Characteristics of Coherent Structures in Marine Atmospheric Surface Layer
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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 0.01-0.001 /s 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 degree 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 diameter is up to 40 m. The mean frequency of the shear layers and the transverse vortices is of order of 0.001 /s. 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 0.001 /s. 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 spatial and temporal characteristics of the coherent structures. As the marine atmospheric surface layer becomes more stable the coherent motions will be suppressed. The effect of the temperature difference on the buoyancy related plume and downdraft motions is more evident than on the other shear related coherent motions.
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