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Use of EPS at the Met Office. Ken Mylne and Tim Legg. Outline. Update on verification of First-Guess Early Warnings of severe weather Example of unusual model and EPS behaviour Met Office short-range ensemble development.

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Use of EPS at the Met Office

Ken Mylne and Tim Legg

  • Update on verification of First-Guess Early Warnings of severe weather
  • Example of unusual model and EPS behaviour
  • Met Office short-range ensemble development
aims of this investigation







Aims of this investigation
  • Verification of Early Warnings of Severe Weather presented last year showed a maximum in skill at day 4 for
    • EPS forecasts
    • Deterministic forecasts from T511 and EPS control
  • Robust result but
    • ECMWF could not replicate
    • Very little support in literature
  • Here we report further investigations of:
    • Can same result be replicated with Met Office model (UM)?
    • Definition of weather events.
verification of early warnings
Verification of Early Warnings
  • Early Warnings are verified against ‘Flash’ Warnings issued at short-range for the same events (with a high degree of certainty)
  • Warnings are verified on an “event” basis
    • An event can be a forecast event or an observed event
    • Each event is counted only once however long it lasts
    • This will be discussed further
  • Assessment period: 26 August 2003 to 29 April 2005.
verification of um based early warnings roc results heavy rainfall
Verification of UM-based Early Warnings ROC results (Heavy Rainfall)

Heavy Rainfall events


  • UM results very similar to ECMWF T511
  • UM also has 4-day skill max
  • UM ROC Areas:
  • D+1 0.522
  • D+2 0.561
  • D+3 0.586
  • D+4 0.661
  • D+5 0.557
  • D+6 0.531
roc results for gales and snow
ROC results for Gales and Snow

Severe Gale events (Little evidence) Heavy Snowfall events (Similar to Rainfall)

cost loss results heavy rainfall
Cost/Loss results (Heavy Rainfall)




Met O U.M.

spatial averaging
Spatial Averaging
  • Anders Persson suggested the 4-day skill max could be due to predictability on the spatial scale of the UK
    • This would suggest a shorter period max for smaller regions
    • FGEW also gives probabilities for 12 sub-regions of the UK
      • Verification of these sub-region probs also show the 4-day max
      • We have not found any evidence to support this idea but it does merit further investigation
the effect of how we define an event
The effect of how we define an ‘event’

There are two main ways of defining events for verification

(i) On an ‘event-wise’ basis – when an event occurs, did we have an early warning of it? And when an early warning exists, did an event occur? One contingency-table entry per event.

(ii) On a time-wise basis – at fixed time intervals, look to see whether or not an Early Warning and/or a Flash Warning were in force and complete contingency tables

Early Warnings have always been verified on an event basis:

  • This is different from most standard verification procedures which use method (ii)
  • Could this account for the day 4 skill-max?
definition of events for verification
Definition of events for verification

Events are defined as:

  • An event can be a forecast event or an observed event
  • Each event is counted only once however long it lasts
    • Event spanning 2 days counted for 1st day only
    • Changing this to last day did not affect the day-4 skill max
  • For a warning to be correct (Hit), a warning and a verifying Flash Warning must coincide for part of their validity period
    • Flash with no warning is a Miss
    • Warning with no Flash is a False Alarm
    • Correct Rejections defined for a complete 24-hour period with no warning or Flash (except one already verified for previous day)
results for different event definitions
Results for different event definitions

‘Event-wise’ ‘Time-wise’

for Heavy Rainfall warnings (01 Oct 2003 – 03 Nov 2004)

conclusions on day 4 skill max
Conclusions on day-4 Skill Max

First-guess Early Warnings verification designed to assess the skill of warnings issued to end users:

  • Event-based
    • Events of variable length
    • Each event verified once only
    • Precise timing not required for success – only some overlap
  • Latest results show that it is this definition which leads to the day-4 maximum in skill apparent in the results
    • We do not claim to fully understand why! Could be related to spatial averaging as suggested by Anders.
    • We cannot assume that results from “standard” verification of NWP will apply to user-oriented products based on NWP.

