Direct verification of forecasts from a very high resolution numerical weather prediction (NWP) model

The University of New South Wales (UNSW) High Resolution numerical weather prediction model (HIRES) is run routinely, on a daily basis, at a horizonta l resolution of 25 km. The output is made available to the New South Wales (NSW) regional office of the Australian Bureau of Meteorology in Sydney. HI RES has been used to forecast mean wind direction and speed at a height of 12 metres for a number of events in the past. The opportunity was used in D ecember 1997 to run the model for the annual Sydney to Hobart yacht race ar ea. For the 1997 Sydney to Hobart yacht race the model was run at 25 km horizon tal resolution and the output was made available to all competitors on the morning of the race, namely December 26th. It was also decided by the autho rs to expand the verification to include all observations available both fr om land and sea within the model domain an well as those available from a m oving single point at sea, namely a yacht. After the event, the model was run once at the increased resolution of 10 k m, out to 5 days ahead. Both model runs were subjected to detailed verifica tion by one of the authors (KLB) who participated in the race aboard the ma xi-yacht Nicorette and who carried out a pre-arranged observational program during the race. Surface synoptic weather maps prepared in the NSW office of the Bureau of Meteorology were also consulted in order to extend the ver ification scheme. The model predicted winds were verified on a six-hourly b asis utilising instrumentation on the yacht as well as surface observations plotted in standard World Meterological Organisation (WMO) format on surfa ce synoptic weather. maps. The yacht carried wind sensors situated on top o f the mast at a height of 30 metres above the water. The authors were most interested in the accuracy of the wind velocity forecast by the model. It is important to note that forecasting for points over the ocean at widel y separated time intervals represents a very difficult challenge. The verif ications reveal that the model gave overall forecast guidance of very good to excellent quality and was particularly accurate early in the race, when a Southerly Buster event occurred during the evening of the first day. Howe ver, a caveat should be added that this forecast, despite its remarkable ac curacy, is not a claim that accurate very high resolution regional predicti on is a solved problem. Rather, it is a demonstration that in particular in stances current models are now capable of achieving high levels of skill a number of days ahead.