Numerical prediction of severe convection: comparison with operational forecasts

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1 Meteorol. Appl. 10, (2003) DOI: /S Numerical prediction of severe convection: comparison with operational forecasts Milton S. Speer 1, Lance M. Leslie 2 & L. Qi 2 1 Bureau of Meteorology, PO Box 413 Darlinghurst, NSW 1300 Australia m.speer@bom.gov.au 2 University of New South Wales, Sydney, Australia l.leslie@unsw.edu.au; qlx@maths.unsw.edu.au The prediction of severe convection is a major forecasting problem in Australia during the summer months. In particular, severe convection in the Sydney basin frequently produces heavy rain or hail, flash flooding, and destructive winds. Convective activity is a forecasting challenge for the Sydney basin, mainly from October to April. Currently, there is a need for improved numerical model guidance to supplement the official probabilistic convective outlooks, issued by the operational forecasters. In this study we assess the performance of a very high resolution (2 km) numerical weather prediction (NWP) model in terms of how well it performed in providing guidance on heavy rainfall and hail, as well as other mesoscale features such as low level convergence lines. Two cases are described in which the operational forecasts were incorrect on both occasions. Non-severe thunderstorms were predicted on 1 December 2000 but severe convection occurred. Severe convection was predicted on 8 December 2000, but no convection was reported. In contrast, the numerical model performed well, accurately predicting severe convection on 1 December and no convection on 8 December. These results have encouraged a program aimed at providing an enhanced numerical modelling capability to the operational forecasters for the Sydney basin. 1. Introduction A major forecasting problem at weather centres worldwide is the provision of accurate and timely shortrange numerical weather prediction (NWP) guidance for convection, especially if the convection is expected to reach the severe stage. A weather forecaster will often need to consider a range of possible outcomes including no thunderstorms, non-severe thunderstorms or severe thunderstorms. Currently, convective outlooks are shaded areas on maps, and are referred to as chance of or likely. As such, they are probabilistic forecasts and depend on whether or not the forecaster is convinced, for example, that a lifting mechanism will be strong enough for a parcel to reach its level of free convection, or that the amount of vertical motion will be strong enough to penetrate temperature inversions. There are two main aims of this study. First, we wish to evaluate the potential of a real-time NWP model, tailored for predicting severe convection, for predicting convective and non-convective days. Two days were selected in which convective outlooks (non-severe and severe, respectively) were issued by the operational forecasters but severe convection actually occurred on only the first of these days. The second aim of this study is also intended to show the potential of the NWP model to form part of an integrated system available to the operational forecasters. The Sydney basin with its elevated surrounding area is a natural laboratory for thunderstorms (Figure 1), particularly during the warm season (September to April) when moisture availability is high from the warm waters of the Tasman Sea. The long-term average number of thunderstorm days per annum ranges from 20 to 25 over the Sydney basin, with the chance of large hail or damaging wind occuring during this period. There have been a number of recent severe storms including the hailstorms of 16 February 2002, 14 April 1999 (Bureau of Meteorology 1999) and 18 March 1990 (Bureau of Meteorology 1993). However, there are many more days when conditions appear favourable for severe convection, but either non-severe or no convection occurs. For example, there were nine days during November and December 2000 when severe thunderstorm warnings were issued for Sydney, but severe convection did not occur on most of those days. Airmass boundaries, such as frontal or sea-breeze convergence lines, often occur to the west and southwest of the Sydney basin but convective initiation does not always happen. Therefore, attempting to account for initial condition uncertainty (inter alia) has become increasingly important in developing improved shortrange NWP guidance for convection / no convection. 11

