The depth of Earth’s layers is known, and the Convection Rolls System was derived accordingly.
This drawing can be best understood as a section of the equator plane of the Earth. Then all physical factors are combined in a 2D plane. Convection currents of lower mantle and outer core are represented with circles. Real flowlines are of course different, filling up the space of the cell froming the section of a convection roll. Developing a 3D model, the interaction of rotation of the Earth itself (and thereby centrifugal force) and convection currents have to be considered, in addition to the Coriolis effect along with the curvature of the Earth. As all those factors have been taken into account, the Convection Rolls Model has been completed.