Characteristics of Coherent Structures in Marine Atmospheric Surface Layer

(ABSTRACT) Wind speed data of multi-heights have been examined to investigate the spatial and temporal characteristics of coherent structures in the near neutral marine atmospheric surface layer. With Taylor's hypothesis, the temporal velocity signals have been transformed to spatial fluctuations and then visualize these spatial velocity fluctuations to identify the coherent structures. It has been confirmed that there exist similar coherent structures in the marine atmospheric surface layer to those in laboratory turbulent boundary layer. These similar coherent structures include ejections, sweeps, shear layers, transverse vortices, and combined events of the shear layers and transverse vortices. Besides these similar coherent structures, there exist the plume and downdraft motions in the unstable marine atmospheric surface layer. It has been observed that the streamwise spatial length of the ejections and sweeps is 20–250 m and their mean frequency is of order of 10 −2 –10 −3 s −1 at mean wind speed of 5–12.6 m/s. Between the region of the upstream ejection and downstream sweep motions an inclined shear layer is often seen. The inclined angle of the shear layer has been observed to vary from 30 • to 70 • with the height and length of the the shear layer. The transverse vortices are seen to exist in every region from the wall up to a height of 45 m and their ii diameter is up to 40 m. The mean frequency of the shear layers and the transverse vortices is of order of 10 −3 s −1. In the fully developed stage of the combined event of the shear layer and transverse vortex, the shear layer is generally longer and the diameter of the transverse vortex is larger. The mean frequency of the combined event of the shear layers and the transverse vortices is of order of 10 −3 s −1. The streamwise spatial length of the plume and downdraft motions is generally from 20 m to 50 m. Analysis indicates that the mean wind speed is a dominant factor in affecting the spatial and temporal characteristics of the coherent structures in the near neutral marine atmospheric surface layer. As the mean wind speed increases, the frequency of the shear related coherent events will increase, while the frequency of the buoyancy related coherent events (plumes and downdrafts) will decrease. The temperature difference between higher level of the surface layer and sea surface is the second main factor in affecting the …