Analyses & Numerical Prediction

Analysis and Prediction Operations Bulletin No. 55
Changes to the Operational Sea State Forecast System
27 August 2002

Introduction

The WAM wave model has been the operational sea state model in NMOC (National Meteorological and Oceanographic Centre) since 1 June 1994 and its initial configuration was described in Analysis and Prediction Operations Bulletin No. 29 of 30 May 1994. Details of the model formulation are contained in WAMDI (Wave Model Development and Implementation) Group (1988), while the finite differencing scheme adopted in the Australian implementation of the model is detailed in BMRC (Bureau of Meteorology Research Centre) Research Report No 43.

While the physics and numerics of the model remain essentially unchanged this bulletin describes changes made to the current operational configurations, the move to integrated code across all domains, and some improvements to the altimeter assimilation during the week, 29 July to 3 August.

Descriptions of previous changes to the operational sea state forecast system in the past few years (Bulletin No.53) are available at  NMC Operational Bulletins. Those changes included the discretisation of the wave spectrum which eliminated the possibility of wave energy propagating directly along the east-west and north-south co-ordinates and the implementation of a new bathymetric dataset for which an evaluation showed  a significant improvement in forecast of Significant Wave Height around the Australian coast (Greenslade 2000).

Altimeter Data Assimilation

Significant wave height (SWH) data from the ERS-2 (European Remote-sensing Satellite) fast delivery product set are received in NMOC in near real time. The development of the scheme to incorporate these data into the model has been carried out in the Bureau of Meteorology Research Centre by Diana Greenslade and is described in detail in Greenslade (2001). Using the altimeter data, statistical interpolation is used to construct analysed SWH fields. The improvement in model skill is retained throughout the forecast period.

Recently, comparisons between ERS altimeter measurements of significant wave height and co-located in situ measurements have shown there is a negative bias in the altimeter estimate. An adjustment to the ERS data reduces the RMS error in the data from 0.3 to 0.2m and provides positive impact on the global forecast wave fields (Greenslade 2002). This correction is included in the operational models.

Model Modification

Numerous modifications were made to the operational wave models during 2002 after several months of parallel trial running to assess their impact.

• Modifications to the new version of the Australian domain operational model include:
  1. Resolution: The resolution has been increased to 0.5 degree latitude/longitude from the course grid one of 1.0 degree.
  2. Wind input: The wind input has increased in frequency from 3-hourly to one-hourly.
  3. Wave energy: A modification of total wave energy computation at each grid point.
  4. Swell assimilation: Re-structuring the code for swell assimilation strategies.
  5. Altimeter: New settings based on regression between altimeter and buoy data (as above).
  6. Wave field: Re-structuring the model code to enable efficient multi processing.

   

• The last four modifications above have been included in the global domain model.

   

• A new Meso-scale domain model covering all Australian region at 0.125 degrees latitudes/longitudes, including modifications (3)-(6) above, but charts for the south-east region will still be available.

For each modification a slight improvement in model performance was observed and, overall, there was a significant forecast improvement in resultant wave height values in comparison with buoy observations.

In all cases, efforts have been to make the resolution of the wave models as close as possible to the atmospheric models.

Model Configuration

The model is run twice daily, with base times of 00 and 12 UTC, on three domains: Global, Australian Region and Australian Meso-scale. Details of these are summarised in table 1 below.

Table 1. Model Configuration Summary.

Note: First order numerics are used in the Australian Meso-scale model as the third order up-winding scheme employed in the other models is incompatible with use of the  sea state equations in shallow water.

Each run consists of a 12 hour hindcast period using analysed winds from the relevant atmospheric model followed by a forecast using prognostic winds. Forecasts are made to base time plus 96 hours for the global system and base time plus 36 hours for the other systems.

Output

Forecast wave energy spectra are post-processed to provide output fields of the following variables:

• Significant wave height
• Wave direction
• 10m Wind speed
• 10m Wind direction
• Mean period
• Peak period
• Swell wave height
• Wind wave height
• Mean Swell direction
• Mean Swell period
• Mean Sea direction
• Friction velocity
• Drag coefficient
• Normalised wave stress

Plots of model forecasts compared with buoy observations together with summary tables of forecast verification for the previous 30 days are produced in each run for the following sites:

• North Rankin
• Jurien Bay
• Rottnest Island
• Cape Naturaliste
• Byron Bay
• Coffs Harbour
• Crowdy Head
• Sydney
• Bondi
• Port Kembla
• Batemans Bay
• Eden
• Kingfish B
• Cape Sorell
• Cape Du Couedic
• Brisbane

Access to these fields in all domains is as follows:

Significant wave height, swell wave height and wind wave height charts for all three domains are available.

Further Enquiries

Routine enquiries regarding product availability or access should be made to the NMOC Help Desk on (03) 9662 2182. Further information on the implementation of the WAM model can be obtained from Graham Warren on (03) 9669 4041 or A.Kazempour on (03) 9669 4502.

References

Bender, L.C and Leslie, L.M. (1994). 'Evaluation of a third generation ocean wave model for the Australian region'. BMRC Research Report No 43. Bur. Met. Australia.

Greenslade, D.J.M. (2000). 'Upgrades to the Bureau of Meteorology's ocean wave forecasting system'. BMRC Research Report No 79. Bur. Met. Australia.

Greenslade, D.J.M. (2001). 'The Assimilation of ERS-2 significant wave height data in the Australian region'. Journal of Marine Systems, 28, pp. 141-160.

Greenslade, D.J.M. and Young. I.R. (2002). 'A Validation of ERS-2 Fast Delivery Significant Wave Height'. BMRC Technical Report (in Press), Bur. Met. Australia.

WAMDI Group (1988): Hasselman, S., Hassleman, K., Bauer, E., Janssen, P.A.E.M., Komen. G.J., Bertotti, L., Lionello, P., Guillaume, A., Cardone, V.C., Greenwood, J.A., Reistad, M., Zambresky, L., and Ewing J.A., 'The WAM model - A third generation wave prediction model'. J.Phys. Oceanogr., 18, 1775-1810.

Warren, G.R. (1999): 'Changes to the Operational Sea State Forecast System'. Bulletin No. 47. Bur. Met. Australia.