email: or

eps model
EPS Model
  • Forecaster was surprised and concerned that EPS control and T511 were so different at day 6 in this forecast, with only a difference in resolution
    • Opposite ends of EPS distribution
    • Is this normal/ to be expected?
    • Has the previously reported problem with the EPS model (time-stepping?) been solved?


short range ensembles
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


Cloud Ceiling, Fog

Winds etc


Observation targeting

Multi-model ensemble contribution

LBCs for future storm-scale ensembles

Short-range Ensembles
ensemble prediction developments
Ensemble Prediction Developments


  • Ensemble under development for short-range
    • Regional ensemble over N. Atlantic and Europe (NAE)
    • Nested within global ensemble for LBCs
    • ETKF perturbations
    • Stochastic physics
    • T+72 global, T+36 regional
progress with etkf
Progress with ETKF
  • ETKF set up with global UM
    • Processing all observations used in data assimilation
    • 12-hour cycle (f/c twice per day)
    • Running in conjunction with stochastic physics to propagate effect
    • Encouraging growth rate in case studies
stochastic physics schemes
Stochastic Physics Schemes
  • Three components to current stochastic physics:
    • Installed in current version:
      • Stochastic Convective Vorticity (SCV)
      • Random Parameters (RP)
    • Under test:
      • Stochastic Kinetic Energy Backscatter (SKEB)


Stochastic Kinetic Energy Backscatter (SKEB)

  • Based on original idea and previous work by Shutts (2004)
    • Closely related to ECMWF CASBS scheme
  • Aim: To backscatter (stochastically) into the forecast model some of the energy excessively dissipated by it at scales near the truncation limit
  • In the case of the UM, a total dissipation of 0.75 Wm-2 has been estimated from the Semi-lagrangian and Horizontal diffusion schemes. (Dissipation from Physics to be added later on)
  • Each member of the ensemble is perturbed by a different realization of this backscatter forcing


  • Streamfunction forcing:

K.- Kinetic En.; R.- Random field;

D.- Dissipated en. in a time-step

R is designed to reproduce some statistical properties found with CRMs


u increments at H500

  • Largest at the jets/storm track


Preliminary results:

  • Positive increase in spread (comparable to that seen at ECMWF)

Increase in spread respect to an IC-only ensemble

500 hPa geopotential height



first full case study run 7 8 july 2004
First Full Case Study Run (7-8 July 2004)
  • ETKF spun-up over 7 days
    • Stochastic physics and ETKF interacting
  • Forecasts run to 5 days
  • Spread looks reasonable
t 48 postage stamps from january 8 2005 storm
T+48 Postage Stamps from January 8 2005 storm

Several members have better lowthan control. Member 4 is deeper. NB. This is global EPS.



project progress
Project Progress
  • Milestone: Implementation of demonstration ensemble based on NAE model for assessment by forecasters (August 2005)
    • Global ensemble has now been running in our parallel test suite for almost 2 weeks
    • NAE suite is nearly complete
    • Product generation and Verification systems are under development
  • We are on target to meet the milestone
medium range tigge
Medium-Range (TIGGE)
  • Global forecasts will be extended to 15 days to contribute to THORPEX multi-model EPS research
    • This will run at ECMWF using UK member state time
  • ETKF scheme believed suitable for Medium Range as well as Short Range
    • Perturbations scaled to 12h forecast errors – could be amplified if necessary
  • Planned configuration
    • 90km resolution
    • 20 members twice per day
  • Closer to understanding 4-day skill max in severe weather warnings from EPS
    • Standard NWP verification results may not always translate to user-specific products and verification
  • Good progress with development of ensemble capability at the Met Office
    • Short-range regional ensemble for Europe
    • Contribution to global medium-range ensembles for THORPEX