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 owing 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. 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 spectra follow a -5/3 power-law scaling. The diffusivity induced by turbulent bursts is estimated to be on the order of 10^-1 m² s^-1 and increases from the top to the bottom of the TTL (from ~2 × 10^-2 to ~3 × 10^-1 m² s^-1). Given the uncertainties involved in the estimate, this is in reasonable agreement (about a factor of 3-4 lower) with the parameterized turbulent diffusivity in ERA-Interim, but it disagrees with other observational estimates from radar and radiosondes. The magnitude of the consequent vertical transport depends on the altitude and the tracer; for the species considered, it is generally smaller than that induced by the mean tropical upwelling.