Use of Probabilistic Forecasts

1. Use of Probabilistic Forecasts

2. Ensembles • These are a number of forecasts all run from similar, but slightly different initial conditions • The same forecast model is run many times • The resulting forecasts are grouped together to aid the forecaster • Forecasts from several different models can also form an ensemble (“poor man’s ensemble”)

3. Displacement Small differences here Time

4. BIG differences here Displacement Small differences here Time

5. Ensembles - By running the model many times with small differences in initial conditions (and model formulation) we can: • take account of uncertainty • estimate probabilities and risks (eg. 30 members out of 51 = 60%)

6. Members or Resolution? • Which is more important – to have as many ensemble members as possible or to have a higher resolution, and therefore fewer members? • In practice it will be a compromise between the two!

7. Probabilistic input. Time evolution, one location. PLUMES METEOGRAMS

8. Operational High Resolution model T799 (25 km) Control member 1 member EPS Plume diagram 850 hPa Temp

9. Probability density is shaded. 10-30% of members have temperatures within this range (70-90% outwith). Thinner this band the higher the certainty. Range of temperatures indicated by the ensemble

10. EPS total precipitation rate (mm) in 12 hours Operational High Resolution EPS control 1 Ensemble member

11. Probabilistic input. Individual solutions, one time frame POSTAGE STAMPS

12. Interpreting Ensemble Data • The presentation of results is important • Need to reduce the different solutions to something manageable • Clustering - grouping solutions that are similar • Probability forecasting

13. Carlisle storm, Jan 05, from ECMWF 51-member medium-range ensemble

14. Cyclone plumes OBSERVED Forecasts starting from 00Z 06/01/05

18. Ensembles – estimating risk By running model(s) many times with small differences in initial conditions (and model formulation) we can: • take account of uncertainty • estimate probabilities and risks • eg. 10 members out of 50 = 20%

19. National Warning - Issued on Monday, 2 October 2006 Heavy falls of rain are possible in places in the Overberg, Breede River Valley, Ruens, Garden Route and the Little Karoo, Eastern Cape coast and adjacent interior and KwaZulu-Natal.

20. GFS Ensemble T+96 for 00z Tuesday 3rd October 2006

21. GFS 12-18z Mon 2nd Oct 2006

22. ECMWF 12-18z Mon

23. GFS 12-18z Tue 3rd Oct 2006

24. ECMWF 12-18z Tues

25. EPS Meteograms for Durban (00z Sat-00z Mon)

26. The main difficulties lie in … • Knowing what to look at • Knowing when to stop! • Absorbing and retaining the information long enough to use it …

27. EC Total ppn prob > 20mm 12z Tue – 12z Wed

28. EFI ECMWF for Tue-Wed

29. What Happened? • Margate 136mm. Monthly average 133mm • East London 83mm.    Monthly average 131mm

30. Impacts? • Torrential rains cause havoc in the Eastern Cape • October 04, 2006, 11:15 • Torrential rains in the Transkei region of the Eastern Cape have caused major damage to homes, roads and schools. People in rural areas, who have mud houses, are the hardest hit. Two more schools have collapsed in Butterworth and some rural roads are flooded. Floods caused five deaths in the area last week near Lusikisiki. Near Butterworth, the low-lying Ceru bridge is overflowing. Bennet Malisana, from Butterworth lost his wife during earlier floods in 1993 when she was among 11 people swept away in a bakkie while crossing a river. He says his area has been hit again. "We have been greatly affected by the rain because we desperately need a bridge," he added. http://www.sabcnews.com/south_africa/general/0,2172,136028,00.html

31. TC Boloetse

32. Life history

33. Strike probability map 24th Jan

34. Strike probability map 26th Jan

35. Strike probability map 29th Jan

36. Strike probability map 1st Feb

37. Strike probability map 4th Feb

38. MOGREPS

39. ECMWF EPS has transformed the way we do Medium-Range Forecasting Uncertainty also in short-range: Rapid Cyclogenesis often poorly forecast deterministically (eg Dec 1999) Uncertainty of sub-synoptic systems (eg frontal waves) Many customers most interested in short-range Assess ability to estimate uncertainty in local weather QPF Cloud Ceiling, Fog Winds etc Short-range Ensembles

40. Initial conditions perturbations • Perturbations centred around 4D-Var analysis • Transforms calculated using same set of observations as used in 4D-Var (including all satellite obs) within +/- 3 hours of data time • Ensemble uses 12 hour cycle (data assimilation uses 6 hour cycle)

41. Initial conditions perturbations Differences with ECWMF Singular Vectors: • It focuses on errors growing during the assimilation period, not growing period: - Suitable for Short-range! • Calculated using the same resolution than the forecast • ETKF includes moist processes • Running in conjunction with stochastic physics to propagate effect

42. Model error: parameterisations • QUMP (Murphy et al., 2004) • Initial stoch. Phys. Scheme for the UM (Arribas, 2004) Random parameters

43. MOGREPS products

44. Postage stamp maps

45. Questions & Answers