The Impacts of Gravity Waves and Wind Shear on the Lifecycle of Cirrus Clouds in the Tropical Tropopause Layer

In this study, we use two-dimensional simulations to investigate the impacts of wind shear and gravity wave temperature fluctuations on the lifecycle of thin cirrus clouds in the tropical tropopause layer (TTL). Initial conditions for the simulations are based on a case study from high-altitude aircraft observations of a narrow layer of abundant small ice crystals just after a homogeneous freezing ice nucleation event. Analysis of high-resolution radiosonde observations shows that the TTL is a region where strong wind shear (often 10–$20\times 10^{-3}$ $s^{-1}$) prevails. We find that the strong TTL wind shear alters the structure of TTL cirrus, accelerates the reduction in ice concentration, and shortens the cloud lifetimes. The typically thin laminar structure of the clouds indicated by lidar observations is likely a direct result of the strong wind shear in the TTL. Gravity-wave temperature fluctuations further accelerate the reduction in ice concentration as the clouds evolve. The simulations show that TTL cirrus lifetimes decrease with decreasing initial ice concentration, decreasing initial supersaturation and increasing turbulent diffusivity. Based on simulations with observed TTL wind shear, wave properties, and initial cloud properties, we show that TTL cirrus lifetimes are typically less than 1 day. Clouds produced by heterogeneous nucleation, with lower initial ice concentration and supersaturation, will have even shorter lifetimes. Persistence of TTL cirrus for multiple days would require sustained upward vertical wind with an amplitude of at least $\simeq$0.2 cm $s^{-1}$.