Evaluation of stability corrections in wind speed profiles over the North Sea

Abstract

An improved meteorological method for estimating the wind speed profile over sea is developed. In the proposed “diabatic” method, the Monin-Obukhov similarity theory, which takes into account the stability of the planetary boundary layer, is applied. By choosing parameterization of several components, the over-sea profile parameters ( u ∗, θ ∗, q ∗, z 0 and L ) can be expressed in terms of routine weather data only (wind speed, sea water temperature and air temperature). With these parameters the wind speed as a function of height can be calculated. The diabatic method has been tested using a 4-month data-set consisting of wind speeds measured at heights of 18 and 70 m and temperatures of the air and the sea water. The mean wind speed measured at a height of 70 m over this period was 9.6 m s −1. Using the diabatic method the estimated mean wind speed at the same height was 9.3 m s −1, with an r.m.s. difference of 1.1 m s −1. The estimated mean wind speed using the logarithmic profile (only neutral stratification) was 8.7 m s −1 with an r.m.s. difference of 1.4 m s −1. This shows that the proposed diabatic method gives a better estimate of the mean wind speed than does the logarithmic method. The diabatic method also yields a more accurate estimate of the frequency distribution of the wind speed as characterized by the Weibull shape and scale parameters and of the average diurnal variation in the wind speed. If the two methods are applied to a 4-year period, it is found that the mean wind speed estimated by each method is almost the same. However, the diabatic method gives a much better simulation of the mean seasonal wind speed.