough air on convective organization.

Fig. 15.
Fig. 15.

Vertical cross section from Control (domain 4) along E–F in Fig. 14f at 2000 UTC 29 Oct. The figure shows the westerly component of wind velocity (filled contours, every 2 m s−1), potential temperature (thin black lines, every 1 K), total cloud (cloud water plus all ice types), mixing ratio (thick black line at 0.1 g kg−1), and freezing level (dotted line). “PF” and “UB” indicate the easternmost extent of the prefrontal trough and undular bore, respectively. Note that the undulations in the potential temperature contours around x = 200 km are generated by storm outflow and not the undular bore, which decayed several hours earlier.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

Finally, we note that the convection triggered north of Griffith was near the western edge of the elevated plateau. The topographic forcing provided by the elevated plateau could plausibly play a role in the initiation and/or evolution of PF-2. However, results from No-Plateau show the elevated plateau has no appreciable influence on PF-2, with very similar precipitation accumulations simulated compared to Control (Fig. 16).

Fig. 16.
Fig. 16.

As in Fig. 9d, but for No-Plateau.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

5. Ensemble results: Sensitivity to initial condition perturbations

A 30-member ensemble of simulations (Ensemble) is used to assess the impact of initial condition uncertainties on the simulation of the undular bore and the two prefrontal precipitation events, and help understand the key processes affecting the important mesoscale constituents of the case study. Recall, the Ensemble simulations have small perturbations applied to the initial and boundary conditions of Control, and for computational efficiency they have only three domains. All members produced qualitatively similar synoptic conditions, albeit with some difference in the spatial and temporal evolution of the prefrontal trough and bore.

a. Ensemble evolution of the prefrontal trough and bore

When the prefrontal trough intensified in SE Australia from 0000 UTC 29 October, differences in its location and structure grew rapidly between members of the ensemble. By 1200 UTC 29 October, a bore had formed at the leading edge of the prefrontal trough in every member, as illustrated in Fig. 17. Roughly 75 km separated the westernmost and easternmost locations of the prefrontal trough at 1200 UTC 29 October; assuming the trough was advancing eastward at 8–10 m s−1 (the average speed from 1000 to 1400 UTC 29 October of all members), this distance translates to a maximum time difference (i.e., model spread) of about 2.5 h.

Fig. 17.
Fig. 17.

The location of the leading edge of the prefrontal trough/bore at 1200 UTC 29 Oct for the Control simulation (black with white stripe) and the 30 ensemble members (colored). The leading edge of the prefrontal trough/bore is approximated from surface flow convergence 0.001 m s−1. Model output from domain 3. Terrain contours are every 200 m. The asterisk indicates the location of the profiles shown in Fig. 18.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

Despite the spread in location and structure of the bore at 1200 UTC 29 October, the low-level environment east of the prefrontal trough in each member was supportive of bore development. Figure 18 presents for all members, measured east of the prefrontal trough (at the asterisk in Fig. 17) at 1200 UTC 29 October: a layer of 0 existed between 0.5 and 1 km in every simulation. Over time, each member simulated the bore detaching from and propagating ahead of the prefrontal trough. This implies that, despite the bore being a fundamentally nonlinear phenomenon, its occurrence is predictable (in this case) because the environmental conditions that support its formation and evolution are insensitive to small changes in the initial conditions.

Fig. 18.
Fig. 18.

Vertical profile of (10−6 s−1) measured at the black asterisk in Fig. 17 at 1200 UTC 29 Oct for the Control simulation (black with white stripe) and the 30 ensemble members. Model output from domain 3.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

While the bore in Control spanned uninterrupted for hundreds of kilometers across continental Australia (see the black and white line in Fig. 17), it was broken in around half the members of Ensemble (e.g., the westernmost bore in Fig. 17). These disruptions were caused by local surface pressure minima that formed within the prefrontal trough as it intensified in SE Australia (not shown). This modified the local, low-level flow field of the prefrontal trough and disrupted bore formation at its leading edge.

Finally, the location at which the prefrontal trough became stationary varied by up to 200 km across the Ensemble (not shown). This spread is important because, as discussed in the following sections, we hypothesize the prefrontal trough can disrupt the evolution of PF-2 if it advances too far east.

b. The predictability of the two prefrontal precipitation events

As with Control, no members of Ensemble simulated PF-1. Previously, we suggested the failure of Control to reproduce PF-1 was related to either the premature arrival of the bore in Deniliquin (by 1.5 h), a negative moisture bias, an overly stable temperature profile, or the poor representation of the bore head. Given some members of Ensemble simulated the bore to arrive in Deniliquin around 1 hour later than in Control (and therefore closer to the observed time) yet they still failed to generate any cloud or trigger convection, this provides some support for the notion that a systematic error in the model prevents every simulation from reproducing PF-1. If this is the case, then a larger ensemble is unlikely to address the issue. We also recognize that an ensemble of any size (as it is presently constructed) cannot account for different error structures in the large-scale GFS analysis driving the simulations because the perturbations are added according to an assumed static background error covariance matrix (cv3; see section 3). A more seamless ensemble prediction system would likely do better at representing these different error structures. PF-1 will not be considered further in this paper.

