Numerical simulation of a cut-off low over southern Australia

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1 Meteorol. Atmos. Phys. 74, 103±115 (2000) 1 School of Mathematics, University of New South Wales, Sydney, Australia 2 School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA Numerical simulation of a cut-off low over southern Australia L. Qi 1,Y.Wang 2, and L. M. Leslie 1 With 8 Figures Received August 7, 1999 Revised October 23, 1999 Summary Cut-off low pressure systems over southern Australia are synoptic scale low pressure systems which have a closed circulation at the surface and a deep trough at the 500 hpa level. They usually develop from a deep trough in the westerlies, and often are companied by a Southern Ocean cold front. It is one of the most signi cant types of weather systems over southern Australia. Moderate or heavy rainfall is often associated with cut-off lows. Few studies so far have been conducted on them. In terms of prediction, the most interesting aspects are the position and intensity of the cut-off low pressure systems, the rainfall amount and distribution and sharp winds, when they are present. In this paper, a numerical model developed at the University of New South Wales, HIRES (HIgh RESolution model), is employed to simulate the cut-off low of August 31 to September 2, The modeling results are encouraging, showing that HIRES can forecast very well the location and intensity of the cut-off low system and its associated precipitation. The impact of an improved, explicit physics scheme on the simulation is also examined. The explicit scheme further improves the rainfall prediction. 1. Introduction The southern Australia cut-off lows studied in this paper are among the most important types of weather systems in that region (see Fig. 1). They are of major economic value to Australia's main agricultural area, the Murray-Darling Basin. There are few studies so far in this topic, mainly focusing on the cut-off lows off the south eastern coastline of Australia (Leslie et al., 1987). These are known as Australian east coast lows. Leslie et al. (1987) found that the small scale physical processes play a key role in the development of the east coast lows. High-resolution sea surface temperatures (SSTs) and increased horizontal resolution were shown to improve the intensity and track forecasts. McInnes (1992) also con- rmed the impact of sea surface temperatures on the numerical simulations. In McInnes (1992), the monthly mean sea surface temperature was used in the control run while the enhanced sea surface temperature eld, was chosen from a GCM output under a double CO 2 climate background (Whetton and Pittock, 1991). They found that the enhanced sea surface temperature elds resulted in a drop on the central pressure of the cut-off lows. The enhanced sea surface temperature elds also brought about increased peak rainfall amounts. The success of these earlier simulations of east coast lows encouraged us to conduct a numerical case study of southern Australia cut-off lows. Figure 2 shows us typical example of a southern Australia cut-off low. In our earlier study, a 14- year climatology and a synoptic case study of a cut-off low that occurred in late August/early September 1997, was carried out. The 14 year climatology produces a number of signi cant ndings. It was found that there is a higher

2 104 L. Qi et al. Fig. 1. Location map Fig. 2. A cut-off low case of 1200 UTC 9 October, The solid line represents sea level pressure, and dashed line for geopotential height at the 500 hpa level frequency of cut-off lows in southern Australia in the cooler months, May to October. About one in ve of this kind of cut-off low moves northeastward and produces moderate to heavy precipitation over southeastern Australia. The most active area for the genesis of cut-off lows in southern Australia is the southwestern region (Latitudes 25 S±40 S, Longitudes 110 E± 130 E), which accounts for about three-quarters of all the cut-off lows. The majority (about 4 out of 5) of the cut-off lows move either eastward or southeastward. Some of them move quickly, while others move slowly and can stay in a region for up to two days. The development and maintenance of the blocking high over the Tasman Sea plays a very important role in the formation and intensi cation of the cut-off low, which is located to the west of the blocking systems. The case study of the cut-off low for August 31, 1997 to September 2, 1997 revealed that there is stronger warm and cold advection in the troposphere especially in the mid-upper levels. It is noted that the amplitudes of the trough and ridge increased signi cantly. During the interaction beween the warm and cold air masses, the isogram gradient of potential temperature increased signi cantly. A stronger baroclinic zone between the upper-level trough and ridge existed over southern Australia. From the wind eld at 200 hpa and also the vertical cross section along 125E, an increase in speed of

