Coupled Model Data Assimilation: Building an idealised coupled system Polly Smith, Amos Lawless, Alison Fowler* School of Mathematical and Physical Sciences, University of Reading, UK * funded by NERC

Outline Recap  objectives  tasks & progress Progress since last meeting Next steps

Objective To use an idealised system to gain a greater theoretical understanding of the coupled atmosphere-ocean data assimilation problem: explore different approaches to coupled 4D-Var data assimilation using a single-column, coupled atmosphere-ocean model help guide the design/ implementation of coupled DA methods within full 3D operational scale systems

Building the idealised system Task 1: develop an idealised, single-column, coupled atmosphere-ocean model with a strong-constraint incremental 4D-Var assimilation scheme. The model needs to be simple and quick to run able to represent realistic atmosphere-ocean coupling Atmosphere: simplified version of the ECMWF single column model (SCM) Ocean: single column KPP (K-Profile Parameterisation) mixed-layer ocean model coupled through surface fluxes and SST

Progress December 2012  Successfully compiled and ran the SCM test-case provided by ECMWF.  Simplified the SCM code to remove physical parameterisations.  Tested the simplified atmospheric model using real data.  Coupled the KPP ocean code to the full-physics SCM and successfully ran a test case for the coupled atmosphere-ocean model.  Produced draft documentation for the models.

Progress July 2013  Revisited stripped down SCM code, now includes simplified version of the IFS turbulent diffusion scheme.  Option to run with dynamics + turbulent diffusion only dynamics + full physics (i.e. original code)  Simplified turbulent diffusion scheme computes surface fluxes to pass to ocean model.  KPP ocean code now coupled to the simplified SCM.

Progress July 2013  Produced scripts for creating coupled model input file  atmosphere data from ERA interim  ocean data from MyOcean project  Continued to update documentation for the models.  svn repository now in place.  Tangent linear and adjoint models written and tested.

Test case: 7 day run from 1 st July 2012, (235.5 o E, 24.5 o N)

Atmosphere

ERA interim data (6 hourly)

Ocean

MyOcean data (daily mean)

Things to think about …  Choice of suitable test-case(s) Atmosphere:  relaxation of winds Ocean:  availability of time-series data

Next steps  Development of baseline 4D-Var system  minimisation algorithm  observations  error covariances  assimilation time window Strongly coupled data assimilation experiments  aim is to have some initial results in next few months. Exploration of different coupling strategies being implemented in the ECMWF system.