The properties of randomly rough surface in the rarefied gas flow are investigated on the base of the polyGaussian model. These properties have much to do with surface aerodynamic characteristics. This paper puts forward the representation of probability distribution for z-axis of roughness in the form of the mixture of the Gaussian distributions. Owing to this way we succeeded in deriving analytic expressions for factorial moments of the number of outliers upon a specific level of a trajectory of a gas particle. Those analytic expressions play a key role in finding the scattering function of gas atoms on the surface and the momentum exchange coefficients. The important particular case of the poly-Gaussian processes has been considered, namely – spherically-symmetric random processes, permitting to simplify the conditions. The asymptotic evaluations were found for high level crossings characteristics corresponding to the case of weakly-rough surface. The results obtained allowed to evaluate the influence of surface roughness on the type of gas flow near the surface in comparison to the Gaussian model of roughness.