Island cloud tails

Quasi-stationary cloud bands (or “tails”) commonly develop past tropical islands in the afternoon. These clouds, which may extend downwind of their parent islands for several hundred kilometers, are associated with vigorous boundary layer circulations driven by differential surface heating. Under suitable environmental conditions, they may ascend deep into the troposphere and generate heavy precipitation. In this study, a combination of satellite imagery and high-resolution numerical simulations is used to study the physical characteristics, underlying mechanisms, and sensitivities of these clouds. A year of high-resolution satellite images of the Lesser Antilles is used to document the regional frequency of the bands and the environmental conditions supporting them. Quasi-idealized, cloud-resolving simulations indicate that the clouds are a manifestation of thermally forced subcloud circulations in the island wake, with a vigorous central updraft core that effectively initiates cumulus convection. Sensitivity tests varying the island topography (area, surface fluxes, and terrain height) and environmental conditions (wind velocity and stability profiles) reveal a delicate linear organization that can be strongly disrupted by a variety of physical processes. The vertical velocities within the updraft core are predicted using a thermodynamic heat-engine scaling, which accurately captures the core strength and sensitivities.