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Stratospheric Data Assimilation for NWP

Richard Swinbank

with thanks to:

David Jackson, Mike Keil, Andrew Bushell, Hazel Thornton

and Rick Rawlins

COST-723 Workshop, March 2004

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Stratospheric Data Assimilation for NWP

Variational assimilation: 3D-VAR

New Dynamics

Ozone and water vapour assimilation

Future plans

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Stratospheric Analysis – History

“SSU Analysis” - 1978– Original stratospheric analysis, based on gridded retrievals of

thickness; T and winds derived Analysis Correction Scheme - 1991

– First Met Office stratospheric data assimilation system; asynoptic, repeated insertion

Variational Assimilation – 2000– 3D-VAR assimilation; 6 hour cycle

New Dynamics – 2003– Semi-Lagrangian Dynamics, on height grid

4D-VAR – planned

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3D-VAR for the stratosphere

Direct assimilation of ATOVS & TOVS radiances– radiance bias correction

Background error covariances using “NMC method”– use rotated vertical modes in stratosphere

Prototype for future extended global forecast system, spanning stratosphere

– 40-level model, based on the then-current global 30-L model

– most testing done at medium resolution (0.83°x1.25°), rather than usual stratospheric low resolution (2.5°x3.75°)

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New Dynamics Old Dynamics

Semi-Lagrangian

Semi-implicit (predictor-corrector)

Arakawa C-grid

Height based: hybrid terrain-following grid

Charney-Phillips

Full 3D Helmholtz solver

Explicit Heun

Split-explicit (2 time-level)

Arakawa B-grid

Pressure based: hybrid sigma-pressure grid

Lorenz

Reference state profile

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New Dynamics Configurations

38-level, N216 (0.55o x 0.83o) Top at 39km

Operational (NWP) in August 2002

50-level, N48 (2.5o x 3.75o) Methane oxidation and spectral GWD

Top at 64 km

Operational (NWP) in October 2003

Positive benefit on forecast and analysis skill

Current ND levels

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Ozone Assimilation

Potential benefits for NWP

– Improved radiance assimilation (HIRS, AIRS, IASI)

– Improved radiative heating rates

– Possible impact on UTLS wind fields

– Improved forecasts of surface UV

Exploitation of research satellite data (especially Envisat data, through ASSET)

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Ozone assimilation - status

Initial trial completed with New Dynamics, using HIRS-9 and SBUV

Some bugs remain with Envisat version of suite Further analysis and testing needed:

– Test with Cariolle ozone, cold tracer

– Test with spectral GWD

– Migrate to RTTOV-7 forward model

– Use NMC method for B

– Feedback ozone on radiation

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Stratospheric Humidity Assimilation

Previously untried

Ill-conditioned vertical transform of background error matrix B

Excessive stratospheric increments (due to spurious vertical correlations)

Run 3D-VAR experiments for 1 cycle

VAR humidity tests6 hr cycle – humidity data only in troposphere

q (bg) q (an)

q inc T inc

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Proposed Solutions

Change vertical weighting to improve conditioning of B

Scale back variances, and reduce vertical correlations

New humidity variable (after Dee and da Silva, 2003)

Impact of Dee and DaSilva scheme

q(analysis) q increment

Default

Dee & daSilva ..

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Future Plans

4D-Variational Assimilation– Very promising results from trials of Global

forecast model

– To be implemented soon

Extend Global Assimilation to span the stratosphere

– Improve assimilation of satellite radiances

– Avoids need for separate operational stratospheric configuration

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Data Assimilation

4D-Var in outline

Run forecast from background field

T-3 T-2 T-1 T+0

T+1

T+2

T+3

T+144

Background field

Analysis Backgroun

d field for next forecast

Change starting field to reduce distance between forecast trajectory and observations (error)

Repeat until satisfactory fit

NWP index

H500 anomaly correlation (N216)

Current and Future(??) levels

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Summary

The operational stratospheric assimilation system has developed rapidly over the past few years (3D-VAR in 2000, New Dynamics in 2003, porting to NEC SX-6)

The system has been extended to assimilate ozone and water vapour – in particular as part of ASSET project

Future developments include adoption of 4D-VAR, and merging global and stratospheric assimilation systems