An idealised model of convective hydration of the tropical tropopause layer. Part I: Model formulation and diagnostic approach

Convection that overshoots the tropical cold‐point tropopause can irreversibly transport water vapour, providing an important pathway for hydration of the tropical lower stratosphere. We formulate an idealised model of this process by treating a convective overshoot as a buoyant plume penetrating a stably stratified layer carrying tracers that represent vapour and ice. We use large‐eddy simulations to study a scaled‐down model flow, which resolves turbulent tracer transport better compared with earlier numerical studies. The vapour and ice tracers evolve according to a minimal moisture model, with equal and opposite rates of condensation and sublimation and a fixed sedimentation velocity for the ice tracer. We explore the behaviour of the model in two experiments where the sedimentation velocity and partitioning of moisture into vapour and ice are varied. To understand the influence of mixing, we extend a formalism that captures the simultaneous modification of buoyancy and tracer concentration to include terms representing the effect of moist processes. We find that our model is an effective representation of the hydration mechanism observed in the tropical tropopause layer when the plume penetrates the stratified layer carrying large amounts of ice, which are mixed with the warmer environment and sublimated to form vapour. Hydration of the stratified layer beyond the direct transport of vapour by the plume requires ice to remain in suspension during mixing in the plume cap, which is controlled by the relative sizes of the sedimentation velocity and turbulent eddy velocity.