Upscale Impact of Mesoscale Disturbances of Tropical Convection on Convectively Coupled Kelvin Waves

Abstract

Tropical convection associated with convectively coupled Kelvin waves (CCKWs) is typically organized by an eastward-moving synoptic-scale convective envelope with numerous embedded westward-moving mesoscale disturbances. Such a multiscale structure of tropical convection is a challenge for present-day cloud-resolving simulations and its representation in global climate models. It is of central importance to assess the upscale impact of mesoscale disturbances on CCKWs as mesoscale disturbances propagate at various tilt angles and speeds. Besides, it is still poorly understood whether the front-to-rear-tilted vertical structure of CCKWs can be induced by the upscale impact of mesoscale disturbances in the presence of upright mean heating. Here, a simple multiscale model is used to capture this multiscale structure, where mesoscale fluctuations are directly driven by mesoscale heating and synoptic-scale circulation is forced by mean heating and eddy transfer of momentum and temperature. The results show that the upscale impact of mesoscale disturbances that propagate at tilt angles of 110°–250° induces negative lower-tropospheric potential temperature anomalies in the leading edge, providing favorable conditions for shallow convection in a moist environment, while the remaining tilt-angle cases have opposite effects. Even in the presence of upright mean heating, the front-to-rear-tilted synoptic-scale circulation can still be induced by eddy terms at tilt angles of 120°–240°. In the case with fast-propagating mesoscale heating, positive potential temperature anomalies are induced in the lower troposphere, suppressing convection in a moist environment. This simple model also reproduces convective momentum transport and CCKWs in agreement with results from a recent cloud-resolving simulation.