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

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

Marine Weather Forecasts  What tools do we use and how do we prepare them?

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

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

Global Atmospheric Model  Solves fundamental equations describing the evolution of the atmosphere

 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

 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

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

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

Ensemble Forecasting – types of output

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

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

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.

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

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

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

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

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

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