The impact of atmospheric stability on the near-surface wind over sea in storm conditions.

We study the influence of boundary layer stability on the near-surface wind speed, especially for high-wind conditions. An analysis of the wind speed ratio between two vertical levels observed at tall masts in the North Sea and The Netherlands demonstrates that over sea non-neutral conditions commonly occur, even when the 10-m wind speed is
7 Bft or higher (at least 13.9 m s-1). Over land, stability conditions are always close to neutral for these strong wind conditions. This is because over land large verticaltemperature differences are rare in these conditions. An analysis of 30 years of station data shows that the ratio of the 10-m wind speed between sea and land depends systematically on the difference between the air temperature and the sea surface temperature. For 10-m wind speeds of 9 Bft or higher (at least 20.9 m s-1) this ratio decreases with almost 4 % per degree air-water temperature difference. The observational
results are reproduced by HARMONIE, a state-of-the-art Numerical Weather Prediction model, although the impact of stability is smaller than in the observations. A model sensitivity analysis for a severe storm shows that the near-surface wind speed over sea can vary by 10% depending on the difference between the air temperature and the sea
surface temperature. The results presented in this study indicate that even in conditions that are usually classified as ‘(near) neutral’, small variations in stability may have a significant impact on the wind profile. They also indicate that for high wind speeds the sea-to-land wind speed ratio is dominated by the stability over sea as in these conditions the stability over land is close to neutral.