Clear Air Turbulence Observed Across a Tropopause Fold Over the Drake Passage-A Case Study

An aircraft turbulence encounter over the Drake Passage is investigated by combining unique high-frequency flight level data, vertical profiles of a near-simultaneous radiosonde profile and numerical results from global and regional numerical weather prediction (NWP) models. Meteorological analysis reveals an intense polar low propagating from the Bellinghausen Sea toward the Drake Passage. A small and deep stratospheric intrusion formed a tropopause fold that promoted strong upper-level frontogenesis and enhanced shear and horizontal deformation of the upper tropospheric and lower stratospheric (UTLS) airflow. In this region, the Basic HALO Measurement and Sensor System (BAHAMAS) aboard the HALO research aircraft flying at FL450 detected large peak-to-peak variations in all meteorological parameters. The computed Energy dissipation rate (EDR) values (cubic root of the eddy dissipation rate) indicate moderate to severe turbulence. The location of this turbulence encounter was well-predicted by the clear air turbulence (CAT) indices derived from the NWP results. The enhanced CAT indices emphasize the large shear and horizontal deformation of the airflow as the cause of the turbulence. Horizontal and vertical energy spectra calculated from the 10 Hz BAHAMAS data show a well-defined energy cascade toward small scales with Kolmogorov scaling. Maximum EDR values of about 0.35 derived both from the spectra and structure functions for the wind speed agree quantitatively very well. In addition, the structure functions support the detection of turbulent atmospheric conditions with signatures of flow anisotropy generated by enhanced thermal stratification in the UTLS. The scales involved are between the buoyancy length scale L-B approximate to 1,500 m and the Ozmidov scale L-O approximate to 111 m.