www.cmar.csiro.au/bluelink/

Potential impact of HF radar and gliders on ocean forecast system

Peter OkeJune 2009CSIRO Marine and Atmospheric ResearchCentre for Australian Weather and Climate Research

Talk Outline

• NSW node of the Australian Integrated Marine Observing System

• Method

• Assessment of Bluelink background and analysis error estimates

• Estimation of likely analysis estimates with new observations

• Summary & conclusions

Australian Integrated Marine Observing System: NSW node

xx

xx

HFradar

X X - mooring

glider

approved

wish-list

xxxx

xx

The Bluelink System

Ocean Model•OFAM

•MOM4p0d

•Global model

•10 m vert res over top 200 m

Data Assimilation•BODAS

•EnOI

•120 member ensemble

•Localised covariances

•Assimilates along-track ALTIM, coastal SLA, SST & in situ T and S1/101/10oo 9/109/10oo 22oo

Method

To assimilate observations we must estimate: the background error covariance (Pb) … errors in the model the observation operator (H) … where & what the obs are the observation error covariance (R) … errors in the obs

Given this information we can estimate: the analysis error covariance (Pa) … errors in the analysis

Pa = [I – Pb HT (HPb HT + R)-1 H] Pb

Bluelink uses an static ensemble, A = [a1 a2 … an], to approximate Pb = A AT / (n-1).

The background ensemble A, with covariance Pb, can be efficiently transformed into the analysis ensemble Aa, with covariance Pa, using a transformation from ensemble square root filter theory.

This is most efficiently done serially – one observation at a time.

Assumed observation errors, R

… includes instrument + representation error.

Experiments

We consider cases with:

•GOOS + new platform

•just new platform

•just GOOS

Where the GOOS is assumed to be altimeters + SST + Argo/XBT

Impact of sampling error – varying obs distribution

Data distribution impacts the results – so we perform each calculation using the distribution of atSLA, SST and T/S

from the GOOS for each week of 2006.

Evaluation of estimates: SLA

•Estimated / Theoretical errors are based on the assumed and estimated BGF and analysis errors

•Actual (BRAN) errors are computed from differences with observations over a 3-year reanalysis

•Obs assimilated every 7-days include: AMSRE SST atSLA Argo T/S XBT

Assumed BGF errors and estimates analysis errors for all variables

Estimates % improvements for different HF radar arrays

HF radar + GOOS

% error reduction relative to GOOS with

only ALTIM+SST+Argo

HF radars are assumed to measure daily-mean surface velocity every day with no data gaps.

Estimates % improvements for different glider sections (or CTD or mooring lines)

Gliders + GOOS

% error reduction relative to GOOS with

only ALTIM+SST+Argo

Gliders are assumed to “fly” along a fixed

latitude within 200 km of the coast with a repeat cycles of 2-

weeks.

Actual glider path … not exactly the

same as the assumed E-W

tracks.

Comparison of different options: showing analysis errors averaged over local regions

Conclusions

HF radar obs may reduce U/V errors by as much as 80%; and T/S and sea-level errors by ~60% near the observed regions.

T and S obs near the shelf are likely to provide a more modest benefit … because they are somewhat redundant. assumed glider tracks are clearly unrealistic – and are more

akin to repeat ship-borne CTD sections.

As a result of this study Bluelink will likely develop the capability to assimilate HF radar data (initially in research mode) once data is available – but will likely retain glider observations for validation.

Sakov, P., and P. R. Oke 2008: Objective array design: Application to the tropical Indian Ocean. Journal of Atmospheric and Oceanic Technology, 25, 794-807.

Oke, P. R., P. Sakov and E. Schulz, 2009: A comparison of shelf observation platforms for assimilation into an eddy-resolving ocean model. Dynamics of Atmospheres and Oceans, in press.

END

Evaluation of estimates: SST

•Estimated / Theoretical errors are based on the assumed and estimated BGF and analysis errors

•Actual (BRAN) errors are computed from differences with observations over a 3-year reanalysis

•Obs assimilated every 7-days include: AMSRE SST atSLA Argo T/S XBT