generation methods and skill of subseasonalpredictions in ... · ensemble generation methods and...

Ensemble generation methods and Skill of Subseasonal predictions in the

NCEP GEFS

Malaquías Peña1, Yuejian Zhu, Kate Zhou1, Dingchen Hou, Richard Wobus1, Qin Zhang2

EMC, 1IMSG at EMC, 2CPCAcknowledgment: EMC’s Global C&W Modeling Branch

18 Aug 2014 WWOSC, Montreal, Canada

Global operational ensemble prediction systems at NCEP

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CFSv2 (out to 10 mo) GEFS (out to 16days)

Coupled Yes Atmos‐Land

Ensemble size 16 80

Resolution T126L64 T254L42

Year of operation 2011 2012

Ensemble Generation Lagged ETR

Community served Climate Weather

Hindcast CFSRR (Saha et al) ESRL (Hamill et al)

CFS included in the NMME, IMME systemsGEFS included in the NAEFS

Version Implementation

Initial uncertainty

TSrelocation

Modeluncertainty

Resolution Forecast length

Ensemble members

Daily frequency

V1.0 1992.12 BV None None T62L18 12 2 00UTC

V2.0 1994.3 T62L18 16 10(00UTC)4(12UTC)

00,12UTC

V3.0 2000.6 T126L28(0-2.5)T62L28(2.5-16)

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V4.0 2001.1 T126(0-3.5)T62L28(3.5-16)

V5.0 2004.3 T126L28(0-7.5)T62L28(7.5-16)

00,06,12,18UTC

V6.0 2005.8 TSR T126L28

V7.0 2006.5 BV- ETR 14

V8.0 2007.3 20

V9.0 2010.2 STTP T190L28

V10.0 2012.2 T254L42 (0-8)T190L42 (8-16)

V11.0 2014.12 EnKF (f06) T574L64 (0-8)T382L64 (8-16)

Evolution of NCEP GEFS configuration

Is there any lingering prediction skill beyond 2 weeks in the GEFS?

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• Wheeler and Hendon MJO index

• GEFS (2006): T120L42 Resolution

• Decaying average BC in solid line;

• CFS (v1; 2003; T62L64) in grey dashed lines

Can this good skill be reproduced for other periods?

• Period coincided with strong MJO signal

AC score

WH Time Series of Mag(RMM1, RMM2)

Time series of Skill

• Forecasts at Day 10

in the extratropics

• Apr 2011‐ Oct 2013

• Multi‐model

average (NUOPC)

CRPSS



• Forecasts at Day 10

in the extratropics

• Apr 2011‐ Oct 2013

• Multi‐model

average (NUOPC)

Time series of Skill

Experimental Setup

Three forecast segments:High Resolution: T254L42 (55km); 0‐8 daysLow Resolution: T190L42 (73km); 8 to 16 days

Extended Resolution: T126L42 (105km); 16 to 45 days

• Daily runs initialized at 00Z, no cycling• First two segments same as operational GEFS and ESRL’s Reforecast

• Last segment has additional 10% computational cost to current GEFS

• Hybrid EnKF‐3DVar initial conditions (and ETR to perturb them)

Three experimental periods

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Strongsignal

Weak signal

Regular signal

Wheeler and Hendon‐MJO Forecast Skill

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Weak Signal CaseGEFS Ensemble Mean, Raw

Benchmark climatological AC of raw CFSv2

WH‐MJO Forecast Skill

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Regular CaseGEFS Ensemble Mean Raw

WH‐MJO Forecast AC Skill

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Strong CaseGEFS Ensemble Mean Raw

Notice AC at lead 0. Clearly, large initial errors does not imply bad long‐range predictions

Only whenthe MJO signal isstrong, theAC skillexceeds thebenchmark

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Strong

Weak

CRPSS

Skill of predictionalsoincreases in someextratropicalvariables

Recent experiments

• Using the parallel GEFS model• Focus on last Winter 2013‐2014

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Three forecast segments:High Resolution: T574L64 (Phys lower, 34km); 0‐8 daysLow Resolution: T382L64 (Phys lower, 55km); 8 to 16 days

