Dependence of the Kelvin-Helmholtz instability with density contrast of 2 on the speed of sound reduction rate with ξ = const. (panel a) and C_max = const (panel b). In each panel, the maximum amplitude of the y-component of the velocity at time = 1.5 with the PVS form (red with diamond) and the PMS form (blue with cross) are shown. The horizontal dashed line shows the reference simulation results without the RSST (ξ = 1) in 4096 × 4096 grids.