Cloud Sync in Response to Wave-Like Large-Scale Forcings

Abstract One crucial aspect of the gravity wave–convection coupling problem is how a cloud ensemble responds to the wave. This paper investigates the cloud response problem using cloud-permitting simulations. A small domain is used to simulate the cloud ensemble, and a large-scale oscillatory vertical motion is used to simulate the gravity wave. Simulation results show that the wave not only modulates the strength of individual convection but also synchronizes convective life cycles. The latter can be quantified with a synchronization index. To understand the synchronization mechanism, we construct a microscopic model comprising multiple clouds. Their life cycle is parameterized as a dual-threshold oscillator of the boundary layer equivalent potential temperature ( θ e ), and their interaction is parameterized as white noise. For each cloud, the transition from shallow to deep convective stages occurs when θ e is sufficiently high to eliminate convective inhibition (CIN); the transition from deep to shallow stages occurs when θ e is sufficiently low to exhaust convective available potential energy (CAPE). The presence of the two thresholds allows the wave to adjust the convective phase through its vertical advection. The synchronization effect of the wave balances with the desynchronization effect of noise to produce a balanced cloud response state. An analytical solution for the synchronization index is derived, capturing key features of synchronization in cloud-permitting simulations. From a macroscopic perspective, the synchronization is essentially the resonant growth of the cloud ensemble under the influence of noise, which approximately obeys a damped harmonic oscillator.