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SRNWP Lead Centre Report on Data Assimilation 2005

for EWGLAM/SRNWP Annual Meeting 2005

3-5 October 2005, Ljubljana, SloveniaMike Bush

NWP

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1. SRNWP/Met Office/HIRLAM workshop in Exeter

2. Global model 4D-VAR

3. NAE model 4D-VAR

4. U.K 4km model 3D-VAR

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SRNWP/Met Office/HIRLAM Workshop

Joint SRNWP/Met Office/HIRLAM Workshop on High resolution data assimilation: towards 1-4km resolution

Held at the Met Office, Exeter 15 -17 November 2004

What is different for the convective scale?

High resolution observations – their assimilation & utility

Assimilating cloud and precipitation observations

Analysis techniques, 3D-Var and error covariances

4D-Var issues

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1. SRNWP/Met Office/HIRLAM workshop in Exeter

2. Global model 4D-VAR

3. NAE model 4D-VAR

4. U.K 4km model 3D-VAR

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4D-VAR: The Perturbation Forecast (PF) model

PF model is an approximation to the tangent linear model

Dynamical core:

Start from continuous equations

ECMWF start from the discrete equations for the dynamical core

scale analysis

Choose appropriate terms at an asymptotic limit

then discretize

Physics:

Simplified physics

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4D-VAR : PF model physics

Simplified physics:

Boundary layer: friction and vertical diffusion of heat and momentum (following Buizza).

Latent heating scheme (simplified microphysics).

No convection.

No radiation.

No gravity wave drag

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Global 4D-VAR

G33 5th October 2004Introduction of 4D-VAR.Single outer loopHalf model resolution (N108)Positive benefit – greatest impact in the SHSatellite data is asynoptic and so particularly suited to 4D-VAR

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PMSL NH T+24 RMS Error against analysis

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4DVAR4DVAR

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500hPa Height SH T+24 RMS Error against analysis

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4DVAR

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250hPa Wind SH T+72 RMS Vector Error vs analysis

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4DVAR4DVAR

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1. SRNWP/Met Office/HIRLAM workshop in Exeter

2. Global model 4D-VAR

3. NAE model 4D-VAR

4. U.K 4km model 3D-VAR

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NAE 4D-VAR: how to treat the boundaries

Increments zero at the lateral boundaries

Weights of 0, 0, 0, 0, 0, 0.25, 0.5, 0.75 applied going away from the rim

Simple but cheap way of treating the boundaries

HIRLAM have a more sophisticated system using periodic boundaries

Only obs within ½ gridlength of the boundary are excluded

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NAE 4D-VAR testing

2-week run at half-resolution was completed

Encouraging benefit to PMSL verification.

Other fields are more neutral/slight detriments.

Global 4DVAR does not assimilate screen level data, nor does it assimilate hourly sequences of the surface obs that it does use.

These new aspects of regional 4DVAR might need further tuning.

No problems with using persistence as the PF forecast for aerosol in the 2-week run

This means that experiments with visibility assimilation can go ahead

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NAE 3D-VAR vs 4D-VAR PMSL Bias and RMSE

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3D-VAR vs 4D-VAR Temperature Bias and RMSE

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1. SRNWP/Met Office/HIRLAM workshop in Exeter

2. Global model 4D-VAR

3. NAE model 4D-VAR

4. U.K 4km model 3D-VAR

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U.K 4km model: 3D-VAR

Initial solution is to introduce 3D-VAR assimilation system (plus Analysis Correction scheme for cloud and precipitation)

So far 12 case studies run from June – September 2005

Parallel Suite running scheduled for October 2005

Implementation scheduled for November 2005

Later upgrade will address consistency with larger scale increments from the driving model

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U.K 4km model 3D-VAR impact: PMSL

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U.K 4km model 3D-VAR impact: 1.5m Temperature

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U.K 4km model 3D-VAR impact: 10m wind

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U.K 4km model 3D-VAR impact: cloud

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Questions?