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The role of the stratosphere in extended-range forecasting Thomas Jung Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research Germany. Overview. Early experiments Relaxation experiments Extended-range predictions Dynamics of two recent cold European winters - PowerPoint PPT Presentation
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The role of the stratosphere in extended-range forecasting
Thomas Jung
Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research
Germany
Overview
Early experiments Relaxation experiments
• Extended-range predictions• Dynamics of two recent cold European winters
o The winter of 2005/06o The winter of 2009/10
Case study: The January 2009 sudden stratospheric warming event
Early experiments: Motivation
Early experiments
F is the mean optimal forcing (adjoint method, 18 cases)
F is applied in the stratosphere only Three 40-day forecasts for each of the winters 1980-
2001• Control• Positive forcing (strong vortex)• Negative forcing (weak vortex)
TL95L60 (cycle 28r1)
Stratospheric response (Z50)
Jung and Barkmeijer (2006); MWR
Stratospheric response (Z1000)
Jung and Barkmeijer (2006), MWR
Downward propagation: ERA-40
Jung and Leutbecher (2007), QJRMS
Downward propagation: TL95L91
Jung and Leutbecher (2007), QJRMS
Conclusions: Part I
“Classical” stratospheric forcing experiments indicate downward control of stratospheric circulation anomalies in the ECMWF model
Downward control is also found in long unforced integrations
The magnitude of the response in unforced experiments looks smaller than in forced experiments
Relaxation experiments
Study the impact of Northern Hemisphere stratosphere on tropospheric forecast skill
Strategy:
• Perform set of control experiments (standard NWP setting)
• Perform additional experiments with forecast error suppressed in the NH stratosphere
• Perform similar experiments for other areas (e.g. tropics)
Experimental setup
TL159L60 (32r1)
88 30-day forecasts (15th of Nov, Dec, Jan and Feb. of the period 1980/81-2000/01)
Initial and boundary conditions from ERA-40 Relaxation towards ERA-40 Persisted SST and sea ice Control, stratospheric and tropical relaxation
experiments
Relaxation regions
Stratospheric relaxation Tropical relaxation
NH Z500 scores
Jung et al (2010a), MWR
NH Z500 forecast error reduction
Jung et al (2010a), MWR
Conclusions: Part II
Relaxation experiments suggest that reducing stratospheric forecast error leads to better tropospheric forecasts
Overall the impacts on Northern Hemisphere forecast skill moderate
Locally larger impacts (e.g. North America and North West Europe)
Results in good agreement with Newman and Sardeshmukh (LIMs)
The cold European winter 2005/06
Jung et al. (2010b), MWR
Seasonal forecasts
Statistical techniques predicted negative NAO Dynamical techniques also predicted negative NAO (ie cold
winter) However, observed circulation anomaly not strictly the NAO
Suggested mechanisms
Folland et al. (2006)• Reemergence of North Atlantic SST anomalies
Scaife and Knight (2008)• Atlantic SST• January SSW
Croci-Maspoli and Davies (2009)• SST and 2mT anomalies in the western North
Atlantic and close to the eastern US seaboard Bader and Latif (2003)
• Cold SST anomalies Indian ocean NAO- Greatbatch and Jung (2007)
• La Nina NAO+
Experimental setup
TL95L60 (32r1) Atmosphere-only with observed SST/sea ice Lagged ensemble (17 members) started in the middle
of November 2005 Calibrations runs with and without relaxation (1990-
2006) Relaxation experiments (various regions)
Z500 anomalies: DJF 2005/06
Z50 anomalies: DJF 2005/06
Z500 anomalies: DJF 2005/06
Ensemble mean anomalies: Polar Z50
Conclusions: Part III
Origin of the cold European 2005/06 has been studied Various mechanisms have been proposed including an
important role of the Northern Hemisphere stratosphere
Our results suggest that the tropics have been important (for the NH troposphere and stratosphere)• Tropical stratosphere: QBO • Tropical troposphere
o South Americao Indian Ocean
The extreme negative NAO winter 2009/10
Jung et al. (2011), GRL
Operational ECMWF forecasts
Jung et al. (2011), GRL
Experimental setup
Jung et al. (2011), GRL
TL159L60 (36R1)
ECMWF Monthly Forecasting System Forecasts started on 1 November 2009 40 Ensemble members Control integration Various sensitivity experiments Hindcasts for each of the configurations
• 1991-2008• 4 ensemble members
Sensitivity experiments I
Jung et al. (2011), GRL
Sensitivity experiments II
D+18-D+32
D+18-D+32
Verifying analysis
Jung et al. (2011), GRL
Conclusions: Part IV
The winter 2009/10 was one of the most negative NAO winters on record
Extremely good monthly and seasonal forecast skill during the second half of the winter
Sensitivity experiments imply that external forcing and the Northern Hemisphere stratosphere played a secondary role
Internal tropospheric dynamics (highly predictable)?
The January 2009 SSW: A case study
Sensitivity experiments have been strongly forced Consider unforced experiments Case selection
• Good extended-range forecasts during the late part of the winter 2008/09
• Strong SSW in January 2009• Did the SSW contribute to the high skill?
Work with Frederic Vitart
Forecasts data
ECMWF VarEPS• T255 up to D+9• T399 from D+10
62 levels in the vertical (up to 5 hPa) Atmosphere-ocean coupling from day-10 50 ensemble members (operationally)
The January 2009 SSW
Start: 8. January 2009 Start: 15. January 2009
Separation into good and poor forecasts
Focus on forecasts started on 15. January 2009 Increase ensemble size to 100 members Separate into best and worst 10 SSW cases
Z500 anomalies
Z500 forecast anomalies (initialized on 15. January 2009)
The role of vertical resolution
Start: 8. January 2009 Start: 15. January 2009
Conclusions: Part V
Major SSW occurred in January 2009 Extended-range forecasts for Europe have been quite
skilful following the SSW Role of the SSW has been assessed by splitting
ensemble into two sub-groups (good and poor SSW forecasts)
Impact of the SSW seem to have played a secondary role
More cases need to be considered in future studies
Some general thoughts
Stratosphere provides a source of extended-range forecast skill in the Northern Hemisphere troposphere
Some areas might benefit in particular (e.g. Europe) However, the influence is likely to be moderate at best
(disagreement with Douville et al?) It is somewhat surprising that there is such a spread
regarding possible mechanisms of recent cold European winters
Further reading: