An Overview of the UK Met Office Weymouth Bay wind model for the 2012 Summer OlympicsMark Weeks

1. INTRODUCTIONIn the summer of 2012 a very high resolution (333m) configuration of the Unified model was run operationally at the UK Met Office, in order to provide real-time wind forecasts to the organisers, sailors and safety crews involved in the Olympic and Paralympic sailing events at Weymouth Bay in Southern England.The location of Weymouth Bay, Dorset is sheltered from the prevailing westerly and south-westerly winds by steep cliffs on the Isle of Portland, which has a significant effect upon the local winds and general conditions for sailing. This provided a strong motivation to run at a very high resolution of 333m in order to represent the complex topography surrounding the bay, and resolve the very localised winds. A schedule of 8 runs per day was installed within the operational suite over the summer, that downscaled from the operational UKV 1.5km model. Site specific data feeds (at 5 minute intervals) of wind speed, direction and gusts was provided to the forecasting team in Weymouth.

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5. Summary

•First sub-1km scale configuration of the Unified model run operationally at the UK Met Office.

•Tailored forecast products for sailing provided in real time to the Olympic forecasters and organisation committee.

•Improved performance in 10m wind speed compared to UKV 1.5km operational model.

•Significant run to run variability in light to moderate sea breeze conditions.

3. MODEL DESCRIPTION•70 vertical levels from the surface to 40 km with horizontal resolution of 333m x 333 m over Weymouth Bay (100km x 100km) and 10 second time-step.

• Initial conditions and 15 minute boundary conditions provided from the operational UKV 1.5km.

•Science based upon the UKV configuration with some exceptions including 3D Smagorinsky sub-grid turbulence scheme, RH threshold for saturation and boundary layer stability functions.

•8 runs per day (0z,3z,6z,9z,12z,15z,18z,21z) to allow use of lagged ensemble approach.•Model output at 15minute intervals with 10m winds and gusts at 5 minute intervals to provide ranges of the variability of wind direction every hour.

4. CASE STUDY – Sea breeze•Ridge of high pressure moving SE, gradient wind NNW 17 KT at 0900 Z. A cold night but the temperature rose quickly over land in morning.•Earlier Weymouth runs (21Z and 0Z) indicated the relatively early onset of a WSW sea breeze, however this was delayed in subsequent runs (03Z and 06Z).•Reason not clear but possibly due to lower starting temperatures over land as dawn values were assimilated by UKV driving model.•Marked wind shift from 340 to 260 degrees with mean speed 10 KT across Weymouth Harbour between 1150z and 1210z, which reached the Weymouth Bay buoy by 1250z.•Different wind details run-to-run and difficult to assess what may caused the differences, although initial starting conditions may have played a part. 06Z run provided the poorest guidance for the early to mid afternoon period. However the availability of earlier runs allowed the forecasters to communicate the uncertainty to race organisers and sailors.

PMSL Analysis on 31/8/12 at 12z

Olympic Sailing Course locations

2.FORECASTING REQUIREMENTS FOR SAILING•7 sailing classes with wind race limits from 5 knots to 20 knots.•Course location, length and shape for races heavily influenced by forecast.•Race management very sensitive to changes in wind direction, particularly in the immediate run up to race to provide an unbiased start line.•Site specific forecast of wind speed, direction, gust strength and the hourly variability in direction required for each course location (see map) during Olympic period.•All Olympic competitors have access to online forecasts and briefed en-masse with no contact with forecasters allowed after the main briefing.

5. Verification of 10m winds against UKV

•Observational wind data from the Portland Harbour Wall anemometer (observations not quality controlled).

•Verification period From July 12th to September 11th 2012 covering Olympics and Paralympics.

•Reduced rms error (see plot on right) and reduced negative bias in mean error of forecast against observation in comparison to UKV 1.5km.

Real-time model verification (Key: Observations/ Weymouth 333m / UKV 1.5kmWeymouth Bay Topography

10m wind speed at 13z on 31/8/12: T+13 & T+10 Portland Harbour Wall wind Observations on 31/8