Regeneration of the Southerly Buster of Southeast Australia
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432 WEATHER AND FORECASTING VOLUME 15 Regeneration of the Southerly Buster of Southeast Australia HELEN J. REID School of Mathematics, University of New South Wales, Sydney, Australia (Manuscript received 3 November 1999, in ®nal form 3 April 2000) ABSTRACT The southerly buster has been successfully simulated using a numerical weather prediction (NWP) model and veri®ed (particularly the sea level pressure ®eld). This simulation was then used to study the behavior of the southerly buster in the region of the Hunter Valley, New South Wales, Australia, with the reintensi®cation of the surge. In simulating the dynamics of the southerly buster in the vicinity of the Hunter Valley, both the horizontal and vertical resolution of the NWP are important. This was found through a series of simulations of a case study of 27 February 1998. The best simulation was achieved with 20 vertical levels, a coarse nesting into the Australian Bureau of Meteorology Limited Area Prediction System model, then down to ®ner grids in progressively higher resolutions. The pressure ridge associated with the southerly buster is induced by the southerly ¯ow up the southern parts of the Great Dividing Range resulting in anticyclonic vorticity that creates a region of high pressure ahead of the main high pressure cell behind the frontal system. This same mechanism is used to explain the reintensi®cation of the surge at the northern part of the Great Dividing Range, which is characterized by a renewed peak in wind speed north of the Hunter Valley. 1. Introduction of the NWP model, HIRES, are in section 4 with thor- ough veri®cation of the simulations in section 5. The The southerly buster along the coast of New South sea level pressure (SLP) pattern is veri®ed in consid- Wales (NSW), Australia, has been the focus of many erable detail as it provides the basis for the investigation studies. In Reid and Leslie (1999, hereafter RL99) and of the pressure ridge of the southerly buster dealt with Reid (2000), a numerical weather prediction (NWP) in section 6, before the conclusions and ideas for further model capable of running at high resolution (HIRES) work are presented in section 7. was employed to simulate the southerly buster with very RL99 focussed on the arrival time and Reid (2000) good results. The region of the Hunter Valley was shown addressed the intensity of the southerly buster. This to have a signi®cant effect on the propagation of the study will endeavor to address the SLP pattern in re- southerly buster. Reid (2000) demonstrated that the lation to the propagation of the southerly buster. As a weaker surge across the Hunter Valley mouth (see Fig. result, section 5b will verify the model in more detail 1) reintensi®es to the north after a period of several in terms of SLP and consider how this is related to the hours. In many cases there is a ¯ow up the Hunter Valley southerly buster. prior to the coastal locations of similar latitude. The question of the extent of northward propagation is often posed (e.g., Baines 1980) and there have been many 2. Southerly busters studies on the development (e.g., McBride and McInnes 1993) but none on the decay. It is proposed that the Southerly busters occur during the spring and summer reintensi®cation of the southerly buster surge occurs in months in the southeast of Australia to the east of the a manner similar to that of the original development of Great Dividing Range along the coast of NSW (see Fig. the southerly buster surge in the south of Victoria. 1). They are a strong, sudden, and squally southerly 21 Section 2 provides a brief description of the southerly wind surge of at least 15 m s and have been known 21 buster phenomenon with the case study of 27 February to gust up to 35 m s (Colquhoun et al. 1985) and are 1998, introduced in section 3. Notes on the con®guration con®ned to the coastal regions, trapped against the mountains by the Coriolis force. The depth of the surge is generally less than 1 km and therefore is below the average height of the Great Dividing Range. Corresponding author address: Dr. Helen J. Reid, Bureau of Me- The passage of the southerly buster is occasionally teorology, Level 16, 300 Elizabeth St., 2000 Sydney, Australia. accompanied by a roll cloud and precipitation is not E-mail: [email protected] usual. In addition the passage is notable for the common q 2000 American Meteorological Society Unauthenticated | Downloaded 09/25/21 05:48 AM UTC AUGUST 2000 REID 433 FIG. 2. SLP chart for 0900 27 Feb 1998. a. Synoptic situation The cold front from which the southerly