3 Numerical simulation of a cut-off low over southern Australia 105 the ULJ was revealed. An upper-level jet (ULJ) streak with a wind speed maximum of more than 70m/s was located in Western Australia. It should be pointed out that in this case the cut-off low was located approximately in the right front quadrant of the ULJ stream, a position favorable for synoptic system development (Brook, 1982; Zhu, 1992). From the mixture ratio eld and the wind eld at 850hPa, a low-level jet (LLJ) extended from the tropical ocean surface in the Coral Sea to higher latitudes and, at the same time, a narrow moisture tongue pushed into the same area and from the same direction. The vertical cross section of the mixture ratio and meridional wind component indicated that the moisture layers in the lower troposphere were deeper and the LLJ maximum was more than 15m/s. The above ndings suggest that a water vapor channel existed and the moisture source was in the tropical ocean, the Coral Sea. A weak front on the surface map extended from the centre of the low southwards. The main rainfall area was concentrated in the warm sector rather than near or behind the cold front. In this study we carry out a numerical simulation of the cut-off low of September 1 to September 2, 1997 using the UNSW model, HIRES, which has been employed in many case studies (Batt and Leslie, 1998; Speer and Leslie, 1996) and in real-time forecasting over six years in the bereau of meteorology of Sydney. The main emphasis here is on predicting the location and intensity of the cut-off low and its rainfall distribution and amount. 2. Model description The initial conditions used in this paper was from the LAPS (Limited Area Prediction System) of the Australia, bureau of meteorology. At the time of the study, LAPS had a horizontal resolution of 75 km and 19 vertical levels. The University of New South Wales (UNSW) numerical weather prediction, HIRES, is employed here to conduct the numerical simulation. It is a hydrostatic, primitive equations model in which there are at least 15 sigma levels in the vertical and semi-implicit time integration on an Arakawa C grid. A Mellor-Yamada level 2.25 is employed in the parameterization of the vertical turbulent exchanges. Normally, HIRES is Table 1. Principal features of HIRES model Model features Description Horizontal resolution Optional, typically between 5 km and 100 km Vertical resolution Optional, usually 15±40 sigma levels Temporal differencing Semi-implicit Spatial differencing Energy conserving on an Arakawa C grid Assimilation scheme Six-hourly cycle Orography 1 degree by 1 degree Sea surface temperature Monthly mean Convection scheme Modi ed Kuo scheme Boundary layer Mellor-Yamada, level 2.25 used for short to medium range forecasts, up to 8 days ahead. The grid space is km, covering the domain of 2 S±44 S, 113 E±178 E. The main physical processes in the model include a modi ed Kuo convection scheme, horizontal diffusion, vertical diffusion above surface layer based on mixing length hypothesis, stability dependent boundary layer with eddy diffusivities being functions of bulk Richardson number. The lateral boundary conditions are updated by the LAPS output every six hours. The topography was retrieved from a 1 degree by 1 degree resolution data by interpolation with a light smoothing procedure. Albedo and deep soil temperature were selected from seasonal climologies. Roughness lengths was interpolated to the grid (Garratt, 1977). 3. Synoptic overview At 23 UTC August 31, 1997, the ridge near the east coast was very pronounced at the 500 hpa level (Fig. 3a), with the 5550 hpa isopotential extending to about 39S. It was a strong weather system covering an area from 15S to 50S. In southern and central Australia the 5550 hpa isopotential extended to 28S. The amplitude between the systems is over ten degrees of latitude. Using the 200 hpa geopotential height eld at 23 UTC August 31, 1997 (see Fig. 3b) as an example, it was also found that a ridge developed along the east coast of Australia, while to the west, there was a deepening trough.

4 106 L. Qi et al. Fig. 3a. Geopotential height eld at 500 hpa, 23 UTC, on 31 August Fig. 3b. Geopotential height eld at 200 hpa, 23 UTC on 31 August Fig. 4. Precipitation in Australia for the 24 hours period ending 23 UTC Sept. 1, 1997 The precipitation pattern, shown in Fig. 4, is the rainfall analysis charts for the 24-hour period 23 UTC September With the cut-off low development and the front moving eastwards, there was a large area of precipitation over Queensland, New South Wales, Victoria and the southeast of South Australia. There are three main precipitation centers: the southeast corner of southern Australia, the central area of New South Wales and southern Queensland with