For PF-2, only six members produced a precipitation distribution comparable to that of Control (and observed by the Yarrawonga radar) during the PF-2 accumulation period (1600–0100 UTC 29–30 October). They are hereafter called the “successful six.” In contrast, eight members failed to produce more than 1 mm of precipitation upstream of or within the Alps during this period (the “failed eight”).

Figure 19a presents a map of the 2-mm precipitation exceedance occurrence, where the filled contours indicate the number of ensemble members that generated at least 2 mm of precipitation during the accumulation period of PF-2 (dark blue equals two members). Precipitation upstream of the Alps was clearly very sensitive to the small perturbations imposed on the initial conditions, and compares unfavorably to the 2-mm precipitation exceedance threshold for PF-2 derived from the Yarrawonga radar (Fig. 19b). However, we note there was also little agreement upstream of the Alps between the successful six, likely exaggerated by the pointwise comparison at relatively high horizontal resolution (dx = 3.33 km). In the Alps, where vertical motions from local topographic features play a more dominant role in precipitation generation, there was better agreement between members.

Fig. 19.
Fig. 19.

(a) The 2-mm precipitation exceedance occurrence from the 30 member ensemble, where the filled contours indicate the number of ensemble members that accumulated at least 2 mm of precipitation during the accumulation period of PF-2 (1600–0100 UTC 29–30 Oct). (b) The 2-mm precipitation exceedance threshold area derived from Yarrawonga radar reflectivities (hatched) from 2100 to 0100 UTC 29–30 Oct. Model output in (a) is from domain 3. Terrain contours are every 200 m. Note that the band of yellow and orange on the left side of (a) is related to the approaching cold front, the location of which exhibited significant spread among ensemble members.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

There appear to be two key components required for the successful simulation of PF-2: the initiation of convection north of Griffith, and the subsequent convective organization as the precipitating system moves south toward the Alps. With this in mind, the following appears dynamically important to PF-2: 1) the evolution of the prefrontal trough and bore, including the maximum eastward extent of the prefrontal trough before it becomes stationary; 2) the distribution of moisture east of the prefrontal trough; and 3) the role of the undular bore in triggering convection north of Griffith and preconditioning the environment to be more supportive of convective organization.

c. The role of moisture advection in PF-2

Convection was triggered in Control when the undular bore propagated into a southward-moving moist air mass. At 1200 UTC 29 October (prior to PF-2 initiation), there was consensus among ensemble members that the air north of Griffith was more moist (Fig. 20a). However, at finer scales there was substantial vertical and horizontal variation in the moisture profile (e.g., Fig. 20b). The successful six were all characterized by anomalously moist air north of Griffith (the black circle in Fig. 20c) and positive CAPE800 hPa whereas the failed eight were overall drier (Fig. 20d).

Fig. 20.
Fig. 20.

At 1200 UTC 29 Oct the (a) 800-hPa mixing ratio ensemble mean (filled, every 1 g kg−1 from 4 g kg−1); (b) skew T–logp measured at the white asterisk in (a) for the Control simulation (black with white stripe) and the 30 ensemble members (gray); and the difference between the ensemble mean 800-hPa mixing ratio and the mean for (c) the successful six measured and (d) the failed eight at 1200 UTC 29 Oct (filled, every 0.5 g kg−1). The black dashed line in (a) indicates the average location of the leading edge of the prefrontal trough. The white box in (a) represents the area in which the southward WV flux is calculated (see text for details). Terrain contours are every 200 m. Model output is from domain 3.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

Within two hours of convective initiation in the successful six, the convection had become organized (e.g., Fig. 15). This occurred within a southward-moving moist air mass, and immediately east of the prefrontal trough. Figure 21 summarizes the importance of the southward water vapor flux (WVF) toward the Alps and the location at which the prefrontal trough became stationary. All members of the successful six had a southward WVF stronger than or equal to Control, where the southward WVF is calculated at 1200 UTC 29 October from the surface to 500 hPa inside the white box in Fig. 20a. Additionally, the prefrontal trough does not advance as far east in four members of the successful six compared to Control. For the other two members of the successful six (E9 and E26), the southward WVF was anomalously high, perhaps allowing convection to intensify farther east where the environment was more stable.

Fig. 21.
Fig. 21.

A summary of two environmental attributes important for PF-2 for each member of Ensemble where the prefrontal trough became stationary with respect to Control and the southward WVF (1012 kg s−1). The x axis is categorical (i.e., not continuous) and shows the approximate location of the prefrontal trough with respect to Control. The y axis is continuous and shows the southward WVF for each member calculated within the white box in Fig. 20a at 1200 UTC 29 Oct. The dashed horizontal line shows the WVF of Control for reference. Blue numbers indicate members of the successful six and orange numbers indicate members of the failed eight. See text for further details.

