Study of clear air turbulence (detection, observation, generation) in the Tropical Upper Troposphere-Lower Stratosphere (UTLS)

The role played by turbulent mixing in the vertical transport of constituents in the UTLS is still poorly understood: there is a lack of knowledge of turbulence due to the limited number of observations in this region as well as to the limitations of current observation techniques.

The first part of the present work deals with the detection of small-scale turbulence in the tropical upper troposphere - lower stratosphere from in-situ meteorological measurements collected under super-pressure balloons (SPBs). Eight SPBs were launched during the first Strateole-2 campaign, from November 2019 to March 2020 and flying for several weeks (∼ 3 months). 

Turbulence detection methods relies on the quasi-periodic vertical oscillations (∼ ±15 m) of the SPBs around their equilibrium positions, such oscillations inducing large fluctuations of measured quantities (pressure, temperature, positions) and inferred quantities (density, potential temperature). A first method of detection is based on correlations between the increments of potential temperature δθ and the vertical displacements of the balloons (i.e. of the sensors) δz. Such correlations are expected to be null as ∂θ/∂z → 0 in case of turbulent mixing. A second method relies on the Richardson number criterion, Ri < 0.25. Ri is deduced from the vertical gradients of measured quantities (T , u, v), estimated from covariances between the increments of the considered quantities and the vertical displacements δz. 

Turbulence indexes (true of false) to describe the different states of the flow encountered by the SPBs during their flights (laminar or turbulent), are evaluated. These different indexes, based on independent measurements and on various methods, correlations or linear regressions, are found to be consistent: they differ for less than 3% of the cases. The flow is observed to be turbulent for about 5% of the time, with strong inhomogeneities along the longitude.

The second part of the present work aims to improve our understanding of turbulence, and its impacts, in the tropical UTLS by studying small- to meso-scale processes, i.e. atmospheric waves, deep convection and associated observed turbulence. These are all key processes of the dynamics of the equatorial UTLS. One can evaluate the probability of turbulence occurrences as a function of the distance to deep convection. Such a distance seems to be a good proxy of wave activity generated by deep convection. The occurrence frequency of turbulence is significantly larger when the distance to deep convection is small, i.e. smaller than ~ 200 km.

This research should contribute to a better parametrization of these processes in climate models, and to a better estimation of their impact to vertical transport in the tropical UTLS.