Excitable Madden-Julian Oscillation like behaviour of a simple model of equatorial moist dynamics results from a time step that is too large

A recent article by Vallis and Penn (2020) in this journal presented the behaviour of a simple single-layer model of moist dynamics, identifying an "excitable" behaviour in which a feedback between condensational heating and gravity waves sustained high values of kinetic energy and, when the model was an equatorial beta-plane configuration, a sustained Madden-Julian Oscillation (MJO)-like disturbance, eastward-propagating and including a localised region of condensational heating. Evidence is presented here that both the excitable and MJO-like behaviours are artificial, in the sense that they result from a choice of time step Delta t that it is too large to allow a satisfactory solution of the partial differential equations defining the model.