Measurement Study of Turbulence in a Tropopause Fold

Tropopause folds are known as regions of intense trace gas exchange between the troposphere and the stratosphere. They occur in upper-level fronts and it is known since the 1970s that turbulence plays a major role in their formation. However, only a limited number of turbulence measurements under these conditions exist. In this study, we present a turbulence sounding in an upper-level front measured with the balloon-borne instrument LITOS (Leibniz-Institute Turbulence Observations in the Stratosphere). This instrument infers turbulent kinetic energy dissipation rates from velocity fluctuations at the Taylor microscale. By using a radiosonde on board of the same balloon, we can observe wind fluctuations across multiple spatial scales.

In the classical picture of a tropopause fold from the 1970s, we expect turbulence to occur in both shear zones above and below the tropopause jet. For the time of our measurement on 06 August 2016, a similar turbulence distribution is expected due to low Richardson numbers in the respective areas shown by the ECMWF-IFS. Our in-situ turbulence measurement with LITOS, however, shows a different picture: we find turbulence to occur in the upper shear zone above the jet but not in the lower one located in the stratospheric intrusion. In our contribution, we will examine potential reasons for this difference between theoretical expectations and the observation. Furthermore, we will discuss possible implications of the lack of turbulence in the stratospheric intrusion on the exchange of trace gases across the tropopause.