Impact of radiative heating, wind shear, temperature variability, and microphysical processes on the structure and evolution of thin cirrus in the tropical tropopause layer

Thin cirrus that frequently form in the tropical tropopause layer (TTL) are important for vertical transport through the TTL, regulation of stratospheric humidity, and the Earth's radiation budget. Here, we use a three‐dimensional cloud‐resolving model to investigate the impact of circulations driven by TTL cirrus radiative heating on the cloud evolution. We use observations of TTL environmental conditions (thermal stability and wind shear) and TTL cirrus microphysical properties (ice crystal sizes and concentrations) to constrain the simulations. We show that with ice crystal sizes consistent with available observations (effective radii ≥ 12 μm), typical thermal stability, and moderate wind shear, the ice cloud sediments to lower levels before radiative heating can drive a circulation to maintain the cloud and before small‐scale convection builds up. In this case, the cloud lifetime is controlled by sedimentation of ice crystals into subsaturated air below the initial cloud level, followed by sublimation. Strong wind shear (>10 m s−1 km−1) tends to hasten the cloud dissipation. With relatively weak thermal stability, small‐scale convection builds up rapidly, resulting in mixing at cloud top and extension of the cloud lifetime. We also consider the impact of synoptic‐scale and mesoscale temperature variability on cloud lifetime. Using TTL trajectories with small‐scale wave temperature perturbations superimposed, we show that TTL cirrus will often dissipate within 12–24 h simply as a result of background temperature variability. Key Points TTL cirrus sedimentation limits radiative‐dynamical interactions Wind shear is important for TTL cirrus structure and evolution Under low‐stability conditions, radiation dynamics are important