The 3 November Tornadic Event During Sydney 2000: Storm Evolution and the Role of Low-Level Boundaries
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22 WEATHER AND FORECASTING VOLUME 19 The 3 November Tornadic Event during Sydney 2000: Storm Evolution and the Role of Low-Level Boundaries DAVID M. L. SILLS,* JAMES W. W ILSON,1 PAUL I. JOE,* DONALD W. B URGESS,# ROBERT M. WEBB,@ AND NEIL I. FOX&,** *Cloud Physics Research Division, Meteorological Service of Canada, King City, Ontario, Canada 1National Center for Atmospheric Research, Boulder, Colorado #NOAA/National Severe Storms Laboratory, Norman, Oklahoma @Bureau of Meteorology, Sydney, New South Wales, Australia &University of Salford, Greater Manchester, United Kingdom (Manuscript received 20 June 2002, in ®nal form 4 March 2003) ABSTRACT Several severe thunderstorms, including a tornadic supercell, developed on the afternoon of 3 November 2000, during the Sydney 2000 Forecast Demonstration Project. Severe weather included three tornadoes, damaging wind gusts, hail to 7-cm diameter, and heavy rain causing ¯ash ¯ooding. A unique dataset was collected including data from two Doppler radars, a surface mesonet, enhanced upper-air pro®ling, storm photography, and a storm damage survey. Synoptic-scale forcing was weak and mesoscale factors were central to the development of severe weather. In particular, low-level boundaries such as gust fronts and the sea-breeze front played critical roles in the initiation and enhancement of storms, the motion of storms, and the generation of rotation at low levels. The complex and often subtle boundary interactions that led to the development of the tornadic supercell in this case highlight the need for advanced detection and prediction tools to improve the warning capacity for such events. 1. Introduction 3 November 2000 and moved through the project area The Sydney 2000 Forecast Demonstration Project within close range of two Doppler radars. The strongest (FDP) was undertaken to demonstrate both the capa- of these stormsÐan intense supercellÐproduced three bilities of modern nowcasting systems and the bene®ts weak tornadoes, damaging wind gusts, giant hail, and associated with their application in real time (Keenan heavy rain in the western suburbs of Sydney resulting et al. 2002). Meteorological instrumentation used to sup- in damage to about 300 properties. Numerous boundary port this project was located in the Sydney region of layer convergence lines were detected (hereafter re- eastern New South Wales (NSW), Australia, and con- ferred to as boundaries), including gust fronts and the sisted of three radars, a mesonet, and upper-air pro®ling sea-breeze front, and their interactions played a critical systems. The project ran from 5 September 2000 to 16 role in the development of severe weather on this day. November 2000 and included nowcasting support dur- Thus, the event yielded a unique dataset for the inves- ing the Sydney Summer Olympic Games. tigation of tornadic supercell evolution and boundary Severe thunderstorms1 developed on the afternoon of interactions in a region (indeed a hemisphere) that is not currently well represented in the related refereed literature. 1 In Australia, severe thunderstorms are de®ned as those that pro- Section 2 discusses previous research relevant to this duce any of the following: hailstones with a diameter of 2 cm or more, wind gusts of 90 km h21 or greater, ¯ash ¯ooding, and tornadoes study. Section 3 provides the sources of data and the (BoM 1999). methodology used for this investigation. Section 4 de- scribes the prestorm synoptic and mesoscale environ- ments. Section 5 examines in detail the severe storms ** Current af®liation: Department of Atmospheric Science, Uni- versity of MissouriÐColumbia, Columbia, Missouri. and their evolution. Section 6 discusses the different ways in which boundaries contributed to this event and provides suggestions for improved operational now- Corresponding author address: Dr. David M. L. Sills, Cloud Phys- casting of such events. The study's conclusions are pre- ics Research Division, Meteorological Service of Canada, King Weather Radar Research Facility, 14780 Jane St., King City, ON L7B sented in section 7. The performance of FDP severe 1A3, Canada. weather algorithms on this day is examined by Joe et E-mail: [email protected] al. (2004, in this issue) and will not be discussed here. q 2004 American Meteorological Society Unauthenticated | Downloaded 10/04/21 08:42 AM UTC FEBRUARY 2004 SILLS ET AL. 23 FDP forecasting and nowcasting issues associated with storm itself). Maddox et al. (1980) also found preex- this event are further examined by Fox et al. (2004, in isting boundaries to be a source of vertical vorticity for this issue) and Wilson et al. (2004, in this issue). tornadic storms. In addition, they established that in- tense tornadoes associated with storms moving along or 2. Background parallel to a boundary had longer lifetimes than those associated with storms moving across a boundary into Severe thunderstorms occur on a regular basis in cooler air. NSW and are most common between the months of With supercell thunderstorms, tornadoes are