Icing simulations for wind turbine blades should consider the roughness-induced flow transition. Adding a transport equation for 'roughness amplification' to the Langtry-Menter model, the roughness-induced transition can be predicted for rough flat plates. However, this approach exhibits a limitation that it cannot predict the skin friction in the shadow zone of blunt bodies. Such an approach depends on the boundary condition(s) of specific dissipation rate (omega). Typically boundary conditions for turbulent kinetic energy (k) and omega have been investigated for various roughness heights, but have been applied only for fully turbulent conditions. This study introduces an approach to predict the flow transition and the skin friction for a roughened surface, whereby the Langtry-Menter model including roughness amplification is coupled with the k and u boundary conditions. The proposed method shows good agreement with the experiments for turbulent onset and the distributions of skin friction and heat convection for a roughened flat plate and a circular cylinder. Using the turbulent models under fully turbulent and transitional assumptions, the effects of the flow transition on the ice accretion shape on a rotating wind turbine are compared. The modified turbulent model showed better performance for the icing simulations without any tuning. (c) 2020 Elsevier Ltd. All rights reserved.