[firedrake] Perturbed extruded meshes

Dear firdrakers, how can I perturb an extruded mesh? I want to add 'mountains' (i.e. bumps) to my spherical icosahedral mesh, so I want to change the radius according to something like r = r(theta) = R_earth+H*exp(-0.5*theta^2/D^2) Can I do this by moving around the underlying 2d host grid and then extruding that? How can I perturb the 2d grid? Thanks a lot, Eike