consid- November and February (BoM 1999). The east coast of ered much more likely if the midlevel mesocyclone is NSW is particularly susceptible to severe thunder- accompanied by a separate low-level (;0±3 km) me- storms, and Sydney, with its high population density, is socyclone (Davies-Jones and Brooks 1993; Brooks et vulnerable to large amounts of damage. In fact, the Syd- al. 1994). The way in which a thunderstorm develops ney area has experienced a number of signi®cant severe a midlevel mesocyclone has been con®rmed: low-level thunderstorm events in recent times with extensive dam- horizontal vorticity associated with strong environmen- age due to giant hail and violent winds (e.g., Mitchell tal vertical wind shear is tilted by the storm's updraft and Grif®ths 1993; BoM 1995, 1999). Many of these (see Davies-Jones et al. 2001). However, research has storms were intense supercells. The Sydney region also pointed to the low-level environment in the vicinity of has the highest average annual tornado incidence in boundaries as the source of vorticity for low-level me- 2 Australia at six per 26 000 km , though most tornadoes socyclones. are usually weak and short lived (Geerts and Noke- Numerical modeling studies have shown that a low- Raico 1995). level mesocyclone develops when baroclinically gen- The effect of boundaries, such as the sea-breeze and erated horizontal vorticity, acquired by an air parcel gust fronts, on thunderstorms in this region has received moving along the cool side of a storm-generated bound- little formal attention. However, research in North ary, is tilted and stretched by the storm updraft (e.g., America has shown that boundaries are preferred lo- Rotunno and Klemp 1985; Davies-Jones and Brooks cations for convective initiation due mainly to enhanced 1993). Atkins et al. (1999) used a numerical model to lift, and can act to enhance the intensity of storms, in- simulate the evolution of supercell thunderstorms in- cluding those that produce severe weather. Purdom teracting with boundaries. They found that, when a pre- (1976) used satellite imagery to show that intersecting existing boundary was present, air from the cool side boundaries often initiate intense convective develop- of this boundary provided much of the horizontal vor- ment. Wilson and Schreiber (1986) found that 79% of ticity necessary for low-level mesocyclogenesis, while storms in their study were initiated in association with the horizontal vorticity associated with storm-generated radar-observed boundaries. This increased to 95% for storms with radar re¯ectivities of 60 dBZ or greater. boundaries played only a minor role. Numerical mod- Several recent ®eld experiments have continued to ex- eling studies have also suggested a variety of methods amine the issue of convective initiation at boundaries by which tornadogenesis occurs following the devel- (e.g., Sills et al. 2002; Weckwerth and Parsons 2002). opment of the low-level mesocyclone, including a Boundaries are also known to have a large impact on downward-building vortex via the ``dynamic pipe ef- the structure, duration, and movement of thunderstorms. fect'' (Trapp and Davies-Jones 1997), two-celled vortex The organization and motion of severe storms was found instabilities within the low-level mesocyclone (Rotunno by Weaver (1979) to be in¯uenced more by intense con- 1986), and increasing mesocyclonic rotation that in- vergence at boundaries than by upper-level winds. Cor- duces low-level convergence and intensi®es vortex ®di (1998) showed that mesoscale convective systems stretching (Wicker and Wilhelmson 1995). propagate in the direction of the greatest system-relative Field observations tend to support the idea that pre- low-level convergence. This convergence is typically existing boundaries are frequently the source of vorticity associated with a low-level jet but can also be provided for low-level mesocyclones and subsequent tornadoes. by boundaries. Wilson and Megenhardt (1997), among Markowski et al. (1998) found that nearly 70% of sig- others, have shown that a storm's organization and life- ni®cant supercell tornadoes during the 1995 Veri®cation time are greatly enhanced when storm motion is roughly of the Origins of Rotation in Tornadoes Experiment equal to that of the storm's gust front. (VORTEX; see Rasmussen et al. 1994) occurred near Finally, it has been found that boundaries can provide preexisting boundaries. Wakimoto et al. (1998), Ras- the vorticity necessary for the development of rotation mussen et al. (2000), Monteverdi et al. (2001), and Zie- at low levels within a storm. Wakimoto and Wilson gler et al. (2001), among others, have also documented (1989) and Brady and Szoke (1989) showed that a thun- cases of tornadic supercell storms involving preexisting derstorm without the persistent midlevel (;3±7 km boundaries. Additional numerical modeling and obser- AGL) mesocyclone that de®nes a supercell can produce vational studies are clearly needed to verify the theories a tornado by stretching vertical vorticity located along related to low-level mesocyclogenesis and tornadoge- a preexisting boundary (i.e., one not generated by the nesis described above.