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On 3 July 2001, the four Cluster satellites traversed along the dawnside magnetospheric flank and observed large variations in all plasma parameters. The estimated magnetopause boundary normals were oscillating in the <i>z</i>-direction and the normal component of the magnetic field showed systematic &nbsp;2&ndash;3 min bipolar variations for 1 h when the IMF had a small positive <i>b_z</i>-component and a Parker-spiral orientation in the <i>x,y</i>-plane. Brief &nbsp;33 s intervals with excellent deHoffman Teller frames were observed satisfying the Walén relation. Detailed comparisons with 2-D MHD simulations indicate that Cluster encountered rotational discontinuities generated by Kelvin-Helmholtz instability. We estimate a wave length of &nbsp;6 <i>R_E</i> and a wave vector with a significant <i>z</i>-component.