Upgrades to the Operational Sea State Forecast System

Transcription

1 NMOC Operations Bulletin No. 53 Upgrades to the Operational Sea State Forecast System Introduction 25 August 2000 The WAM wave model (WAMDI, 1988, Komen et al., 1994) has been the operational sea state model in NMOC since 1 June 1994 and its current configuration is described in NMOC Operations Bulletins No 47. On August 28 th 2000, two significant changes will be introduced to the operational system, firstly, the discretisation of the wave spectrum will be altered and secondly, a new bathymetric dataset will be implemented. These changes coincide with the transfer of the operational version of the wave models to the new NEC SX-5 supercomputer. Spectral Discretisation Previously, the wave spectrum, F(f, theta ) was discretised into 24 frequency bins with central values of f i+1 = 1.1* f i, with f 1 = Hz, and 12 directional bins at 30 o resolution centred at 0 o, 30 o, 60 o,... from North. The frequency discretisation remains unchanged, but the wave spectrum has now been rotated clockwise by 15 o, so that the 12 directional bins are now centred at 15 o, 45 o, 75 o,... from North. This spectral rotation eliminates the possibility of wave energy propagating directly along the east-west and north-south co-ordinate axes, which can result in elongated north-south patterns of Significant Wave Height (SWH) as seen in Figure 1(a). The parallel contours in the north-east corner of this figure are due to northerly-propagating wave energy being blocked by the Solomon Islands (at around 160 o E, 10 o S) at alternate grid-points. The new discretisation has been running on the SX-5 in parallel to operations and the equivalent SWH forecast to Figure 1(a) is shown in Figure 1(b). The parallel contours in the north-east corner are no longer visible and there are minor changes in the rest of the SWH field. This wave spectrum discretisation has been implemented at ECMWF since May 1997 (Bidlot et al., 1997) and has been evaluated for the Bureau of Meteorology's system in BMRC. The impact on monthly verification statistics was found to be minimal (Greenslade, 2000). Bathymetry 1

2 A version of WAM which includes depth effects has been running operationally over the south-east of Australia since July 24 th The bathymetric dataset originally implemented was the ETOPO5 bathymetry (National Geophysical Data Center, 1988), the same as that used in the deep-water global and regional versions of WAM. Since the implementation of the shallow-water version, however, it has become evident that there is a need to update the bathymetry. A high-resolution (1/120th o ) version of the bathymetry of the Australian continental shelf has recently become available from the Australian Geological Survey Organisation (AGSO). A new global 1/12th o bathymetric dataset has been constructed within BMRC by Diana Greenslade, consisting of the AGSO data in the Australian region (108 o E to 158 o E, 9 o S to 45 o S) and the ETOPO5 data elsewhere. Evaluation of this new bathymetric dataset (Greenslade, 2000) shows that it provides a significant improvement in forecasts of SWH around the Australian coast. The greatest impact is found at the location of the Port Kembla waverider buoy on the New South Wales coast (see Figure 2). The new bathymetric dataset has been implemented operationally within all wave model domains (i.e., global, regional and mesoscale) as improvements in the location of the Australian coastline are also seen. References Bidlot, J. R., Janssen, P.A.E.M., Hansen B. And Günther, H A modified set-up of the advection scheme in the ECMWF wave model. ECMWF Technical Memorandum 237, ECMWF, Reading. Greenslade, D Upgrades to the Bureau of Meteorology's ocean wave forecasting system. BMRC Research Report No 79. Bur. Met. Australia. Komen, G.J., Cavaleri, L., Donelan, M., Hasselmann, K., Hasselmann, S. and Janssen, P.A.E.M Dynamics and modelling of ocean waves. Cambridge University Press, Cambridge, UK, 532 pp. National Geophysical Data Center, Digital relief of the surface of the earth, Data Announcement 88-MGG-02, NOAA, Boulder, Colorado, USA. WAMDI Group, 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,

3 1.(a) 3

4 1.(b) Figure 1. (a) Previous operational 24-hour forecast of SWH for the Australian region valid at 1200UTC on August 22nd, (b) Same as (a) but with the new spectral discretisation 4

5 Figure hour forecasts of SWH at Port Kembla during January 2000 for the wave model run with original bathymetry (ETOPO5) and with the new bathymetry (AGSO). 5