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 systemTask 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-windwith 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: temperatureactual

SCM without forcing

SCM with forcing

ERA interim data: humidityactual (log)

SCM without forcing

SCM with forcing

ERA interim data: u-windactual

SCM without forcing

SCM with forcing

ERA interim data: v-windactual

SCM without forcing

SCM with forcing

Coupled model: full physics

Coupled model: full physics

Coupled model: radiation, cloud & convection off

Coupled model: radiation, cloud & convection off

Next steps• Initial results with simple coupled atmosphere-ocean model

expected by end of January 2013.

• Development of prototype 4D-Var system - tangent linear and adjoint modelspreliminary results by April/May 2013.

• Exploration of different coupling strategies being implemented in the ECMWF system - identical twin experiments