Turbulent fractions in the Tropical Tropopause Layer using STRATEOLE-2 long-duration balloon measurements

The Tropical Tropopause Layer (TTL) is a gateway for momentum fluxes and atmospheric components to the global stratosphere. The dynamical processes in the TTL remain challenging to characterize, particularly at turbulent length scales, and are still poorly understood. Current estimates of turbulence frequency and intensity vary considerably between observations in this region, although they are crucial for designing efficient and accurate model parameterizations.

 

Quasi-Lagrangian in situ measurements of thermodynamic parameters and GPS are obtained from STRATEOLE-2 long-duration balloons drifting at isopycnal level around 20 km altitude during several months over the equator. The balloons' vertical oscillations around their density equilibrium position in a stratified environment allow us to estimate local vertical gradients in temperature, pressure and winds. From these estimates we evaluate Richardson numbers, which enable us to characterize the flow as turbulent or laminar during each flight, and thus to estimate turbulent fractions. 

 

Various methods were tested to evaluate local gradient estimates that can be applied directly to the detection of turbulent episodes. For example, using the envelope defined by the local extrema of the time series, we estimate instantaneous local gradients from the ratio of the variables amplitude to the vertical displacement amplitude. This approach enables the reconstruction of temperature and wind increments time series. By calculating correlations between observed and reconstructed increments, we show that our gradients are quite consistent, especially for shear winds estimates. 

 

We deduce the turbulent fraction from the ratio of the mean lifetime of turbulent episodes to the mean interval between two successive ones. Additionally, we describe the distribution of these estimates.