1. www.ncof.gov.uk Results of the assimilation of sea ice concentration and velocity into a sea-ice-ocean model. John Stark, Mike Bell, Matt Martin, Adrian Hines, Alistair Sellar, Jeff Ridley.

2. www.ncof.gov.uk Motivation & Outline of Talk. Motivation Improve operational forecasts of sea-ice and ocean Improve seasonal and decadal forecasts Improve NWP forecasts by using better sea-ice data Reduce uncertainties in sea-ice models by detailed intercomparison of models & data. Overview of talk Overview of the assimilation system. –Ice concentration assimilation. –Ice velocity assimilation. Results from year-long reanalysis integrations –Period from October 1999 December 2000. Summary

3. www.ncof.gov.uk The Sea-Ice-Ocean Model 1° Global FOAM. Ocean component is identical to the operational FOAM model. 6-hourly NWP fluxes. Ice thickness distribution (ITD), 5 categories (Lipscomb 2001). Elastic viscous plastic rheology (Hunke & Dukowicz 1997) 1° FOAM, Operational since 1997 Derived from the CICE model (Hunke & Lipscomb, LANL) Configuration of sea ice model is identical to our latest climate model, HadGEM1. Regular Lat-Long grid (Polar island participates in ice flow)

4. www.ncof.gov.uk Sea Ice Concentration Assimilation Scheme Obs from SSM/I (passive microwave). ASI algorithm converts brightness temperature to ice concentration estimates. Optimal interpolation used : –Can process many observations efficiently. –Can cope with poor estimates of the error. –Does not require accurate knowledge of error correlations (but these can be used). –Does not require an adjoint or inverse model or ensemble runs. ASI algorithm & Q.C. O.I. Analysis Forecast NWP surface temperature SST & Ice Ice concentration Swath data Analysis Increments

5. www.ncof.gov.uk Observation Pre-Processing Very large number of observations : more than 300,000 per day. –Sub-sampled to include only observations with all 7 channels (85GHz is sampled at twice the frequency in both directions). –Helps to reduce error correlation, since beam footprint is larger than spatial sampling. Used a maximum ice extent mask to reduce spurious observations in sub-tropics and reduce the total number of observations. –Land masks extended to 100km Ice extent mask for March

6. www.ncof.gov.uk Ice Conc. Observation Pre-Processing Observations that fail the weather filter are treated as 0% ice cover. Swath Obs, Tb NASA-Team Weather Filter Fail Conc=0% NTA > 30% ? Discard Conc(ASI) Ice Extent Filter Error Estimate NWP Surface T. Fail

7. www.ncof.gov.uk Effect of 30% NTA threshold. Threshold = 5%Threshold = 30% Trade off between spurious ice observations & resolving the marginal ice zone:

8. www.ncof.gov.uk Application of Analysis Increments Preferentially altering the thinnest ice. –Smallest change in heat content –Represents thermodynamic changes to the sea ice. Scaling all categories –Smallest RMS change in category ice area. –Represents convergence / divergence in the flow. –Can result in large changes to thick ice. The analysis scheme provides an estimate of total sea ice concentration –This must be mapped onto the ice thickness distribution (ITD) in the model. Ice Thickness Fractional Cover

9. www.ncof.gov.uk Application of Analysis Increments Scaling all categories Red = Free run Cyan = Assim Red = Free run Cyan = Assim Ice Thickness Fractional Cover Ice Thickness Fractional Cover

10. www.ncof.gov.uk Ice Velocity Assimilation Scheme Obs Q.C. Analysis Analysis increments Balancing stress Computed using free drift  i Forecast Model ice velocity bias Ice-motion data from CERSAT- IFREMER and NSIDC Ice motion forecast

11. www.ncof.gov.uk Ice Vel. Observation Pre-Processing CERSAT Obs (NetCDF) Discard Quality Flag==1 ? Fail Fowler Obs (ASCII) Coordinate Conversion & Vector Rotation Discard V > 1m/s ? Fail Model N-day Mean Velocities Interpolate to Obs Combine with Ob Errors, output to Obs file.

12. www.ncof.gov.uk Velocity analysis scheme Assimilation –Assimilation scheme based on OI, similar to ice concentration. –Assimilation scheme able to control divergence introduced by the assimilation. –Relative amount of divergence can be controlled. –Uses a ‘balancing stress’ based on free drift dynamics to apply the increments. –Uses 2 background error length scales. Quality Control –Observations near coastlines have reduced impact to prevent velocities into land.

13. www.ncof.gov.uk Ice Velocity : Application of Increments EVP scheme rapidly adjusts to maintain equilibrium with external forcing on ice. –Direct application of the analysis increments to the velocity fails. –Previous work (Meier, Zhang) has diagnosed an additional velocity which is added to the model and plays no direct part in the dynamics. –We used an alternative approach : balancing stress. Ice velocity after a perturbation compared to a control run.

14. www.ncof.gov.uk Ice Velocity : Application of Increments Ice velocity assimilation using a ‘balancing stress’ An additional term was added to the model sea-ice momentum equation to apply the velocity increments. The additional stress term is computed by assuming the sea-ice is in free drift. The first term on the right is the additional stress required to balance the ice-ocean stress, the second term comes from the Coriolis term. This is a non-linear term since it depends on the sea ice velocity and not just the increment.

15. www.ncof.gov.uk Outline of Talk. Overview of the assimilation system. –Ice concentration assimilation. –Ice velocity assimilation. Results from year-long reanalysis integrations –Period from October 1999 December 2000. Summary

16. www.ncof.gov.uk Snap Shots : March 1, 2000 After 150 days of assim. Control Conc. Assim. Vel. Assim Conc. & Vel. Assim  ASI Gridded.  Ice Concentration

17. www.ncof.gov.uk Snap Shots : Sept 1, 2000 After 335 days of assim. Control Conc. Assim. Conc. & Vel. Assim  ASI Gridded.  Ice Concentration Vel. Assim

18. www.ncof.gov.uk Ice concentration statistics. Significant improvement with ice conc. assimilation. Small difference with ice vel. assimilation. Melt season a particular problem. 1° Global 1/3° Arctic

19. www.ncof.gov.uk Velocity Comparison with Buoy Motion Motion computed offline from 1-day mean model fields. Assimilation improves representation of buoy motion during winter. Less improvement during summer.

20. www.ncof.gov.uk Impact of velocity assim. on thickness. Velocity assimilation led to changes in thickness of order 30cm. Thicker ice around Greenland and Fram Strait. Thinner in Beaufort Gyre. Annual avg. thickness change (m) (assim. – control)

21. www.ncof.gov.uk Correlation between velocity increments and wind stress. Zonal Wind / Assim, October 1999Meridional Wind / Assim, October 1999 Zonal Wind / Assim, March 2000Meridional Wind / Assim, March 2000 Strong negative correlation in many areas indicates wind stress too strong.

22. www.ncof.gov.uk Summary The ice concentration and velocity assimilation has been shown to give quantitative improvements in modelled sea ice. Very little coupling between ice concentration and ice velocity in this model. –Ice dynamics has little impact on ice concentration in this model. Velocity analysis increments applied indirectly, via a balancing stress. –Ice velocity cannot be directly assimilated using an EVP model. Use of bias correction techniques allows forecast improvement & reduces RMS errors. Diagnosis of deficiencies in the model and forcing are possible using the assimilation.