a) Power spectra of v_h obtained from IMaX (solid line) and simulations (dashed line). The dot-dashed line represents the average power spectrum of 500 simulated random walks. b) Power spectra of v_z obtained from IMaX and simulations. Each spectrum is the average of spectra of twenty individual features. The vertical continuous and dashed lines indicate the lowest frequency due to the total length of the time series for the vertical velocity and the horizontal velocity, respectively. c) Amplitudes of v_h. d) Amplitudes of v_z. The vertical continuous and dashed lines indicate the mean value for IMaX and the simulation, respectively.