New Zealand Heavy Rainfall and Floods

Transcription

1 New Zealand Heavy Rainfall and Floods 1. Introduction Three days of heavy rainfall associated with a deep upper-level low (Fig. 1) brought flooding to portions of New Zealand (Fig. 2). The flooding was widespread across North Otago, Dunedin, and the Taieri Plain 1.Flooding in Otago in southern New Zealand. There was flooding along the Temuka River near Timaru in Southern New Zealand. Rainfall amounts over 140 mm were observed during the event. The heavy rainfall was associated with a deep 500 hpa trough which approached the region from the west on 23 May Figure 1. NCEP GFS 00 hour forecasts of conditions valid at 1200 UTC 24 May 2010 over New Zealand. Data shown include a) 250 hpa winds and wind anomalies, b) 850 hpa winds and wind anomalies, c) 500 hpa heights and height anomalies, and d) precipitable water (mm) and precipitable water anomalies. 1 wet big chill Otago Daily news Thursday 27 May Ahead of this system, strong northeasterly flow brought a surge of deep moisture from the tropical Pacific (Fig. 1d). This

2 configuration suggested a Maddox Synoptic (Maddox et al. 1979) event type with a sharp surge of tropical moisture poleward ahead of an approaching cyclone. The surge of high precipitable water air (PW) has the appearance of an Atmospheric River (AR: Neiman et al and Boa et. al 2006). This AR originated over the tropical Pacific and was pulled southward into New Zealand by a migratory upper-level cyclone. This note will document the New Zealand heavy rainfall event of May The purpose here is to show the AR and the value of anomalies in identifying these systems. 2. Methods The 500 hpa heights, 850 hpa temperatures and winds, other standard level fields were derived from the NCEP GFS, GEFS, and the NCEP/NCAR (Kalnay et al. 1996) reanalysis data. The means and standard deviations used to compute the standardized anomalies were from the NCEP/NCAR data as described by Hart and Grumm (2001). Anomalies were displayed in standard deviations from normal, as standardized anomalies. All data were displayed using GrADS (Doty and Kinter 1995). The standardized anomalies computed as: Figure 2. Map of New Zealand show location of some of the flooding near Otago. SD = (F M)/σ ( ) Where F is the value from the reanalysis data at each grid point, M is the mean for the specified date and time at each grid point and σ is the value of 1 standard deviation at each grid point. Model and ensemble data shown here were primarily limited to the GFS and GEFS. The 1.25x1.25 degree JMA data may be used when it becomes available. The NAM and SREF data were also available for use in this study. Displays will focus on the observed pattern and some forecast issues associated with the pattern.

3 For brevity, times will be displayed in day and hour format such at 24/0000 UTC signifies 24 May 2010 at 0000 UTC. 3. Results i. Pattern Figures 3-6 show the pattern over New Zealand as the event evolved. The 500 hpa pattern showed a sharp subtropical ridge off the northeast (Fig. 3a) and a short-wave moving eastward. This wave deepened as it moved eastward (Fig. 3a-f) and developed a 5460m closed low with -2 to -3SD height anomalies as it moved over New Zealand. At the surface a weak cyclone was evident (Fig.4) which slowly evolved into a strong cyclone. The GFS analyzed a 992 hpa cyclone with -3SD pressure anomalies at 25/0000 UTC and then a closed 988 hpa low east of New Zealand at 12/1200 UTC. The gradient in the pressure field implied strong south-southeasterly flow. Not surprisingly, the PW (Fig. 5) field showed a surge of high PW air with 2 to 4 SD above normal PW values surging toward New Zealand. These data are in 12-hourly increments, but capture the sense of the surge of high PW air into the region. The 850 hpa winds (Fig. 6) show the track of the 850 hpa cyclone and the surge of strong south-southeasterly winds, with the high PW air, into New Zealand. The wind anomalies peaked at 4 to 5SDs above normal at 25/0000 UTC. Though not shown, the v-winds were strong and the v-wind anomalies were highly negative, -4 to -5SDs from normal. This strong flow into the region implied strong forcing and the potential for heavy rainfall. Other fields could be shown; however the pattern here is a classic heavy rainfall pattern which should be relatively predictable. ii. Forecasts The NCEP GFS QPF s valid at 26/0000 UTC are shown in Figure 7. These data show that the pattern indicating heavy rainfall (section 3i) was handled well by the NCEP GFS. The model accumulated QPF showed impressive rainfall amounts over New Zealand. Due to forecast accumulation intervals this is not easy to see. But the longer range forecasts indicated areas of over 128 mm (5+ inches) over southern New Zealand. The pattern predicted by the GFS was similar to that shown in its own analysis and is not presented. The precipitation field shows that the forecasts were likely representative based on the distribution and amounts of QPF. iii. Ensemble forecasts NCEP GEFS forecasts, similar to the high resolution parent model, correctly predicted the pattern over New Zealand and indicated the potential for heavy rainfall. Due to the coarser resolution of the GEFS it obviously lacked the details and higher end amounts reflected in the GFS forecasts shown in Figure. 7.

