© crown copyright met office uk report for govst matt martin govst-v, beijing, october 2014

© Crown copyright Met Office

UK report for GOVST

Matt Martin

GOVST-V, Beijing, October 2014.


Contents

• Developments for new version of FOAM (v13).

• On-going developments

• Summary


Development of new global FOAM versionOverview

• FOAM V12 is reported in Blockley et al., 2013:• NEMO model (vn 3.2)• NEMOVAR data assimilation (3DVar-FGAT)• ¼° global, 1/12° regional (N. Atlantic, Indian Ocean, Med).

• Main short-comings which were identified (partly through GOV class 4 inter-comparisons):

• Model mixing biases.• Sea-ice issues (particularly sea-ice thickness).• Inability of DA to control the larger-scale biases in data-sparse areas.

• FOAM V13 (at NEMO vn3.4) aims to address these issues through:• Changes to the TKE mixing scheme (mainly parameter tuning).• Sea-ice model (CICE) parameter tuning (increasing the ice albedo and the

conductivity of the snow and ice).• Implementing multiple length-scales in the NEMOVAR data assimilation

scheme. Also increased the number of iterations in the minimisation of the DA.


Model changes (v12 vs v13)Temperature biases

warming at 100m

cooling at 500m

FOAM v12(like GO1)

New model(GO5)

Old model(GO1)

model drifts

model bias and RMS

From D. Storkey


freshening in top 400m

salinification at depth

FOAM v12

model driftsmodel bias and RMS

From D. Storkey

Old model(GO1)

New model(GO5)

Model changes (v12 vs v13)Salinity biases

Development of new global FOAM versionData assimilation developments

• The model biases described before imply a large-scale component to the errors.

• In FOAM V12, the NEMOVAR system only had a small-scale covariance model, with the errors being corrected over length-scales of the Rossby radius.

• In FOAM V13, we implemented code in NEMOVAR to include multiple length-scales in the correlation model. Two length-scales are now modelled, the smaller one is still based on the Rossby radius, the larger one is 400km everywhere. The ratio of the variances associated with each scale determines the effective length-scale.

• With a larger-scale component, we also increased the number of iterations in the minimisation of the 3DVar cost function.

Old error correlation length-scale (km)

Work by I. Mirouze

Large length-scale (400km)

Small length-scale

Effective length-scale


Development of new global FOAM versionResults: temperature profile errors

Significant reduction in mean temperature errors between 50-200m depth.• Mainly due to model mixing improvements.• Also improved by the 2 length-scales in the DA.

RMS errors also slightly reduced.

V12 system global errors V13 system global errors

From E. Blockley


Development of new global FOAM versionResults: salinity profile errors

V12 system global errors V13 system global errors

Significant reduction in mean salinity errors over the top 300m.• Mainly due to the 2 length-scales in the DA.• Improvements mainly in the Southern Ocean.

RMS errors also slightly reduced.From E. Blockley


Development of new global FOAM versionRelative impact of model & DA changes

• Model changes improve the temperature biases, particularly in areas such as the South Pacific • DA changes further reduce biases and RMS errors.• A small increase in deeper errors (~1000m) -> further tuning of error covariances required.

• Model changes improve the salinity biases and RMS errors, particularly in areas such as the Southern Ocean.• DA changes further reduce biases and RMS errors.

BLACK => Old model, old DABLUE => New model, old DARED => New model, new DA


Development of new global FOAM versionResults: sea-ice volume and extent

• Ice volume increased from model tuning (albedo and conductivity).

• Much better agreement with PIOMAS volume.

• Ice extent now has smaller drift in the forecasts than the v12 system.

From A. McLaren and E. Blockley


Development of new global FOAM versionStatus and plans

• Results of V13 vs V12 trials:

• Significant improvements in sub-surface T&S biases

• SSH slightly degraded in the Southern Ocean regions -> could be balance issues in the DA given the large improvements in salinity there.

• No/little impact on the surface current forecast accuracy.

• Improvements in sea-ice extent and volume.

• Small reduction in SST RMS error from 0.49 to 0.47K.

• Operational implementation of the V13 system expected early 2015.

• Long reanalysis (1989 – 2013) with the V13 system also being produced (for seasonal forecast calibration purposes).


