Abstract: The effects of baroclinicity on the geostrophic momentum Ekman flow are investigated in this paper. By making use of the two-variable singular perturbation method, the uniformly valid asymptotic solutions of the first two orders of wind structure and the vertical velocity at the top of the boundary layer are obtained. The baroclinic effects are clearly reflected in these solutions. Especially, the first-order corrections of the solutions are completely induced by thermal wind. Three new pumping factors due to baroclinic effects are derived in the solution of vertical velocity at the top of the boundary layer. The first is thermal wind deformation, the second, thermal wind vorticity, and the third, the cross term of thermal wind vorticity. It is pointed out that only in such baroclinic systems of horizontally nonuniform temperature gradient as frontal zones will the three new pumping factors come into effect. Furthermore, the dynamic properties of the three pumping factors are carefully analysed with respect to a frontal zone. The quantitative results of the baroclinic effects will appear in the next paper.