buster de- veloped was over the Great Australian Bight during the afternoon of 26 February 1998. Over the next 2 days, it progressed farther east and out into the Tasman Sea. The frontal system on this occasion was complex. A prefrontal trough became dominant as it progressed FIG. 1. A map of southeast Australia over the domain of the 30- across Victoria, although it weakened as it approached km resolution model simulation indicating the location of the Great Dividing Range (shaded region over 500 m), New South Wales, Vic- the southern coast of NSW and the front merged with toria, Queensland, the Hunter Valley, Tasmania, and the Tasman Sea. the weakened trough as the system progressed up the coast of NSW during 27 February 1998. A region of low pressure existed over the northwest of the continent and a high pressure cell moved across the southeast of wind shift from northwesterly to southerly, as well as Australia directly behind the frontal system. Figure 2 for the sudden drop of temperature, which can be up to illustrates the broadscale synoptic situation in which the 158C within minutes (Howells and Kuo 1988; Mass and southerly buster was embedded; however, it does not Albright 1987). Also, a signi®cant rise in the SLP occurs show the detail of the 3-hourly hand analyses from the as a ridge of high pressure follows the cold frontal sys- Sydney of®ce of the Australian Bureau of Meteorology tem. This study will focus on the origins and movement on which the above comments were based. of this ridge. A southerly buster is generated when a cold front is blocked and experiences anticyclonic deformation near b. Observational summary the Great Dividing Range (McInnes 1993). The surge Data were obtained for eight locations in NSW to of air propagates northward as a coastally trapped oro- verify the HIRES NWP model simulations. These lo- graphic jet up the east coast of Australia, the duration cations are Bellambi, Sydney, Norah Head, William- of which is generally less than 24 h, from the time the town, Taree, and Coffs Harbour on the coast, while cold front reaches the Great Dividing Range in southern Cessnock and Scone are inland in the region known as NSW to its dissipation on the north coast or southern the Hunter Valley. The locations are illustrated in Fig. coast of Queensland (Baines 1980; Howells and Kuo 3 with an observational summary in Table 1. More de- 1988; McInnes et al. 1994). tails of this case can be found in Reid (2000). The tem- perature drop associated with the southerly buster tend- 3. Case study ed to occur over an hour-long period. Pressure rises tended to continue for several hours, with the main in- This section describes the southerly buster of 27 Feb- crease over a 5-h period. ruary 1998 both synoptically and in terms of surface observations. This event was near the end of the south- 4. HIRES erly buster season, which coincides with midspring to late summer. Times are standard times for the east of To look at the detail of the southerly buster surge, a Australia (UTC 1 1000). NWP model that can be run at high resolution (HIRES) Unauthenticated | Downloaded 09/25/21 05:48 AM UTC 434 WEATHER AND FORECASTING VOLUME 15 boundary layer is treated as stability dependent with eddy diffusivities as functions of the bulk Richardson number. There is a surface heat budget with a prognostic equation for surface temperature and a modi®ed Kuo scheme for convection. Large-scale precipitation schemes as well as monthly average sea surface tem- perature, as obtained from the nesting data ®les, are used. In RL99, HIRES was run over a domain at 20- km horizontal resolution with 16 levels in the vertical. In Reid (2000), the horizontal resolution increased to 10 km and the vertical resolution increased to 20 levels, to incorporate more levels near the surface as the south- erly buster is a shallow phenomenon. This demonstrated an increase in the accuracy of the simulation with the higher resolution. However, various features of the local ¯ow were not well resolved. For example, the sea-breeze circulations during the afternoon were not clearly de- ®ned. In this study, HIRES has been run over a domain with 10- or 5-km resolution. The higher-resolution (5 km) simulations used computational routines that in- cluded an explicit treatment of moist processes (Qi et al. 2000). Also addressed was the effect of higher res- olution in the vertical, increasing the use of 20 sigma levels to 28 levels, to better resolve the middle tropo- sphere and tropopause. The following 20 levels are used: 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.75, 0.8, 0.825, 0.85, 0.875, 0.90, 0.925, 0.95, 0.975, 0.99, 0.995, 0.998, and 0.999.