5 Numerical simulation of a cut-off low over southern Australia 107 Fig. 5a. Wind speed at 200 hpa at 23 UTC August 31, 1997 maximum 24 hour rainfall totals of over 25 mm and in some parts over 50 mm. The main rain belt was a long, narrow, band and persisted over southeastern Australia for most of the period. Clearly, the precipitation area over southeastern of South Australia corresponded closely with both the low pressure center and the topography. The wind speeds in the upper troposphere became larger at the same time. At the 200 hpa level, the maximum wind speeds was 60m/s at 23 UTC August 31 (Fig. 5a), then 70m/s at 23 UTC September 1 (Fig. 5b). The jet streak was located near 25S, and the low centre at (135E, 35S). The low centre was at the front right quadrant of the jet streak. At 23 UTC September 1 (Fig. 5c), there was a moisture tongue on the mixing ratio chart at the 850hPa level. It stretched from the Paci c Ocean to inland Australia, and the maximum value was 10g/kg located at (20S, 145E). The 9g/kg isogram extended southward as far as to 29S, forming a typical moisture tongue with a drier area to its west which was coincident with the cold air in the south. At 00 UTC 30 August 1997, a low centre was located to the west of Adelaide (Fig. 6a). At 00 UTC September 1 (Fig. 6b), the low moved to the northwest of Adelaide, and the central pressure was still 1009 hpa. At 12 UTC September 1, the low centre was to the east of Adelaide with a central pressure of 1004 hpa. At 00 UTC September 2 (Fig. 6c), it had moved to southwest of Melbourne, and the central pressure went down to 1000 hpa accompanied by a large area of precipitation in South and East Australia.

6 108 L. Qi et al. Fig. 5b. Wind speed at 200 hpa at 23 UTC September 1, 1997 Fig. 5c. Mixing ratio eld at 850 hpa at 23 UTC September 1, Results of the numerical simulation Figure 7a is the 24-hour rainfall simulation by HIRES model. The rain area covers New South Wales, Victoria and part of South Australia and Queensland. There are three major rainfall centers, and they are located at Adelaide, the northeast border of Victoria and east coast of New South Wales near Sydney. The maximum 24-hours rainfall amount is over 45 mm. Compared with the observation (Fig. 4), the forecast rainfall pattern is close to the observed, especially the rainfall center over Adelaide, in which the observed rainfall amount is 25 mm to 50 mm while the forecast is 25 mm to 30 mm. The rainfall forecast is also good along the east coast of Queensland. The observation is 0 mm to 1 mm and the forecast is 0 mm to 5 mm. Turning to the wind eld, the forecast (Fig. 7b) simulated the LLJ very well. The simulated LLJ is formed by two branches, one of which extended over Queensland and New South Wales while another one is developed along the border

7 Numerical simulation of a cut-off low over southern Australia 109 Fig. 6a. The surface analyses chart at 00 UTC August between New South Wales and Victoria. The maximum surface wind speed is over 16m/s. At the same time there is a moisture tongue extending from Coral Sea (Fig. 7c). Compared the wind eld and the mixing ratio eld, the moisture tongue cooperate quite well with strong wind (12m/s). For the 24-hour forecast, the simulated position of the low center is at 38S, 139E (Fig. 7d) and the central pressure falls from 1010 hpa to less than 1005 hpa in 24 hours, which is quite close to the observation (Fig. 6c). The simulated SLP eld also shows a ridge along the east coastline of Australia. In the temperature eld, the weak front along 145E also was simulated quite well (Fig. 7e). At 200 hpa level (Fig. 7f), there is a simulated maximum region of wind speed over West Australia which corresponds very well with observation (Fig. 5b). Generally, the HIRES model with the Kuo scheme has captured most of the synoptic features of this cut-off low process and the precipitation predictions are good. Fig. 6b. The surface analyses chart at 00 UTC September Fig. 6c. The surface analyses chart at 00 UTC September Impact of the explicit cloud microphysics process In this case study, allthough the HIRES-model with the Kuo scheme was quite good, there were still some problems on the precipitation forecast over central New South Wales, so an explicit cloud microphysics scheme in the model (developed by the second author, Yuqing Wang) also was employed to check its impact on the simulated results, in which the grid resolved precipitation and evaporation are calculated by the scheme of Wang (1999), Lin et al. (1983), and Rutledge and Hobbs (1983, 1984). The cloud microphysics scheme condsiders water vapor, cloud water, cloud ice, snow, rain water and graupel as prognostic variables, including aggregation, accretion, evaporation, deposition, sublimation, melting and freezing processes. The simulated results show that the explicit scheme can improve the rainfall forecast signi cantly (Fig. 8a). It is noteworthy to see that there is now a maximum rainfall region over central New South Wales and the rainfall amount is over 55 mm. The observation of the rainfall amount over that region is over 50 mm. The wind and SLP elds (Fig. 8b and 8c) are similar to the results of the Kuo scheme.