2 M S Speer, L M Leslie and L Qi Figure 1. Location map of southeastern Australia and the Sydney basin highlighting the topography. Topographical contours are in metres. To this end, the authors have been undertaking a program of short-range forecasting over the past five years in relation to severe and non-severe weather events (see, for example, Speer & Leslie 1997; Leslie & Speer 1998; Speer & Leslie 2000). The focus here is on two particular days, 1 and 8 December 2000, for which severe convection was predicted, in a probabilistic sense, for the Sydney area by forecasters from the Bureau of Meteorology in Sydney. That is, likely / probable forecasts were issued. Figures 2a and 2b are the official convective outlooks covering Sydney and New South Wales, issued during the mornings of 1 December and 8 December for the rest of those respective days. On 1 December, severe thunderstorms did develop, verifying as heavy rain, large hail (> 2 cm) and damaging winds (> 48 knots or 24 m/s). In contrast, no convection occurred on 8 December. For the latter case, the expectation was that there would be sufficient buoyancy for afternoon convection to occur and that it would reach the severe stage. However, extensive cumulus development in the morning dissipated during the day. Afternoon convection failed to materialise, despite a pre-existing boundary between moist northeast winds and drier northwest winds on the ranges just to the southwest of the Sydney area. The environment became virtually cloud-free in the afternoon, assisted by decreasing moisture in the low to mid-tropospheric levels. The environmental setting for the two cases is described in section 2. In section 3 the methodology is 12 presented, describing the model characteristics along with the data used in the study. Model results are described in section 4 highlighting important mesoscale mechanisms in addition to precipitation forecasts. A discussion of the implications of the results for convective model guidance is given in section 5. The main findings from the results also are summarised in section Environmental setting November and December 2000 were very active months for thunderstorms in the Sydney area. A total of nine severe thunderstorm warnings were issued on nine days during the two-month period. Typically, severe thunderstorm warnings are issued within 1 2 hours of the expected event. On 1 December there were several reports of pea-sized hail, one report each of marble- and golfball-sized hail, reports of wind damage, and locally heavy rain was recorded (defined for the region as totals greater than or equal to 26mm in 20 minutes). In contrast, on 8 December there were no thunderstorms reported in the Sydney metropolitan area. The general synoptic setting for both days was dominated by the northward movement of a cold front along the coast of southeast Australia and the adjacent Pacific Ocean (Figures 3a and b). Cold fronts often trigger severe thunderstorms in the Sydney basin, especially in the post-frontal air (Speer & Leslie 1997).

3 Predicting severe convection Figure 2. Thunderstorm outlooks for NSW, including Sydney, issued by the Bureau of Meteorology, NSW: (a) at 1040 am on 1 December 2000 valid until midnight 1 December; (b) at 1020 am on 8 December 2000 valid until midnight 8 December. 2.1 Case 1: 1 December The vertical temperature profile at Sydney Airport recorded at 6 am loca1 time on 1 December 2000 is shown in Figure 3c. A modified vertical temperature profile (not shown), based on the forecast afternoon maximum surface air temperature of 25 ºC and a dewpoint of 17 ºC, producing CAPE values of about 1250 J kg 1, would provide sufficient buoyancy for convective updrafts. During mid-afternoon on 1 December, thunderstorms developed within the Sydney basin in post-frontal surface southerly air. Figure 3d is a World Weather Research Project (WWRP) wind analysis display approximately 50 m above ground level combined 13

M S Speer, L M Leslie and L Qi Figure 3a. MSLP pattern indicating a prefrontal airmass over Sydney in which convection/severe convection typically develops. Figure 3c.

