1 00/XXXX © Crown copyright 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

2 00/XXXX © Crown copyright Stratospheric Data Assimilation for NWP Variational assimilation: 3D-VAR New Dynamics Ozone and water vapour assimilation Future plans

3 00/XXXX © Crown copyright Stratospheric Analysis – History “SSU Analysis” –Original stratospheric analysis, based on gridded retrievals of thickness; T and winds derived Analysis Correction Scheme –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

4 00/XXXX © Crown copyright 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°)

7 00/XXXX © Crown copyright 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

8 00/XXXX © Crown copyright Equation Set Options

9 00/XXXX © Crown copyright New Dynamics Equation Set

10 00/XXXX © Crown copyright Physics Package (HadGAM) New Dynamics (Cullen et al. 1997) 2-stream Radiation (Edwards & Slingo,1996) Mass flux convection (Gregory & Rowntree, 1990) Non-local Boundary Layer (Lock et al. 2000) Sub-grid Orography and GWD (Webster et al., 2003) Statistical cloud scheme (Smith, 1990) Prognostic Ice Microphysics (Wilson & Ballard, 1999) Met Office Surface Exchange (Cox et al., 1999) Cubic Monotonic Tracer Advection.

11 00/XXXX © Crown copyright New Dynamics Configurations  38-level, N216 (0.55 o x 0.83 o )  Top at 39km  Operational (NWP) in August 2002  50-level, N48 (2.5 o x 3.75 o )  Methane oxidation and spectral GWD  Top at 64 km  Operational (NWP) in October 2003 Positive benefit on forecast and analysis skill

Current ND levels

13 00/XXXX © Crown copyright Methane Oxidation  Conservation of ( 2CH 4 + H 2 O ) = 6 ppmv  Idealised oxidation rate as a function of height based on ECMWF’s scheme  Only operates in middle atmosphere  Photolysis of water vapour at higher levels.

Effect of Methane Oxidation 10 year mean Jan UARS observed

15 00/XXXX © Crown copyright Gravity wave drag Typical model errors are alleviated using a parametrization of drag due to breaking gravity waves A version of the USSP scheme (Warner and McIntyre, 2000) has been implemented in the UM (Scaife et al., 2000) Isotropic and homogeneous source of gravity waves in the lower atmosphere Launch spectrum proportional to m -3 at large m Hydrostatic, non-rotating dispersion relation:  /k=N/m “Transparent” upper boundary.

Equatorial Zonal Wind for L50 VN 5.4 Assim Obs

17 00/XXXX © Crown copyright 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)

18 00/XXXX © Crown copyright 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

19 00/XXXX © Crown copyright 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 tests 6 hr cycle – humidity data only in troposphere q (bg) q (an) q inc T inc

21 00/XXXX © Crown copyright 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..

23 00/XXXX © Crown copyright 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

24 00/XXXX © Crown copyright Data Assimilation 4D-Var in outline Run forecast from background field T-3T-2T-1T+0T+1T+2T+3 T+144 Background field Analysis Background 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

28 00/XXXX © Crown copyright 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