Performance of different forecast systems in an exceptional storm in the Western Mediterranean Sea



We consider an exceptional storm—‘Klaus’ (January 2009)—its evolution on the Western Mediterranean Sea, and how the associated wind and wave conditions were modelled by seven of the major systems presently operational in this area. We intercompare the model results and then verify them and the related model ensemble versus the available measured data.

Working with short-term forecasts (24 h) only, as expected, each model correctly anticipates the incoming of an exceptional storm. However, even at such limited range, we have found substantial differences among the results of the different models. The differences concern the time the storm should have entered the Western Mediterranean Sea, the peak values of wind speed and significant wave height, the general distribution of the fields, and the locations where the maxima were achieved.

We have compared the model results versus the available measured data, wind from scatterometer, waves from altimeter, plus a few buoy data. We have found some inconsistencies in the results, model wind data being on average larger than the measured one, while the opposite was true for wave heights. However, the limited amount of data available and its different times and positions, at and off the centre of the storm, impede the drawing of any definite conclusion in this respect.

On the whole we feel that our results, although related to a single storm, cast doubts on the reliability of a single forecast system to provide sufficiently reliable and accurate forecasts in case of an incoming exceptional storm. The results, both for wind and waves, have improved using an ensemble of the seven considered models. This suggests that there is no relevant systematic error in the used models except, as possibly suggested by our results, in the case of wave generation under very strong wind and very young sea conditions. Copyright © 2011 Royal Meteorological Society and British Crown Copyright, the Met Office