Turbulence motions are often perceived as chaotic and unstructured. It has been observed however that the turbulence is frequently organized in semi-regular,persistent and large-scale coherent structures in realistic boundary layers. This study investigates such coherent structures, called rolls, in the non-stratified (Ekman) boundary layer on a rotated planet (the Earth). Large-eddy simulation numerical model is applied to reveal the roll structure, evolution and sensitivity to the flow direction and latitude. Two linearized stability mechanisms (by Lilly and by Leibovich and Lele) are considered. It was demonstrated that both mechanisms are at work in the Ekman layer but the Lilly mechanism dominate only in high latitudes whereas Leibovich and Lele mechanism dominates in low and middle latitudes.