1. Page 1© Crown copyright 2005 DEVELOPMENT OF 1- 4KM RESOLUTION DATA ASSIMILATION FOR NOWCASTING AT THE MET OFFICE Sue Ballard, September 2005 Z. Li, M. Dixon, S.Swarbrick, O.Stiller and H. Lean Met Office, JCMM, Reading University

2. Page 2© Crown copyright 2005 Contents  Aim of high resolution convective scale system  4km UK 2005, 1km ~ 2008-2010  Trial system – small domains  Data assimilation options  Trial results  Impact of data assimilation  Impact of relative humidity and latent heat nudging  Scale selective assimilation  Conclusions

3. Page 3© Crown copyright 2005 High Resolution Trial Model 1 km 76 levels Resolved convection 4 km 38 levels Mass-limited convection

4. Page 4© Crown copyright 2005 Current HRTM Assimilation Options 12 km 3D-Var Data Assimilation With or without moisture and Latent Heat Nudging (LHN) using AC scheme (referred to as MOPS data – moisture observation processing system) i.e. spin up 4km, 1km from 12km T+1 each cycle. 4km 3D-VAR with continuous cycles with or without MOPS 1km with nudged reconfigured 4km increments using IAU With or without LHN and moisture nudging using AC scheme  IAU – increments output from 3D-Var and fixed over time window  AC scheme – increments depend on latest model fields so vary with timestep through weighting factor and model evolution/impact of data

5. Page 5© Crown copyright 2005 T-3 T-1T+0T+1 T+3 AC scheme/UM 3D cloud fraction Surface rainrate IAU 3 hour f/c: background Hourly ModelOb Next analysisPrevious analysis Nudging RH & Latent heat T+2T-2 Conventional observations 3D-Var (FGAT) Obs window 3D-Var system including MOPS RH and LH nudging via AC scheme

6. 6hr accumulations from13Z to 19Z 16/8/04 from 12UTC analysis Rain rates at 14.30 UTC from 12UTC analysis With 4km 3D-Var +MOPSradar Spun-up from 12km T+1

7. Accumulated precipitation 19-20UTC 27 th April 2004 From 18UTC analysis radar With MOPS No MOPS Impact of MOPS data

8. Page 8© Crown copyright 2005 Spin - up Assimilation 12 and 4km with MOPS Assimilation 12 and 4km No MOPS 1km domain average Rainrate 15UTC 13 th May 1003

9. Page 9© Crown copyright 2005 Domain average rain rates 4 Cases x 8 cycles summer 2003 1km no MOPS Black – radar Blue 12 km Green – 4km Orange -1km Dashed – spinup 7 Cases x 4 cycles summer 2004 Prognostic rain (1 and 4km) Increased wt MOPS (12,4,1km)

10. Page 10© Crown copyright 2005 Domain average rain rates 4 Cases x 8 cycles summer 2003 Prognostic rain (1 and 4km) Increased wt MOPS (12,4,1km) 7 Cases x 4 cycles summer 2004 Prognostic rain (1 and 4km) Increased wt MOPS (12,4,1km) Black – radar Blue 12 km Green – 4km Orange -1km Dashed – spinup

11. Page 11© Crown copyright 2005 T+2 Forecast precipitation rate for 20UTC 27April 2004 Water Loading + Reduced MOPS wts 12km 4km 1km radar

12. Page 12© Crown copyright 2005 T+4 Forecast precipitation rate for 22UTC 27April 2004 12km 4km 1km radar Water Loading + Reduced MOPS wts

13. Page 13© Crown copyright 2005 Statistics for 4 cycles of assimilation and forecast data time9,12,15,18UTC 27 April 2004 Area average Precipitation rate For radar area Skill score for radar area for accumulation greater than 2mm against sampling radius Orange dashed 4km Orange 4km Water loading Reduced MOPS wt Blue 12km Black radar Orange 4km blue12km Orange 4km Water loading Reduced wt MOPS blue 12km

14. Page 14© Crown copyright 2005 theta inc 12km analysis “new” 4km analysis /12km back “new” 4km analysis /4km back & half length standard 4km analysis doesn’t see obs outside domain Zero at boundary 4km short wave analysis12km long waves Problem of small domain Scale selective analysis

15. Page 15© Crown copyright 2005 Impact of S-band radar radial wind data v-component of velocity at 410m 12km analysis 4km standard Scale selective Radar data In 12km and 4km Scale Selective Radar in 4km only

16. Page 16© Crown copyright 2005 Conclusions  Need 1-4km to capture small scale severe events  Need DA to overcome spin-up of explicit convection  However long range forecasts can be better than short range  MOPS RH & LHN improves locations but amounts too high  At 1-4km and worse with prognostic rain  Needs further development but ideally want to use assimilation of cloud and precipitation in 3D-Var and 4D-Var  Model explicit convection overdoes rates (?)  Need to combine synoptic scale and convective analysis  Need to move towards full analysis for 1km model  Ideally want 4D-Var or EnKF - Start with 3D-Var  need to resolve problems seen with 4km first  Use reduced vertical resolution (and horizontal?) for analysis?  High resolution data  Balance and background errors

17. Page 17© Crown copyright 2005 Questions & Answers