1. 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)
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2. 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
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3. 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.
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4. 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

5. 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…

6. 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
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7. 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
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8. SI Potential - Daily Skill scores Previous run & MOGREPS members MSLP
25% members better than BaU
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9. SI Potential - Daily Skill scores Previous run & MOGREPS members Temperature
25% members better than BaU
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10. 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
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11. Single day multi-variable plot Can be calculated for any member
Different forecasts better for different variables
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12. 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
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13. 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
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14. 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

15. 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

16. 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
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17. 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!
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