On-going developmentsDevelopment of 1/12° global configuration

1/4° 1/12°

Surface current speed (5 day mean)

Boundary currents and extensions change dramatically between ¼° and 1/12° with potentially important implications for biases and air-sea fluxes/coupling.

• The ORCA12 NEMO model configuration has been run at the Met Office. Based on a configuration developed within the DRAKKAR consortium.

• NEMOVAR now being implemented in ORCA12.

P. Hyder and J. Waters


• Coupled forecasts currently being produced operationally:• ¼° ocean/sea-ice, ~60km atmosphere/land.• Initialised using FOAM ocean/sea-ice analysis and Met Office NWP

atmosphere/land analysis.• Ocean forecasts from the coupled system provided through MyOcean.

• Developments:• Weakly coupled data assimilation system.• Higher resolution model (1/12° ocean/sea-ice, 17km atmosphere/land).• Improving the coupling through implicit solver for SST to improve diurnal cycle.

• Weakly coupled DA system planned to be implemented as a demonstration operational system in 2015.

On-going developmentsCoupled prediction


• New FOAM system (v13) planned to be implemented in early 2015.

• Coupled predictions (¼° ocean/ice ~60km atmosphere/land) using FOAM as initial ocean conditions and NWP analyses as initial atmospheric conditions now operational (forecasts for MyOcean).

• New weakly coupled DA system (at the same resolution) being developed to improve initialisation of these coupled forecasts.

• Now developing higher resolution coupled system (1/12° ocean/ice ~17km atmosphere/land) which could provide NWP and ocean forecasts in a few years.

• Move to new supercomputer, starting in 2015. Expected to provide an order of magnitude increase in computing power overall in about 2 years from now (phased increase).

Summary


Thank you


Additional slides

© Crown copyright Met OfficeDec 2010 Dec 2012

Dec 2010 Dec 2012

Development of new global FOAM version

SST statistics:Global reduction in RMS error from 0.49 to 0.47

Sea-ice concentration statistics:Global increase in RMS error from 2.9% to 3%


Development of new global FOAM version

SSH statistics:Global increase in RMS error from 7.1cm to 7.5cmMainly due to degradation in the Southern Ocean

Dec 2010 Dec 2012

Dec 2010 Dec 2012


Development of new global FOAM versionSurface currents compared to drifters

• Little impact on the surface current forecasts or analyses.


Monthly mean W 06/2011 – equatorial sections

Free run

DA with no pcbias

DA + inst pcbias + orig pcbias


Monthly W standard deviation 06/2011 –equatorial sections

Free run

DA with no pcbias

DA + inst pcbias + orig pcbias


Impact of horizontal resolutionCurrent speed (5 day mean)

• Demonstrates the impact of horizontal resolution on surface ocean currents. • For example, boundary currents and extensions change dramatically between ¼ and 1/12th with potentially important implications for biases and air-sea fluxes/coupling.

1 deg ¼ deg 1/12th deg

From Pat Hyder


Global 1/12° NEMOVAR in FOAMStatus

• The ORCA12 NEMO model configuration has been run at the Met Office. Based on a configuration developed within the DRAKKAR consortium.

• Basic technical work to implement NEMOVAR within this configuration has been undertaken.

• For an example day:• Observation operator has been run.• Basic version of NEMOVAR has been run.• Increments have been added into the model.

• Some work was required to make the observation operator and the NEMOVAR step more efficient:

• Reading observations in batches and assign to correct processor.• Removal of some global arrays and implementation of alternative

techniques for locating observations on the model grid.


• In the tropics, data assimilation causes spurious vertical motions which disrupt the velocities (horizontal and vertical).

• In FOAM a pressure correction is applied to maintain balance between the wind stress and the ocean density field in tropical regions to alleviate this.

• The pressure correction is a slowly evolving bias field which is updated at each analysis step using the temperature and salinity increments.

• However, even with the pressure correction we still get a strong equatorial response with spurious vertical velocities and equatorial waves.

• We are investigating the application of an “instantaneous pressure correction” to attempt to reduce the impact of the DA on the velocities in the tropics.

On-going developmentsData assimilation in

the tropics

© crown copyright met office uk report for govst matt martin govst-v, beijing, october 2014

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