Dear firedrakers (and Colin), is there an easy way of building a PETSc-based block-Jacobi preconditioner for a mixed system in firedrake? So if I have the mixed system in velocity, buoyancy, pressure, can I tell PETSc that the dofs in one vertical column belong together, i.e. the full vector has a block-structure, and then use a block-Jacobi preconditioner? Or does that make no sense if I have a horizontally continuous space? Colin, in your full model, do you use the Schur-complement preconditioner and have you tried replacing it with a block-Jacobi)? Do you have a non-linear shallow water code lying around, in which we would test replacing the Schur-complement preconditioner with point-Jacobi? We’re thinking about using this as a short term alternative for the full Schur-complement preconditioner in GungHo. Thanks, Eike