Citation: Monthly Weather Review 144, 7; 10.1175/MWR-D-15-0355.1

In contrast, seven members of the failed eight had either a weaker southward WVF than Control, or a prefrontal trough that moved further east than Control. The outstanding member of the failed eight, which had both a strong southward WVF and a prefrontal trough that did not advance beyond that of Control, was E13. The reason for its failure appears related to the undular bore and is discussed in the next section.

d. The role of the undular bore in PF-2

It has been shown (e.g., Koch and Clark 1999) that an undular bore can transport moist air aloft, which preconditions the environment for convection at a later time. The failure of E13 to reproduce PF-2 provides some evidence that the undular bore was important to the evolution of PF-2 through the preconditioning mechanism.

E13 did not produce a bore in the between Griffith and the Alps because a local surface pressure minima developed within the prefrontal trough near Deniliquin. Therefore, despite the anomalously high moisture flux toward the Alps in E13, the weak and shallow convection initially triggered north of Griffith remained weak and shallow and very little precipitation accumulated. We hypothesize that had a bore existed south of Griffith and eroded the convective inhibition through the vertical transport of moisture, convection within the moist air mass would have become organized and PF-2 would have been reproduced in some capacity.

Unfortunately, E13 appears to be the only member that provides strong support for this hypothesis. Although the ensemble comprised 30 members, it appears insufficiently broad to explore all possible scenarios related to PF-2. Nonetheless, these simulations provide insight into the dynamics of the events and the predictability of relevant mesoscale processes. In particular, this ensemble reveals a surprisingly high predictability for undular bore formation in SE Australia for this event.

6. Summary and discussion

This study has examined the mesoscale processes and predictability of a prefrontal environment in which two distinct precipitation events were observed in the Australian Alps from 29 to 30 October 2010. While the resulting precipitation accumulations were modest, the observations and subsequent model simulations expose a series of events that highlight the complexity of the processes at play.

A prefrontal trough migrated in advance of a cold front across southern Australia. When the prefrontal trough entered SE Australia, it was shown to resemble a density current advancing into a stable nocturnal layer, forming a bore at its leading edge. The bore then detached from and propagated ahead of the prefrontal trough and became undular, supported by a wave-ducting mechanism. The undular bore was observed by the Deniliquin weather station (three distinct undulations in surface pressure) and in the Doppler wind field of the Yarrawonga radar. There was also evidence that the undular bore triggered convection from radar and surface observations. While previous studies have documented and numerically modeled undular bores, this is the first documented case in southeast Australia associated with significant precipitation.

A series of high-resolution model simulations suggest the undular bore was both a predictable feature of the case study and/or played an integral role in the initiation and evolution of two observed prefrontal precipitation events. Although no model simulations (including all members of the ensemble) reproduced the first prefrontal precipitation event (PF-1), aspects of the second prefrontal precipitation event (PF-2) were reproduced by the control simulation and six members (20%) of the ensemble.

With respect to PF-2, the ensemble was particularly useful to dissect and identify the flow kinematics critical to the initiation and evolution of the event. To summarize, the southward WVF was effectively a primer for convection whereas the bore was the trigger. The prefrontal trough, on the other hand, was important to generate the bore, although it also had the ability to prevent the necessary convective organization by pushing the convection further east into a more stable environment. The undular bore was also important in supporting convective organization by lofting moist, surface-based air south of the moist air mass. We stress that these are interpretations from the model simulations, used to supplement our understanding from the limited observations.

Finally, some weather stations in southeast Australia observed triple wind changes over a 12-h period as the bore, prefrontal trough, and then the cold front passed through the region. The ensemble revealed substantial spread in the simulated arrival time of each wind change (up to 2 h for the cold front), and also substantial spread in the timing and intensity of the cold front (see Watson 2013). This has obvious implications for forecasting during periods of heavy rainfall and fire weather.

Acknowledgments

Campbell D. Watson was supported by a Melbourne Research Scholarship. Computing was performed at the National Computational Infrastructure (NCI) Facility. The work benefitted from computational support provided by the ARC Centre of Excellence for Climate System Science (CE110001028). We thank Alan Seed for providing the radar-derived rainfall fields from the Yarrawonga radar. Thanks also to Daniel Kirshbaum and three anonymous reviewers for their constructive feedback on an earlier version of this manuscript.

APPENDIX

Topography of No-Plateau

Control uses topography as given by the U. S. Geological Survey (USGS) dataset (Fig. 8b). The elevated plateau that extends north from the VIC Alps is removed for No-Plateau by applying a weighting function, , to the default terrain elevation, , at the grid points inside the solid box in Fig. 8c. Following Langhans et al. (2011), the weighting function is described by
ea1
where
ea2
ea3
Here, , , , and specify the start and end points of the target areas in which the terrain is flattened to a new mean terrain height of = 80 m.

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