Strategic Intervention A novel use of Ensembles in Forecast Guidance Ken Mylne (Ensemble Forecasting Applications Manager) & Nick Grahame (Head of the Guidance Unit, Operations Centre) © Crown copyright Met Office

Motivation – the Problem How to use forecasters’ guidance in automated production? Automated forecasts based on post-processed NWP verify best, but… No means for forecaster to correct rare forecast errors – busts How to edit or correct 5000 UK sites? Forecasters use Metmorph to produce modified guidance, but… Metmorph cannot feed automated 5000 sites One direct feed to BBC graphics © Crown copyright Met Office

The Solution – “Change Once, Use Many” Edgemoor Agreement – forecasters and research met at Edgemoor Hotel and agreed new solution Strategic Intervention – allow forecaster to select an alternative NWP solution that will feed downstream outputs directly. © Crown copyright Met Office

The Edgemoor Agreement Strategic Intervention tool The SI tool will provide: Allows the Chief Forecaster to select an alternative data source to drive the production process Reduced usage of downstream intervention Priorities for usage: Days 1-2 Beyond day 2 should be probabilistic – no intervention Impact – large impact on customers 3 types of errors anticipated: Poor representation of trapped BL cloud/fog in winter or spring anticyclones Errors in precipitation intensity/distribution/type Synoptic scale errors and outliers compared to other models © Crown copyright Met Office NWP Refresher - Ensembles

Intervened Production Probabilistic Production SI Production Flow Standard Production (as now) Intervened Production Probabilistic Production Latest Obs Forecaster Selects NWP guidance MOGREPS members Previous Deterministic NWP run MOGREPS member Latest Deterministic NWP run UKV EPS UKV follows guidance UKV UKV MOGREPS-UK member Post-processing & Automated Production Post-processing & Automated Production Post-processing & Automated Production Ensemble Best-Data Sites, Charts Website Ensemble products Impact model etc… Change once, use many Best-Data Sites, Charts Website Products etc… Best-Data Sites, Charts Website Products etc…

SI Project Phase 1 SI – allows forecaster to select the previous run of the deterministic NWP – delivered April 2011 Phase 2 SI – will allow selection of a member of MOGREPS Controversial! Phase 1 Previous Run Phase 2 MOGREPS Member © Crown copyright Met Office NWP Refresher - Ensembles

Selection of Ensemble Members – a good idea? Ensembles designed for estimating the local forecast pdf Perturbed members intended to be used as a group, NOT as alternative scenarios Perturbed members expected to verify worse than control – factor √2 Identification of the “preferred member” or the “best member” is very difficult: Best member locally at T+6 may not be best at T+24 or T+48 Influences from upstream Different best members for different parameters and locations © Crown copyright Met Office NWP Refresher - Ensembles

SI Potential - Daily Skill scores Previous run & MOGREPS members MSLP 25% members better than BaU © Crown copyright Met Office NWP Refresher - Ensembles

SI Potential - Daily Skill scores Previous run & MOGREPS members Temperature 25% members better than BaU © Crown copyright Met Office NWP Refresher - Ensembles

SI Potential - Daily Skill scores Previous run & MOGREPS members Precipitation Ave(ETS for 6h precip>0.5, 1.0, 4.0mm) 25% members better than BaU © Crown copyright Met Office NWP Refresher - Ensembles

Single day multi-variable plot Can be calculated for any member Different forecasts better for different variables © Crown copyright Met Office NWP Refresher - Ensembles

Potential for improvements MOGREPS often has >25% members better for certain variables Previous run occasionally better than BaU Little correlation between variables Less chance of there being a member which is best for everything Can we identify such a member if it exists? Likely that SI can only correct the worst element, with others being acceptable © Crown copyright Met Office NWP Refresher - Ensembles

Business Rules Using perturbed forecasts regularly would degrade the average verification Restrict SI selection to occasional use to correct severe errors Expect forecast to be degraded away from area targeted for correction Business Rules : BUSTS - Forecaster believes latest NAE is seriously in error having a major impact on UK customers. First 24h only © Crown copyright Met Office NWP Refresher - Ensembles

MOGREPS-UK and the UKV UKV is 1.5km convective-scale model UK 36h forecast in Best Data MOGREPS-UK is new ensemble using 2.2km version of UKV Under SI Phase 2 forecaster will select alternative solution from MOGREPS-UK Replace UKV in Best Data Selection by: Synoptic errors – upstream error detection in analysis and selection from MOGREPS-EU Mesoscale errors – error detection in UKV and selection from MOGREPS-UK © Crown copyright Met Office NWP Refresher - Ensembles

Tools to aid selection Tools to compare imagery with model fields Very difficult with web-based tools Now developing pseudo-imagery from MOGREPS members to allow full comparison with imagery in forecaster workstations © Crown copyright Met Office NWP Refresher - Ensembles

SI Limitations SI is NOT a replacement for Metmorph Used rarely – major busts only Chosen solution will not correct all errors or fully represent guidance Only 12 members to choose from Quite likely worse than latest run AWAY from targeted area Aim is to reduce major high impact error to lesser lower impact errors © Crown copyright Met Office NWP Refresher - Ensembles

Conclusions and Discussion Strategic Intervention allows the forecaster to select an alternative source of NWP to drive the automated forecast production Change Once, Use Many Latest model is usually the best Overuse of SI would be damaging Strict Business Rules for rare severe errors Unconventional use of ensembles Use with care! © Crown copyright Met Office