4 M S Speer, L M Leslie and L Qi Figure 3a. MSLP pattern indicating a prefrontal airmass over Sydney in which convection/severe convection typically develops. Figure 3c. Aerological diagram for 6 am (local time) 1 December 2000 showing the vertical temperature and dewpoint profile at Sydney Airport. The observed vertical wind profile at 9 am on 1 December 2000 is also shown. winds, thereby providing a favoured area in which the thunderstorm cells developed. 2.2 Case 2: 8 December Figure 3b. MSLP pattern indicating a postfrontal airmass over Sydney in which convection/severe convection typically develops. with radar-based 10-minute scan reflectivity data, and is based mainly on the data from a Doppler radar located near Sydney Airport. The main thrust of the WWRP centred on testing weather radar-based nowcasting systems by several international research groups over the Sydney basin prior, during and immediately following the Sydney 2000 Olympics Games. The display shows several convective cells with high rainfall rates in a southerly airstream at 6 pm on 1 December. During the late morning of 1 December, surface and low tropospheric level southerly winds penetrated westwards through the Sydney basin to the ranges bordering the basin. By mid-afternoon a low level convergent zone, sloping eastwards with height across the Sydney area, had developed as a result of the low level south to southeast winds underlying westerly 14 The vertical temperature profile at Sydney Airport recorded at 6 am (local time) 8 December, is shown in Figure 3e. A modified vertical temperature profile (not shown), based on the forecast afternoon maximum surface air temperature of 26 ºC and dewpoint of 17 ºC, producing CAPE values of about 2100 J kg 1, would again provide sufficient buoyancy for strong convective updrafts. Furthermore, during the afternoon intersecting boundaries persisted between moist northeast winds and drier northwest to westerly winds just to the southwest of the metropolitan area. Figure 3f is a WWRP wind analysis display approximately 50 m above ground level at 5 pm 8 December, showing clear intersecting boundaries between northeast and northwest winds and stronger northeast seabreezes. Thunderstorms often develop on or close to the intersection of such boundaries (Wilson & Schreiber 1986). Despite the existence of these mesoscale phenomena throughout the afternoon, no convective development occurred. A cold front that reached Sydney overnight on 8 December was accompanied by thunderstorms outside the Sydney basin. They occurred about 60 km further south of Sydney, and were located near the coast. 3. Methodology 3.1. Model A nesting strategy was used with a mesoscale NWP model developed by the second author (Leslie & Purser

Figure 3f. As for (d), except at 5 pm, 8 December 2000. Note the intersecting boundaries between northeast and northwest winds marked in red (dashed).

These versions of the model will be referred to simply as the 5 km and 2 km versions respectively. 3.2. Data Figure 3e.

5 Predicting severe convection Figure 3d. World Weather Research Project wind analysis display at approximately 50 m above ground level, based mainly on data from a Doppler radar located near Sydney Airport at 6 pm on 1 December Figure 3f. As for (d), except at 5 pm, 8 December Note the intersecting boundaries between northeast and northwest winds marked in red (dashed). Fritsch scheme (Kain & Fritsch 1990) run at 5 km horizontal resolution and an explicit scheme (Buckley et al. 2001) that predicts the various water vapour phases, run at 2 km horizontal resolution. These versions of the model will be referred to simply as the 5 km and 2 km versions respectively Data Figure 3e. Aerological diagram for 6 am (local time) 8 December 2000 showing the vertical temperature and dewpoint profile at Sydney Airport. The observed vertical wind profile at 9 am on 8 December 2000, is also shown. 1995). The model, known as HIRES, produces twicedaily, real-time, high resolution forecasts of wind speed and direction, rainfall and other variables over the Sydney basin in its hydrostatic form. In this form HIRES has successfully produced a number of realistic rainfall distribution and amounts at very high resolutions in case study research mode (see, for example, Speer & Leslie 2000). The HIRES model has various cumulus parameterisation options, including the Kain- Archived operational data were available at a horizontal resolution of 37.5 km, from the Australian Bureau of Meteorology s limited area prediction model (LAPS). These analyses were interpolated at the same resolution as the HIRES model, and passed through the HIRES diabatic dynamic initialisation scheme. A HIRES forecast was produced at this resolution covering the Australian continent and adjacent oceans. A nested forecast was then run over southeast Australia at resolutions of 5 km and 2 km, respectively, covering an area centred on the Sydney basin, as shown in Figure Results 4.1 Case 1: 1 December 2000 Convective rainfall amounts and their pattern of distribution across the metropolitan area verify well when compared to the observed rainfall amounts and distribution. Figure 5a shows the rainfall accumulation and distribution pattern over the Sydney basin between 15