8 110 L. Qi et al. Fig. 7a. 24-hour rainfall (mm) simulated by HIRES with Kuo scheme Fig. 7b. 24-hour surface wind eld (m/s) simulated by HIRES with Kuo scheme

9 Numerical simulation of a cut-off low over southern Australia 111 Fig. 7c. 24-hour surface moisture eld (g/kg) simulated by HIRES with Kuo scheme Fig. 7d. 24-hour SLP eld (hpa) simulated by HIRES with Kuo scheme

10 112 L. Qi et al. Fig. 7e. 24-hour temperture eld ( C) simulated by HIRES with Kuo scheme Fig. 7f. 24-hour wind eld (m/s) at the 200 hpa simulated by HIRES with Kuo scheme

11 Numerical simulation of a cut-off low over southern Australia 113 Fig. 8a. 24-hour rainfall (mm) simulated by HIRES with the explicit scheme Fig. 8b. 24-hour surface wind eld (m/s) simulated by HIRES with the explicit scheme

12 114 L. Qi et al. Fig. 8c. 24-hour SLP eld (hpa) simulated by HIRES with the explicit scheme 6. Discussion and conclusions A simulation has been carried out of the cut-off low of August 31, 1997 to September 2, Based on the results above, it is clear that the HIRES-model performs very well in this case and can catch most of the synoptic features of this process. The 24-hours forecast for the track, intensity and rainfall patterns are quite close to the observations. When an explicit microphysics process was introduced at the model, the forecast can be signi cantly improved, especially the rainfall eld. The sea surface temperture eld used in this model simulation is selected from climate background, monthly mean eld. It may be partly responsible for the simulated low pressure center moving slower than observed. Further study is required to answer this question. Acknowledgement Ths paper is supported by Australia Research Council (ARC) and the U.S. Of ce of Naval Research under Grant No. N References Batt KL, Leslie LM (1998) Veri cation of output from a very high resolution numerical weather prediction model: the 1996 Sydney to Hobart yacht race. Meteorol Appl 5: 321±327 Brook RR (1982) A study of the subtropical jet stream in the Australian region. Aust Meteor Mag 30: 223±239 Garratt JR (1977) Aerodynamic roughness and a mean monthly surface stress over Australia, Australian Division of Atmospheric Physics, Tech Paper No. 29, Melbourne, CSIRO McInnes KL, Leslie LM, McBridge JL (1992) Numerical simulation of cut-off lows on the Australia east coast: Sensitivity to sea-surface temperature. Int J Clim 12: 783±795 Leslie L, Skinner T (1994) Real-time forecasting of the Western Australian summertime trough: evaluation of a new regional model. Wea Forecasting 9: 371±383 Lin YL, Farley RD, Orville HD (1983) Bulk parameteration of the snow eld in a cloud model. J Climate Appl Meteor 22: 1065±1092 Qi L, Leslie LM, Zhao SX (1999) Cut-off low pressure systems over southern Australia: Climatology and case study. Int J Clim (in press) Rutledge SA, Hobbs PV (1983) The mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones. VIII: A model for the

13 Numerical simulation of a cut-off low over southern Australia 115 ``seederfeeder'' process in warm-frontal rainbands. J Atmos Sci 40: 1185±1206 Rutledge SA, Hobbs PV (1984) The mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones. XII: A diagnostic modeling study of precipitation developmnt in narrow cold-frontal rainbands. J Atmos Sci 41: 2949±2972 Speer MS, Leslie LM, Colquhoun JR, Mitchell E (1996) The Sydney Australia wild res of January 1994: meteorological conditions and high resolution numerical modelling experiments. Int J Wildland Fire 6: 145±154 Wang Y (1999) A triply-nested movable mash tropical cyclone model with explicit cloud microphysics- (TCM3), BMRC Research Report No 74, BMRC, Australia Whetton P, Pittock AB (1991) Australian region intercomparison of the results of some general circulation models used in enhanced greenhouse experiments, CSIRO Division of Atmospheric Resear Technical paper No. 21, CSIRO, Australia Zhu QG, Lin JR, Shou SW, Tang DS (1992) Synoptic theory and methods. Beijing: Meteorology Press, pp 275±277 Authors' addresses: L. Qi and L. M. Leslie, Centre for Environmental Modeling and Prediction, School of Mathematics, University of New South Wales, Sydney 2052, Australia ( qlx@maths.unsw.edu.au); Y. Wang, International Paci c Research Center, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA

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