Figure 5b. (a) Doppler frequency spectrum. In this plot, time is along the x axis and Doppler shift frequencies (expressed in units of velocity calculated by using Eq [10]) are along the y axis. The relative intensities of Doppler shift frequencies are depicted by using varying gray scales. This display is inverted: A darker pixel represents a higher relative intensity at that particular frequency and time. s = seconds. (b) Narrow versus broad Doppler spectra. In the narrow spectrum, the frequency content is such that the mean Doppler shift frequency (fmean) is relatively close to the maximum Doppler shift frequency (fmax). The broad spectrum has a frequency content that results in a lower fmean.