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 Objectives Tasks & progress Issues Next steps/ targets for next 6 months

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 strong-constraint incremental 4D-Var

Building the idealised system Task 1: develop an idealised, single-column, coupled atmosphere-ocean model with a strong-constraint incremental 4D-Var assimilation scheme. For this we need a model that is simple and quick to run is able to represent realistic atmosphere-ocean coupling Atmosphere: ECMWF single column model (SCM) Ocean: single column KPP (K-Profile Parameterisation) mixed-layer ocean model

SCM: based on the IFS code includes a full set of physical parameterisations we will use a stripped down version - adiabatic component (+ vertical diffusion?) also want option to use full physics in later experiments KPP model: developed at UoR already within the SCM code but never been used currently only works when full SCM physics scheme is switched on coupling through surface fluxes and SST

Progress so far  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.

Issues Lack of documentation Choice of suitable test-case(s), creation of input file Prescription of model forcing ocean: surface fluxes of heat, moisture & momentum, compute using bulk formulae of Large & Yeager atmosphere: vertical velocity, geostrophic u & v winds, horizontal advection tendencies. Relaxation

SCM test-case: with (top) & without physics (bottom) temperature humidity u-wind v-wind

Relaxation of winds: u-wind with relaxation without relaxation

Issues Lack of documentation Choice of suitable test-case(s), creation of input file Prescription of model forcing ocean: surface fluxes of heat, moisture & momentum, compute using bulk formulae of Large & Yeager atmosphere: vertical velocity, geostrophic u & v winds, horizontal advection tendencies. Relaxation

ERA interim data: temperature actual SCM without forcing SCM with forcing

ERA interim data: humidity actual (log) SCM without forcing SCM with forcing

ERA interim data: u-wind actual SCM without forcing SCM with forcing

ERA interim data: v-wind actual SCM without forcing SCM with forcing

Coupled model: full physics

Coupled model: radiation, cloud & convection off

Next steps Initial results with simple coupled atmosphere-ocean model expected by end of January Development of prototype 4D-Var system - tangent linear and adjoint models preliminary results by April/May Exploration of different coupling strategies being implemented in the ECMWF system - identical twin experiments