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

摘要

Atmospheric turbulence inrensities and timescales have been measured tor one yepr and modeled in a shoreline environment. Measurements were carried 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 TOPS (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 Manna el at. and Hsu method which is based upon z/L values, originating from sea-air temperature differences. The observed values of the lateral windfluctuations σ_v and stability classes for the dataset could not be adequately described in terms of local parameters such as Obukhov length L and friction velocity u_*. Therefore, a simple model, which explicitly considered advection and dissipation inthe turbulent kinetic energy equation, was formulated for the calculation of turbulence intensity at sea for offshore wind flows; dissipation is assumed to scale well with σ_v~3/uT_l. Using this model, observed a, 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 can not be described successfully only in terms of local parameterization. The calculated values of the Eulerian timescale correlate quite well with measured values of σ_v/u at both sites. However, values of th: timescale at sea were larger; this may be caused by differences in roughness lengths between land and sea.