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Developments in the Use of Short and Medium-Range Ensembles at the Met Office

Developments in the Use of Short and Medium-Range Ensembles at the Met Office. Ken Mylne Ensemble Forecasting Manager. Contents. First Guess Early Warnings – again! Example of EPS Use for Confidence New Applications of EPS Drought-planning Forecasts Best-Member Project

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Developments in the Use of Short and Medium-Range Ensembles at the Met Office

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  1. Developments in the Use of Short and Medium-Range Ensembles at the Met Office Ken Mylne Ensemble Forecasting Manager

  2. Contents • First Guess Early Warnings – again! • Example of EPS Use for Confidence • New Applications of EPS • Drought-planning Forecasts • Best-Member Project • Eurorisk PREVIEW Windstorms • MOGREPS – The new Met Office short-range ensemble

  3. First Guess Early Warnings- A confession

  4. FGEW Verification – an error • Over the last few years we have reported on verification results from our First Guess Early Warnings • A key feature was a maximum in skill at 4 days • Much discussion and investigation • With regret and apologies, I have to report that this “result” was caused by an error in our verification scheme • Misalignment of forecast times with verifying times in the verification database • Day 4 was correctly aligned, other days wrong

  5. Corrected Verification - ROC • With the bug corrected the verification shows a near-constant ROC area for days 1-4 of the forecast with a slow decline from day 5 or 6.

  6. Corrected Verification - Reliability • Reliability diagrams show useful resolution at 2 and 4 days.

  7. Corrected Verification – Cost-loss • Cost-loss curves show slow reduction in values at longer lead times • Largest effect for large C/L ratios • Increasing chance of capturing event at high probability

  8. Example of EPS Use for Confidence

  9. Example – low spread, high confidence • Cold period in November ‘06 with snow in SW • ECMWF EPS gave very high confidence of blocking breakdown • Allowed issue of high confidence of return to mild conditions on Wed 30th 5 days ahead • Analysis of 0600 on 30th confirms this was correct

  10. New Applications of EPS - i) Drought Planning Forecasts

  11. Drought planning forecasts • Probabilities for 10-day rainfall totals • Mod-High probability of below normal • Only low risk of very dry (<5mm in 10 days) in SE England

  12. New Applications of EPS - ii) “Best Member” Project

  13. Integrating Ensembles into Operations • We would like to make all forecasts probabilistic • BUT Met Office customers still demand a best-estimate deterministic forecast • Currently use field modification • Link UK GM at T+36 with preferred solution later • Can we choose an ensemble “Best Member”? • Best fit to preferred solution, or Ensemble Mean? • Representative member of largest cluster? • Best Member could give: • Dynamically consistent forecast at all times • Full set of consistent model fields

  14. Best Member project • Project being run with Operations Centre to test: • Can we identify a suitable Best Member? • How long does a ‘good’ EPS member stay ‘good’? • Can we re-calculate probabilities? • Expect BM selected for mesoscale may be different from that for synoptic scale/medium range forecasts. • Use error tracking to identify source regions • Consider short and medium ranges separately, using MOGREPS and EC EPS respectively.

  15. New Applications of EPS - iii) Eurorisk PREVIEW Windstorms

  16. Risk Mapping Data (SMHI) Database (Met Office UK) Website (Met Office UK) Eurorisk PREVIEW Windstorms Multi-model ensemble forecasts for Windstorm Risk Adapted existing site-specific database tables for additional ensembles 196 sites across 9 countries Kalman Filter Ensemble Forecasts PEACE (MeteoFrance) SRNWP-PEPS (DWD) LAMEPS (Met.no) ECMWF EPS (ECMWF) MOGREPS ( Met Office UK) COSMO-LEPS Windstorm Forecast Products Windstorm Risk Mapping Products Wind Gust Calculation (Meteo-France)

  17. Eurorisk PREVIEW Windstorms Products • Traffic light maps for initial alerts • Site-specific details: • Meteograms • Wind-roses

  18. Eurorisk PREVIEW Windstorms • Alert thresholds will be a combination of probability and severity, eg. • Thresholds referenced to site climates • Analysed from ERA40 • Calibrated using site observations

  19. MOGREPS – Met Office Global and Regional EPS

  20. MOGREPS – The Met Office short-range Ensemble NAE • Ensemble designed for short-range • Regional ensemble over N. Atlantic and Europe (NAE) • Nested within global ensemble • ETKF perturbations • Stochastic physics • T+72 global, T+36 regional • Aim to assess uncertainty in short-range, eg.: • Rapid cyclogenesis • Local details (wind etc) • Precipitation • Fog and cloud MOGREPS is on Operational Trial for 1 year from September 2005

  21. Ensemble Transform Kalman Filter (ETKF) 0.9 Pert 1 -0.1 Pert 2 -0.1 Pert 3 -0.1 Pert 4 -0.1 Pert 5 ( - ) + = ( - ) + = ( - ) + = ( - ) + = ( - ) + = T+12 perturbed forecast T+12 ensemble mean forecast Transform matrix Control analysis Perturbed analysis

  22. Stochastic schemes for the UM • Stochastic Convective Vorticity (SCV) • Unresolved impact of organised convection (MCSs) • Not used in the higher resolution regional ensemble • Random Parameters (RP) • Structural error due to tuneable parameters • Stochastic Kinetic Energy Backscatter (SKEB) • Similar to ECMWF CASBS scheme • Excess dissipation of energy at small scales • SKEB implemented in global ensemble in PS11 Impact is propagated to next cycle through the ETKF!

  23. Display system

  24. Products – Postage stamps

  25. Products – Probability charts

  26. MOGREPS Site-specific forecasts • EPS Meteograms • MOGREPS Plume • Kalman filter MOS is being implemented for MOGREPS forecasts

  27. Verification • Verification to date is very basic • Verification is being implemented within the Area-based Verification system (ABV) and Site-specific Verification system (SBV) • Verification performed over NAE area for forecasts from global ensemble • Performed (except where stated) against analysis • For 111 cycles between 17/10/05 and 9/1/06

  28. 500hPa height – spread and RMSE Spread and RMSE for 500hPa GPH • Spread optimised by variable inflation factor against observations in u, v, T and RH at T+12 • Appears too large because verified against analysis (tbc) • Ensemble mean skill does not currently improve on control skill

  29. 500hPa height – rank histogram Rank Histogram at T+72 for 500hPa GPH Solid – operational, Dotted – PS11 upgrade • Rank histogram is encouragingly flat • Close to ideal • Suggests that ETKF perturbations are representative of genuine analysis errors • This performance seems much improved on ECMWF ensemble

  30. Verification - T+36 6hr total > 0.5mm Global Ensemble Regional Ensemble

  31. Verification - T+36 6hr total > 5mm Global Ensemble Regional Ensemble

  32. Verification - T+36 6hr total > 0.5mm Global Ensemble 0.84 0.88 Global Regional

  33. Conclusions • FGEW verification resolved! • EPS continues to become more integrated in forecasting procedures • First-choice for new services (eg drought planning) • How to produce best deterministic forecast? • Basis of new risk-management services for EU • MOGREPS provides a promising new short-range ensemble capability • complements EPS for medium-range

  34. Any questions?

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