4 Figure 8 show the GEFS PW forecasts with the spread and the lower panels show the mean and standardized anomalies. Figure show the 925 hpa winds and wind anomalies. These data show the strong winds into New Zealand with the plume or AR of deep moist air. With the correct sense of the large scale flow and the strong north-northeasterly winds, the GEFS produced heavy rainfall as shown in Figures These forecasts showed a large area of 2 inches (50mm) of QPF and a large areas covered by the 2.5 inch contour ( 62.5mm). Though hard to visualize there are several 3 inch (75mm) contours. These data show several members produced over 4 inches (100 mm) over 3 discrete areas of New Zealand. Figures show GEFS mean forecasts from 9 different forecast cycles. These data imply that the pattern was relatively well predicted and thus predictable by the GEFS. Clearly, the plume of high PW surging poleward into New Zealand (Fig. 12) was well predicted as was the strong low-level jet (LLJ:Fig. 13). Not surprisingly the GEFS predicted heavy rainfall (Fig. 14), albeit less than the high resolution GFS (Fig.7). 4. Conclusions A deepening mid-tropospheric cyclone moved over New Zealand on May 2010 producing heavy rainfall and flooding. The eastward propagating tapped an AR of deep moisture. The plume of high PW in this AR contained PW values over 3SDs above normal. Upon interacting with the terrain of New Zealand, this warm moist air mass produced widespread areas of 100 to 150 mm of rain. The rain led to flooding. The surge of high PW and the strong poleward 850 hpa winds are common in the eastern United States with heavy rainfall events. It would appear that a similar mechanism and pattern can produce locally heavy rainfall across New Zealand. The archetype for this type of event and the terrain influence are quite common throughout the world and are rather well predicted by most numerical weather predictions systems. The NCEP GFS did respectable in terms of where the and when the heavy rain would fall. Get the pattern, get the precipitation. Additionally, the GEFS produced excellent forecasts for a global ensemble. These course systems tend to under predict high end QPF amounts and relative to there high resolution models. But they provide patterns and probabilities which in turn provide confidence in the forecasts. It is likely that high resolution 12 and 4km models would have provided insights into the locally heavier rainfall amounts. There is a clear terrain signal in the QPF and there is distinct terrain which high resolution models would provide additional details to forecasters in shorter time ranges. 5. Acknowledgements Aaron Tyburski (NWS/CTP) for providing information about the case in general.

5 6. References Bao, J.-W., S. A. Michelson, P. J. Neiman, F. M. Ralph, and J. M. Wilczak, 2006: Interpretation of enhanced integrated water vapor bands associated with extratropical cyclones: Their formation and connection to tropical moisture. Mon. Wea. Rev., 134, Maddox,R.A., C.F Chappell, and L.R. Hoxit. 1979: Synoptic and meso-alpha aspects of flash flood events. Bull. Amer. Meteor. Soc., 60, Neiman, P.J., F.M. Ralph, G.A. Wick, J. D. Lundquist, and M. D. Dettinger, 2008: Meteorological characteristics and overland precipitation impacts of atmospheric rivers affecting the west coast of North America based on eight years of SSMI/satellite observations. J. Hydrometeor., 9,

6 Figure 3. GFS 00 hour analysis of 500 hpa heights (m) and height anomalies (σ) from GFS initialized at a) 0000 UTC 23 May, b) 1200 UTC 23 May, c) 0000 UTC 24 May, d) 1200 UTC 24 May, e) 0000 UTC 25 May and f) 1200 UTC 25 May 2010.

7 Figure 4. As in Figure 3 except for GFS mean sea level pressure (hpa) and pressure anomalies.

8 Figure 5. As in Figure 3 except for GFS precipitable water (mm) and precipitable water anomalies.

9 Figure 6. As in Figure 3 except for GFS 850 hpa winds (kts) and 850 hpa wind anomalies.

10 Figure 7. GFS forecasts of total accumulated precipitation (mm) valid at 0000 UTC 26 May 2010 from forecast initialized at a) 0000 UTC 23 May, b) 1200 UTC 23 May, c) 0000 UTC 24 May, d) 1200 UTC 24 May, e) 0000 UTC 25 May and f) 1200 UTC 25 May 2010.

11 Figure 8. NCEP GEFS forecasts form 1200 UTC 22 May 2010 valid at left) 0000 UTC 25 May and right) 1200 UTC 25 May Upper panels show each member 12.5 and 25mm contour.

12 Figure 9. As in Figure 8 except for GEFS ensemble mean 850 hpa winds (kts) and u wind anomalies and 850 hpa winds and v wind anomalies in the lower panels.

13 Figure 10. As in Figure 9 except for 48 hour accumulated rainfall valid at 1800 UTC 24 May and 1200 UTC 25 May. Upper panels show the probability of 4 inches or more QPF. Lower panel s show the ensemble mean QPF and each member s 4 inch contour color coded by member.

14 Figure 11. As in Figure 10 except valid for the 48 hours ending at 0000 UTC 26 May 2010.

15 Figure 12. GEFS forecasts valid at 0000 UTC 25 May 2010 showing GEFS mean precipitable water and precipitable water anomalies. Data from forecasts initialized at a) 1200 UTC 21 May 2010, b) 0000 UTC 22 May, c) 1200 UTC 22 May, d) 1800 UC 22 May, e) UTC 23 May, f) 0600 UTC 23 May, g) 1200 UTC 23 May, h) 1800 UTC 23 May and i) 0000 UTC 24 May 2010.

16 Figure 13. As in Figure 12 except for 850 hpa winds and total wind anomalies.

17 Figure 14. As in Figure 12 except for accumulated QPF for the period ending at 0000 UTC 26 May 2010.