Extended Resolution: T254L64 (73km); 16 to 35 days

• Daily runs initialized at 00Z• New hybrid EnKF‐3DVar analysis• Initial perturbations from F06h Hybrid EnKF‐3DVar, centered • New version of the GFS (Semi‐Lagrangian, daily high‐resolution SST and

ice)

9.1d9.1d 9.6d

76%

74%

12 hours improvement of skillful forecast

2% improvement of 7‐day forecast AC score

Treatments of SSTs1. Initial anomalies damped towards climatology

– Skill benchmark

2. One‐way (AMIP‐type) runs– Prescribing analysis SST every 24h as integration progresses

3. Two‐tiered runsIn preparation

Comparison of 16‐d fcst for NH 500hPa height

Operation (black); Parallel (red); SST (green)(67 cases)

SST impact:Based on these 67 cases, the SST variations(realistic) impacts northern hemisphere mid-latitude forecast in terms of ensemble meanand probabilistic forecast. An mainimprovement happens for week-2 forecast

Anomaly correlation RMS error

And spread

CRPS

NH 500hPa height anomaly correlation for ensemble mean (raw)

6 months (control) Damping vsprescribed

12.5 days (50% AC)

Sampling size is small

Polar Vortex for 2013‐2014 winter500hPa height

Forecast leads – 34 daysValid at: Jan 5th 2014

Verifying Analysis

34‐days Ensemble mean forecast

Northern Hemisphere Blocking Indices for Winter (2013‐2014)GEFS ensemble mean (raw) ‐ production

10‐day

12‐day

5‐day

7‐day

1‐day

OBS

OBS

OBS

Fields are smoothed out in the ensemble

mean

Northern Hemisphere Blocking Indices for Winter (2013‐2014)GEFS probabilistic forecast (10%) – parallel run

15‐day

10‐day

12‐day

7‐day

OBS

OBS

5‐day1‐dayOBS

Northern Hemisphere Blocking Indices for Winter (2013‐2014)GEFS probabilistic forecast (10%) – parallel extended fcst

24‐day

26‐day

22‐day

18‐day

16‐day

OBS

OBS

OBS

28‐day

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September 01, 2013 through Feb 28, 2014

Equatorial (5S to 5N) 850U band‐passed

Analysis 5‐day Forecasts

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Priorities

• Reduction of systematic errors• Test two‐tier approach• Improve stochastic parameterization• Test surface perturbations • Sensitivity of performance to surface anomalies

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Summary

• Experimental GEFS integrations are been carried out toexplore their feasibility and potential utility at extended range

• Three periods during 2011‐2012, corresponding toregular, strong and weak MJO signal were completed. – Key preliminary result: Skill of extratropical variables is largewhen the MJO signal is strong.

• A new experiment is ongoing to assess the impact of SST. – Key preliminary result: Prescribing realistic SSTs improve skillat medium and extended range time‐scales.

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Building capability to– Capitalize on couple modeling tools at NCEP and othercenters

• The CFSv2 couples atmosphere, land, ocean and ice• NMME, IMME, hindcast data

– Produce surface perturbations• Uncertainty in the boundary with ocean, land, ice

– Broader statistical post‐processing methods (wea‐clim)• Larger, seasonaly dependent biases• Generation and untilization of hindcasts

– Process‐oriented evaluation methods, besidesprediction skill measures

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Science issues in the Extended GEFS• I.C. Frequency• Lead time (32)• Coupling with ocean• Membership (use of lagged forecasts?)• Vertical Resolution• Horizontal Resolution• Reforecast

– Length– IC frequency– Membership

Limited by computer resources

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!Number of correct Yes forecast divided by the total number of occasions on!which that event was forecast and/or observed.

generation methods and skill of subseasonalpredictions in ... · ensemble generation methods and...

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