Atmospheric Turbulence above Coastal Waters: Determination of Stability Classes and a Simple Model for Offshore Flow Including Advection and Dissipation

Abstract

Atmospheric turbulence intensities and timescales have been measured for one year and modeled in a shoreline environment. Measurements were carded out at two sites on both sides of the shoreline, about 10 km from the beach. The frequency distribution of Pasquill stability classes [determined using the COPS (calculation of Pasquill stability) method by analyzing wind fluctuations] was compared with similar observations at the land site. A shift to the more stable classes is observed. Moreover, a large shift to the stable classes was shown at the overwater site for the COPS method in comparison to the Hanna et al. and Hsu method, which is based upon z/L values, originating from sea-air temperature differences. The observed values of the lateral wind fluctuations σθ and stability classes for the dataset could not be adequately described in terms of local parameters such as Obukhov length L and friction velocity σ*. Therefore, a simple model, which explicitly considered advection and dissipation in the turbulent kinetic energy equation, was formulated for the calculation of turbulence intensity at sea for offshare wind flows; dissipation is assumed to scale well with σ3v/uTl. Using this model, observed σθ. values were explained remarkably well. It is concluded that atmospheric turbulence intensity above sea in the vicinity of the shoreline is strongly influenced by horizontal gradients and cannot be described successfully only in terms of local parameterization. The calculated values of the Eulerian timescale correlate quite well with measured values of σθ/u at both sites. However, values of the timscale at sea were larger, this may be caused by differences in roughness lengths between land and sea.