Cloud-Edge Motion by a Ducted Gravity Wave

Abstract

Abstract The reflection of a wave at a fluid interface is fundamental to the atmospheric wave duct. As latent heating distinguishes the buoyancy response between cloudy and clear air, cloud edges can serve as a ducting interface for gravity waves. However, advection of the thermodynamic conditions by the ducted wave itself can cause evaporation or condensation, where the motion of cloud edges results from shrinking or enlarging regions of saturated air. For an idealized ducted wave mode trapped by a cloud layer, a linear Boussinesq analysis shows that its vertical motions produce a sinusoidal corrugation of the cloud edge that travels with the wave. When thermodynamic conditions are continuously varying, the cloud edge propagates as a moving onset of phase change and not as a material interface. Using this Boussinesq solution to initialize the full-physics Cloud Model 1 (CM1), the simulation confirms the amplitude and speed of the cloud-edge wave. In a comparison of simulations for domains of decreasing height, convergence to the Boussinesq ducted wave can be quantitatively established. This demonstration suggests a theory-based convergence benchmark for the motion of a cloud edge by phase change. Significance Statement Waves in the atmosphere can cause evaporation or condensation that results in changes to the shape of an individual cloud. A standard theory for airflow is extended to include the motion of the edges of a cloud. An example wave cloud as approximated by this theory is shown to be reproduced, with high accuracy, in a computer weather forecast model. The quantitative verification of this basic theory for clouds represents a new opportunity for exploring how wave interactions may be involved in the complex processes that shape the clouds that are an important component in our weather and climate systems.