6 M S Speer, L M Leslie and L Qi Figure 4. Model domain for the nested 37.5 km grid. The 5 km and 2 km grid domain is also shown, located over Sydney. 2 pm and 8 pm on 1 December Almost all the rainfall occurred during this period and it closely matches the observed 24 h rainfall amounts (Figure 5b). There are two larger observed amounts of 24 mm and 50 mm at Horsley Park and Holsworthy AWS, respectively, which the model forecast failed to predict. However, for individual recording stations, such inaccuracies are to be expected since these scales are at the level of individual storm cells. Moreover, no additional observations were assimilated. Therefore, only the original archived operational data was used to generate the model initial state for this study. Figure 5a. 2 km resolution HIRES rainfall forecast distribution pattern over Sydney and surrounding area and accumulation (mm), between 2 pm and 8 pm on 1 December The convergence line between low level coastal south to southeasterly winds and westerly winds above is clearly reproduced by the model. Figure 6 shows the 850 hpa forecast wind field at 2 km horizontal resolution for 6 pm on 1 December, initialised nine hours earlier. There is a strong delineation between southerly Figure 5b. Observed 24 h rainfall (mm) to 9 am on 2 December 2000 for the Sydney basin. Note that all the rainfall occurred as a result of the thunderstorms, i.e. between 2 pm and 8 pm on 1 December. 16

7 Predicting severe convection Figure 6. 2 km resolution HIRES forecast (+9 h) wind field (km/hr) at 850 hpa, focusing on the Sydney basin, for 6 pm on 1 December Note the delineation between southerly and westerly winds shown in the centre lower half of the figure. Figure 7. 2 km resolution HIRES forecast focusing on the Sydney basin and showing the predicted hail-size distribution between 2 pm and 8pm on 1 December Contours are for hail diameter (interval = 1 cm). Note that marble-sized hail was reported at Chipping Norton and that golfball-sized hail was reported at Campbelltown. and westerly winds apparent in the centre lower half of Figure 6. The zone marking the change from westerly to south or southeasterly winds slopes eastwards with increasing height across the Sydney basin. However, comparison with Figure 3d is not possible since at 50 m above ground level this delineation occurs much further to the west and hence out of the model domain. There were no significant differences in rainfall amount and distribution between the modified Kain-Fritsch parameterisation scheme and the cloud microphysics scheme, or in the structure and location of the low level convergence line. The non-hydrostatic model forecast at 2 km horizontal resolution did provide valuable additional information in the form of predicting isolated hail (maximum diameter about 1 inch or just above 2 cm) in the metropolitan area (Figure 7). The report of marble-sized hail at Chipping Norton and the report of golfball-sized hail at Campbelltown are marked on Figure 7, and fall within the 2 cm and 1 cm diameter contours, respectively. This is an encouraging result considering that Sydney, and NSW in general, is highly prone to hail damage. Any hail forecast of this quality is a significant contribution to the probability of predicting hail. of the 41 official Bureau of Meteorology rainfall stations. Also, the significant low level intersecting boundaries between northeast and northwest to westerly winds to the southwest of Sydney, which were such a prominent and persistent observed radar feature during the afternoon, were realistically reproduced by the model (Figure 8). In the model there was a decrease in low to mid-level tropospheric moisture owing to cold air advection from over the ranges to the westsouth-west of Sydney, thereby inhibiting afternoon 4.2 Case 2: 8 December, 2000 The most significant result here was that the model forecast produced no rainfall over the Sydney basin, which verifies exactly with no recorded 24 h falls at any Figure 8. 2 km resolution HIRES forecast (+8 h) of the wind field (km/hr) approximately 50 m above ground level for 5 pm, 8 December Note the intersecting boundaries (dashed lines). Compare with Figure 3f. 17

