Limited-Area Ensemble Prediction: the ARPA-SMR LEPS system Stefano Tibaldi, Tiziana Paccagnella,

1. Limited-Area Ensemble Prediction: the ARPA-SMR LEPS system Stefano Tibaldi, Tiziana Paccagnella, Chiara Marsigli, Fabrizio Nerozzi, Andrea Montani ARPA-SMR With contributions from F.Molteni, R.Buizza and H.Hersbach, all at ECMWF, at some time

2. OUTLINE • The need for the regionalisation of scenarios • The LEPS approach • Methodology • Some case studies • Statistical evaluation • COSMO-LEPS • Concluding remarks

3. THE NEED FOR REGIONALISATION OF SCENARIOS (1) Despite the recent increase of computer power resources, which have allowed the development of more and more sophisticated NWP models, the accurate forecast of extreme weather conditions, especially when related to intense and localised precipitation structures, is still difficult. This limitation is due, among other reasons, to the inherent low degree of deterministic predictability associated to this kind of phenomena.

4. THE NEED FOR REGIONALISATION OF SCENARIOS (2) Global-model ensemble systems have been shown to be important tools to tackle the predictability problem beyond day 3. Operational ensemble systems are usually run at a coarser resolution with respect to single deterministic model integrations for obvious economy reasons. The EPS skill in producing quantitative forecasts of intense and localised events in the short- and early-medium-range is still limited, although growing.

5. THE LEPS APPROACH • The main purpose of the LEPS project is to produce a system capable of providing the forecaster some probabilistic guidance to identify the possible occurrence of severe weather conditions in the time range: • “late-short-range (>48h) up to early-medium-range (120h)”.

6. THE LEPS APPROACH LEPS is designed to combine in a single system the (supposed) ability of a global ensemble prediction system to generate an exhaustive set of large-scale evolution scenarios (through an adequate sampling of the phase-space in the neighbourhood of the best available initial conditions) With the (supposed) capability of a LAM of detailing atmospheric phenomena on the local scales, particularly the precipitation field, in regions with complex orography

7. The B.F. (Brute Force) APPROACH The obvious solution: one LAM integration for each global EPS member All the information from the global EPS is retained BUT it is hardly feasible on an operational basis (at least at ARPA-SMR)

8. B.F.: REQUIRED RESOURCES (evaluated on COSMO-LEPS configuration) • For each LAM run: • Computer time: • +120 hours of LAM integration • 306 x 258 x 32 grid points • 13 hours cpu on vpp5000 • Data volume (IC and BCs): • 0.9 GB for each member 51 runs 663 hours cpu! 46 GB!

9. THE LEPS APPROACH The LAM is nested in only a limited number of members selected from the global EPS, the Representative Members Some of the information from global EPS is lost BUT the operation becomes feasible on an operational basis

10. Most Representative Member one per cluster choice is based on selected 3D fields: has to be: the closest to the mean of its own cluster AND the most distant to the other clusters’ means 5 runs instead of 51, 102 or 153!!

11. Dim 2 Possible evolution scenarios Initial conditions Dim 1 LAM scenario Dim 2 LAM scenario LAM integrations driven by RMs LAM scenario Dim 1 Initial conditions LEPS – Limited area Ensemble Prediction System EPS and ensemble size reduction Cluster members chosen as representative members (RMs)

12. precipitation scenarios PROBABILITY MAPS: WEIGHTING?????? Dim 2 Dim 1 Initial conditions LEPS – Limited area Ensemble Prediction System LAM scenario LAM scenario LAM scenario

13. The basic idea was to leave the task of exploring the phase space to the global EPS, while the LAM has to zoom in the forecast, producingadequately intense local phenomena (e.g. precipitation maxima),but….

14. Precip outliers91 ECMWF EPS DJF 1999/2000

15. QUESTIONS: Are we adequately sampling the space of possibilities (i. e. the phase space around the initial conditions)?

16. Spread: EPS started at 14 May 1999, 00 UTC The same 700hPa isoline plotted for the 51 Members at + 72 The same 700hPa isoline plotted for the 51 Members at + 120

17. Spread vs error Mean square error/mean square spread

18. How can we improve our exploration of the phase space, increasing the available number of EPS forecasts? By resurrecting the old concept of time-lagged ensemble forecast! The Super-Ensemble!

19. Precip outliers91 ECMWF EPS DJF 1999/2000

20. clustering time start of integrations Day n-2 EPS 102 members Day n EPS Day n-2 Day n Verification period LEPS super-ensemble

21. Summary of the LEPS methodology Super ensemble: 2 global ensembles starting 5/3 days before the verification time 102 members (50 + 1)*2 Hierarchical Cluster Analysis method: Complete Linkage area: Southern Europe fields: 4 variables at 4 levels (3D cluster) number of clusters: fixed to 5 5 clusters • Representative Member Selection • one per cluster • base on the nearest (3D fields) to the mean of its own cluster AND the most distant to the other clusters’ means 5 representative members (RMs) 5 LAMBO integrations nested on 5 RMs: LEPS - Limited-area (High Resolution) Ensemble Prediction System

22. LEPS:SOME CASE STUDIES

23. Soverato: 8, 9, 10 - Sep - 2000

30. Soverato flood

31. Soverato (Calabria) flood 91 ECMWF proxy of 24h cumulated precipitation 10/09/2000 at 00Z

32. Between 9 and 10 September, rainfall peaks above 300 mm in 24 hours were recorded close to the village of Soverato this causing landslides, great disruption and losses oflife. 8 9 10 8 9 10

33. Soverato (Calabria) flood Ensemble T255 P > 20 mm/ 24 h P > 50 mm/ 24 h P > 100 mm/ 24 h 100 30 ECMWF probability maps for 24h cumulated precipitation exceeding P threshold at +60h

34. P > 20 mm/ 24 h P > 50 mm/ 24 h P > 100 mm/ 24 h Soverato (Calabria) flood LEPS – 5 LAMBO runs driven by the 5 RMs selected from 153 members of the Super-Ensemble TL255 75 42 58 47 LEPS probability maps for 24h cumulated precipitation exceeding P threshold at +60h

35. P > 50 mm / 24 h 23 33 Soverato (Calabria) flood: BF vs LEPS LEPS: 5 weighted LAMBO runs on ensemble EPS TL255L40 Brute-force approach 51 LAMBO runs on ensemble EPS TL255L40 (51 members) P > 50 mm / 24 h 25 54 Probability maps for 24h cumulated precipitation exceeding threshold at +60h

36. MAP IOP2b: 19, 20 - Sep - 1999

41. MAP IOP 2B, 20-21 September 1999 24 hours observed precipitation from 06 to 06 UTC

42. MAP IOP 2B ECMWF EPS forecasts 51 members P > 20mm/24h P > 50mm/24h 98% ECMWF probability maps for 24h cumulated precipitation exceeding threshold at +66 hours

43. MAP IOP 2B LEPS: 5 LAMBO runs at 20 km on Super-Ensemble TEPS TL159 P > 20mm/24h P > 50mm/24h LEPS probability maps for 24h cumulated precipitation exceeding threshold at +66 hours

44. MAP IOP 2b BF vs LEPS LEPS: 5 weighted LAMBO runs at 20 km on ensemble EPS TL159 Brute-force approach 51 LAMBO runs on TEPS TL159 (51 members) P > 50mm/24h P > 50mm/24h Probability maps for 24h cumulated precipitation exceeding threshold at +66 hours