The performance of two flow field decomposition techniques, the linear spatial filter and the proper orthogonal decomposition, in turbulence decomposition from a two-dimensional velocity field was investigated by using experimental data of velocity field beneath laboratory-generated wind waves. The main interest was in the identification of turbulent eddy structures embedded in the unsteady velocity field by the decomposition techniques that are used. The linear spatial filter presented good results showing almost the same vorticity map from the decomposed velocity field as the one from the original velocity field. As long as the size of the window function implemented in the linear spatial filter is adequately determined, turbulent flow field is well extracted by the filter in terms of preservation of turbulent eddy structures. In contrast, somewhat distorted vorticity map was obtained from the turbulent velocity field decomposed by the proper orthogonal decomposition, which implies poor performance of the method in turbulence decomposition.