We conducted direct numerical simulations (DNS) and wind tunnel tests (WTT) to investigate the role of freestream turbulence (FST) in the laminar-turbulent transition of a boundary layer on a swept flat plate with cylindrical roughness. In the DNS, we examined how FST influences the transition process of roughness-induced crossflow vortex. Under short-wavelength FST conditions, hairpin vortices formed behind the roughness even at lower heights compared to long-wavelength FST conditions. Here, the long wavelength is the same length as the most unstable wavelength of a stationary crossflow vortex. Based on the roughness height for hairpin vortex generation, the critical roughness Reynolds number under the short-wavelength FST condition was found to be comparable to that observed in WTT. These findings indicate that high-frequency FST components penetrate the boundary layer in WTT environments. We clarified that the receptivity of high-frequency fluctuations plays a key role in the laminar-turbulent transition process caused by roughness and FST.