Like I’ve written about before, I have had a very strong interest in writing my own simple GCM/dynamical core. After several frustrating initial attempts, partly described here I switched strategy. Rather than attempting to reimplement someone else’s model from their description, I would start from the primitive equations and derive my own model, aiming for as much simplicity as possible in the system of equations. By working in an isobaric (pressure) vertical coordinate, the equations become very simple, but implementing a non-flat surface becomes computationally hard, so I chose simplicity over realism here for the first try.