Small-Scale Wind Fluctuations in the Tropical Tropopause Layer from Aircraft Measurements: Occurrence, Nature, and Impact on Vertical Mixing

The contribution of turbulent mixing to heat and tracer transport in the Tropical Tropopause Layer (TTL) is poorly constrained, partly due to a lack of direct observations. Here, the authors use high-resolution (20 Hz) airborne measurements to study the occurrence and properties of small-scale (<100 m) wind fluctuations in the TTL (14-19 km) over the tropical Pacific. The fluctuations are highly intermittent and appear localized within shallow (100 m) patches. Furthermore, active turbulent events are more frequent at low altitude, near deep convection and within layers of low gradient Richardson number. A case study emphasizes the link between the turbulent events and the occurrence of inertio-gravity waves having small horizontal or vertical scale. In order to evaluate the impact of the observed fluctuations on tracer mixing, their characteristics are examined. During active events, they are in broad agreement with inertial range turbulence theory: the motions are close to 3D-isotropic and the sp...