1. Page 1 NAE 4DVAR Oct 2006 © Crown copyright 2006 Mark Naylor Data Assimilation, NWP NAE 4D-Var – Testing and Issues EWGLAM/SRNWP meeting Zurich 9 th -12 th October 2006

2. Page 2 NAE 4DVAR Oct 2006 © Crown copyright 2006 NAE 4DVar The Met Offices 4DVar NAE went operational on the 14 th March 2006 after much testing and tuning. We will present results from:- Two seasons trials (Spring and Summer 2005) Pre-operational real-time trials – 5 weeks in Dec 2005 Problems, particularly with screen temperature (T2m) scores. Future development plans

3. Page 3 NAE 4DVAR Oct 2006 © Crown copyright 2006 Spring 2005 Trial 6 th – 20 th March Mixed conditions: anticyclonic with widespread frost lows bringing gale force winds very mild south-westerlies

4. Page 4 NAE 4DVAR Oct 2006 © Crown copyright 2006 Spring 2005 Trial 6 th – 20 th March The NWP UK index is used to asses skill in the NAE and consists of verification against screen temperature, visibility, wind, cloud amount and precipitation amount. NWP UK Index (NAE area): +1.5% NWP UK Index (UK Mes area): +2.6% NWP UK Index (UK stations): +1.9%

5. Page 5 NAE 4DVAR Oct 2006 © Crown copyright 2006 Spring 2005 Trial - Pressure

6. Page 6 NAE 4DVAR Oct 2006 © Crown copyright 2006 Spring 2005 Trial – Screen Temperature

7. Page 7 NAE 4DVAR Oct 2006 © Crown copyright 2006 Spring 2005 Trial Summary good positive impacts wind and pressure particularly good hint of improved balance at T+0 detriment in screen temperature fit up to T+12

8. Page 8 NAE 4DVAR Oct 2006 © Crown copyright 2006 Summer 2005 Trial 18 th June – 2 nd July Week 1 anticyclonic with thunderstorms Week 2 anticyclonic and more mobile weather with rain over UK

9. Page 9 NAE 4DVAR Oct 2006 © Crown copyright 2006 Summer 2005 Trial 18 th June – 2 nd July NWP UK Index (NAE area): +2.0% NWP UK Index (UK Mes area): +3.7% NWP UK Index (UK stations): +5.4%

10. Page 10 NAE 4DVAR Oct 2006 © Crown copyright 2006 Summer 2005 Trial - Pressure

11. Page 11 NAE 4DVAR Oct 2006 © Crown copyright 2006 Summer 2005 Trial – Screen Temperature

12. Page 12 NAE 4DVAR Oct 2006 © Crown copyright 2006 Summer 2005 Trial summer rainfall t+9 3DVAR t+9 4DVAR radar

13. Page 13 NAE 4DVAR Oct 2006 © Crown copyright 2006 Summer Trial 2005 Summary again good scores, especially ppn screen level temperature ok after T+0 better balance in analysis

14. Page 14 NAE 4DVAR Oct 2006 © Crown copyright 2006 Real-time trial setup 4DVAR real-time trial (Dec 2005) final settings 12km UM /36km PF Visibility Assimilation on operational 3-hourly 3D-Var control 6 weeks real-time from Dec 2005 4 forecasts per day 4DVar gave consistently positive results, similar to the seasonal trials

15. Page 15 NAE 4DVAR Oct 2006 © Crown copyright 2006 Equitable Threat Score precipitation ETS 3D-Var 4D-Var

16. Page 16 NAE 4DVAR Oct 2006 © Crown copyright 2006 Screen Temperature Scores 3D-Var 4D-Var

17. Page 17 NAE 4DVAR Oct 2006 © Crown copyright 2006 Screen Temperature investigation An investigation into the poor initial screen temperature scores, involving experimental reruns in March 2006, was undertaken. The 1 st week of March 2006 included some especially poor screen temperature forecasts.

18. Page 18 NAE 4DVAR Oct 2006 © Crown copyright 2006 Single reruns 18Z on 2 nd March was particularly bad so was rerun and compared with experiments including:- PF Persistence No Screen Temperature obs 3DVar No T2m obs after T+0 (i.e. obs only from T-3 to T+0) Halving period of all obs (i.e. obs only from T-90m to T+90m)

19. Page 19 NAE 4DVAR Oct 2006 © Crown copyright 2006 Screen Temperature VER results The UM ran to T+6 and RMSs produced:- Operational NAE (3DVar 3-hourly) is still better than 4DVar Control at T+0 RMS fit. PF Persistence has a lower RMS fit than Control 4DVar at T+0 ! Using T obs only upto T+0 increased RMS. 3DVar (with 6 hours obs) is similar to 3DVar Using 3-hours of all obs halves the gap

20. Page 20 NAE 4DVAR Oct 2006 © Crown copyright 2006 Issues with Screen Temperature experiments PF persistence has a lower RMS error to obs at T+0 than 4DVAR. Removing the 2 nd half of the time-window for screen temperature obs increases the RMS error. Halving the number of all the obs (to between T-90m to T+90m) decreases the T2m RMS but has a major detriment on scores for other variables (especially pressure). Is the PF model dealing correctly with Surface Temperature? Stats reflect only one case, but 1-week reruns indicate similar results.

21. Page 21 NAE 4DVAR Oct 2006 © Crown copyright 2006 Linearisation tests In PF-linearisation tests low-level theta in persistence also beats control for the first 2 hours (8 timesteps):- 4DV ar Persistence After the first 2 hours the PF model has a higher correlation. Why isnt this feeding into the analysis?

22. Page 22 NAE 4DVAR Oct 2006 © Crown copyright 2006 Damping Coefficient Theta damping coefficient from analysis increment. The Theta Damping coefficient is the ratio the average PF model increment size to the average UM increment size In the lower levels, the Theta PF increment is up to 20% larger than the UM increment Can we improve the physics to remedy this feature?

23. Page 23 NAE 4DVAR Oct 2006 © Crown copyright 2006 Future Improvements New PF physics package is expected later this year. Includes better boundary layer mixing and PF convection. We wait to see how this will influence the screen temperature near T+0.

24. Page 24 NAE 4DVAR Oct 2006 © Crown copyright 2006 Recent Improvement The Screen temperature skill in Summer was improved through soil moisture modifications We now get soil moisture (outside the UK) from the Global Model nudging scheme replacing climatology

25. Page 25 NAE 4DVAR Oct 2006 © Crown copyright 2006 Questions? Any questions?........