Numerical simulation studies in 2D with the addition of noise are reported for the convection of a supercritical fluid,3He, in a Rayleigh-Be'nard cell where the fluid parameters and cell height L are the same as in published laboratory experiments. The noise addition is to accelerate the instability onset after starting the heat flow across the fluid, so as to bring simulations into better agreement with experimental observations. Homogeneous temperature noise and spatial lateral periodic temperature variations in the top plate were programmed into the simulations. A speed-up in the instability onset was obtained, which was most effective through the spatial temperature variations with a period of 2L, close to the wavelength of a pair of convections rolls. For a small amplitude of 0.5 microK, this perturbation gave a semiquantitative agreement with experimental observations. Results for various noise amplitudes are presented and discussed in relation to predictions by El Khouri and Carle`s.