Weak Temperature Gradient (WTG) modeling using a small cloud-resolving model (CRM) admits multiple equilibria depending upon the initial model conditions. A new kind of equilibrium is presented here which undergoes a periodic and steady-state oscillation that is emergent from static boundary conditions. The periodic oscillation is shown to be a moisture mode with an oscillating vertical motion profile shape generated by the interaction between vertical motion and radiation. A simple two-mode model based on decomposing the vertical motion profile and associated radiative cooling variations can describe the oscillation.