PRESENTS: FORECASTING FOR OPERATIONS AND DESIGN February 16 th 2011 – Aberdeen

1. PRESENTS: FORECASTING FOR OPERATIONS AND DESIGN February 16th 2011 – Aberdeen

2. Marine Weather Forecasts • What tools do we use and how do we prepare them?

3. The meteorological world has moved on a long way .....

4. Nowadays marine forecasts are prepared using output from NWP – Numerical Weather Prediction • Global Atmospheric Model • Ensemble Forecasting • Spectral wave model

5. Global Atmospheric Model • Solves fundamental equations describing the evolution of the atmosphere

6. Fundamental to the success of a forecast is an accurate knowledge of the initial state of the atmosphere • Short-range forecast - 'first guess’ • Observations of actual and derived conditions

9. The model is run forward in time, typically up to a week ahead • During the run, a range of forecast information is produced at set intervals – say 1 or 3 hours • e.g. temperature, humidity, pressure, winds

10. Ensemble Forecasting • Model does not fully describe processes in the atmosphere • Model resolution insufficient to capture all atmospheric features • Initial observations are not available at every point in the atmosphere • Observational data cannot be measured precisely

11. Ensemble Forecasting • One method of quantifying the forecast uncertainty is by running the model a number of times • Each run has slightly different starting conditions • Each run evolves differently with time

12. Ensemble Forecasting – types of output

13. Spectral wave model • Solves the spectral energy density equation • Uses input from the Global Atmospheric Model, e.g. winds, sea and air temperature

14. Spectral wave model • The wave model predicts the energy spectrum at each point at each time step • The energy spectrum is processed to produce significant wave height, wave period, wind wave, swell etc

15. Preparing a Weather Forecast for Marine Operations • Start by examining raw forecast data of wind, waves etc • Compare starting values with actual observed data • Make allowances for model biases • Prepare the final forecast figures • Disseminate the forecast in required format – e.g. table/graph, operations checklist, 3-D plot etc.

18. Providers of Marine Weather Forecasts • Most oil companies will not have their own in-house forecasting service but will contract out the provision of weather services to a specialist provider. • Forecast company – either public or private – will prepare forecasts for a number of clients using their available model data • Forecasting companies will use output from one or more global forecast centres e.g. UK Met Office, NCEP, ECMWF • Many companies have their own in-house wave model capability

19. Extreme Wave Height • Gather information on wave climate from observations of wave height over as long a period as possible • Choose a distribution function to fit the ‘upper tail’ (i.e. the highest waves) of the data • Log-normal • Weibull • Fisher Tippett • Extrapolate to a probability of exceedance of 1 in 50 years

20. Extreme Wave Height (continued) • Calculate the highest likely wave corresponding to the maximum value of 50 year significant wave height • Combine with other sea-level parameters e.g. storm surge, astronomical tide

21. Other Extreme Metocean Parameters • Wind • Current – Tidal, Surge, Residual • Air and Sea Temperature • Snow and Ice

22. Change of Extremes in the Future • Very difficult to predict how the wave climate will respond in the future • Recent changes in North Atlantic wave height have been correlated with the phase of the North Atlantic Oscillation