8 M S Speer, L M Leslie and L Qi Table 1. Model values of dewpoint (ºC) indicating a decrease in moisture at Sydney Airport in the low to mid-tropospheric levels between 9 am and 5 pm (+8 h) on 8 December 2000 (initial model state). Model level Dewpoint Dewpoint (hpa) (9 am) (5 pm) Near surface convection. The decrease in moisture in the model is indicated by the model values of dewpoint at 9 am (initial state) compared to 5 pm (+8 h) shown in Table 1. This drying of the atmosphere is consistent with the observed decrease in dew point values in the low to mid-tropospheric levels from morning to afternoon, shown on the vertical temperature sounding (Figure 3e). 5. Discussion and conclusions Two days in December 2000, when the Sydney office of the Bureau of Meteorology officially predicted severe convection in the Sydney metropolitan area, have been discussed synoptically and modelled using the very high resolution NWP system, HIRES. They were selected in order to assess the utility of the guidance for forecasting severe convection in terms of rainfall and hail amount and distribution, and the depiction of key mesoscale mechanisms such as low tropospheric level convergence lines. In the first case, 1 December, operational forecasters predicted nonsevere thunderstorms, in probabilistic terms. Nonsevere convection was forecast as likely to occur. In fact, severe convection occurred. The HIRES numerical model correctly predicted severe convection in a deterministic sense. In the second case, 8 December, severe convection was again predicted as likely by operational forecasters, but did not eventuate. The HIRES NWP model again correctly predicted that no convection would eventuate in the standard forecast initialised at 9am on 8 December, and even from the earlier time of 9 pm on 7 December also predicted that there would be no convection. Development is now under way of a system that utilises both the probabilistic and/or deterministic forecasts in an integrated manner. The proposed system has three stages. The first stage is the current practice of a probabilistic/subjective approach. The next stage, of which this work represents a feasibility study, is a single forecast deterministic approach. The third stage, and ultimate aim, is the development of an ensemble, or related, approach for determining whether the forecast spread for a given set of initial states is large or small, especially for convection reaching the severe stage. Quantifying forecast uncertainty is an extremely important goal, particularly when the forecast can range from no convection to severe thunderstorms. These three stages together will provide an expert system for the prediction of severe convection. The main conclusion drawn from this study is that the 2 km numerical model predictions have the potential to improve operational forecasting of severe weather. If the HIRES forecasts had been available to the operational forecasters in real-time, the capacity for discriminating between severe and non-severe convection would have been present. However, for the second case, the model guidance by itself might not have be sufficient for forecasters to rule out the possibility of convection or severe convection, given that the soundings and other data available to the forecasters in the morning suggested strongly that severe convection was likely. To augment the probabilistic forecast of no convection/convection/severe convection that is routinely produced by forecasters, it would be highly desirable to cast the model guidance in probabilistic form. After the factors affecting the forecast have been assessed, a range of forecasts based on a set of equally plausible initial conditions could be used to generate probabilities. However, to utilise the model in this manner first requires the numerical forecasts to be producing routine, reliable deterministic predictions. The excellent deterministic model results of this study support the development of such a procedure for predicting convection over the Sydney basin. Acknowledgements The authors wish to acknowledge the support of an Australian Research Council Large Grant in carrying out this work. References Buckley, B. W., Leslie, L. M & Wang, Y. (2001) The Sydney hailstorm of April 14, 1999: Synoptic description and numerical simulation. Meteorol. Atmos. Phys. 76: Bureau of Meteorology (1993) Report on the Sydney Hailstorm of March Bureau of Meteorology, PO Box 1289K, GPO Melbourne, Victoria, Australia, 3001K, June 1993, 44pp. Bureau of Meteorology (1999) Report on the Forecasting and Warning Performance for the Sydney Hailstorm of 14 April Bureau of Meteorology, PO Box 1289K, GPO Melbourne, Victoria, Australia, 3001K, May Kain, J. S. & Fritsch, J. M. (1990) A one-dimensional entraining/detraining plume model and its application in convective parameterization. J. Atmos. Sci. 47: Leslie, L. M. & Purser, R. J. (1995) Three dimensional massconserving semi-lagrangian scheme employing forward trajectories. Mon. Wea. Rev. 123:

9 Predicting severe convection Leslie, L. M. & Speer, M. S. (1998) Short-range ensemble forecasting of explosive Australian east coast cyclogenesis. Wea. Forecasting 3, Speer, M. S. & Leslie, L. M. (1997) A climatology of coastal ridging over southeastern Australia. Int. J. Climatol. 17: Speer, M. S. & Leslie, L. M. (2000) A comparison of five flood rain events over the New South Wales north coast and a case study. Int. J. Climatol. 20: Wilson, J. W. & Schreiber, W. E. (1986) Initiation of convective storms by radar-observed boundary layer convergence lines. Mon. Wea. Rev. 114:

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