7 4 Servicio Meteorológico Nacional, Buenos Aires, Argentina 8 Key Points: 9 • moderate to severe clear air turbulence over the Drake Passage is analyzed from 10 unique 10 Hz flight level in situ observations during a research flight as part of 11 the SOUTHTRAC campaign in 2019 12 • the main source of the turbulence is the enhanced flow deformation and shear 13 in the vicinity of a tropopause fold generated by an eastward propagating polar 14 low 15 • both spectral as well as structure function methods are applied to quantify the 16 eddy-dissipation rates, the intermittency, and the flow anisotropy during the 17 CAT event 18 Corresponding author: Paola Rodriguez Imazio, [email protected] 19 An aircraft turbulence encounter over the Drake Passage is investigated by com-20 bining unique high-frequency flight level data, vertical profiles of a near-simultaneous 21 radiosonde profile and numerical results from global and regional numerical weather 22 prediction (NWP) models. Meteorological analysis reveals an intense polar low prop-23 agating from the Bellinghausen Sea toward the Drake Passage. A small and deep 24 stratospheric intrusion formed a tropopause fold that promoted strong upper-level fron-25 togenesis and enhanced shear and horizontal deformation of the upper tropospheric 26 and lower stratospheric (UTLS) airflow. In this region, the Basic HALO Measure-27 ment and Sensor System (BAHAMAS) aboard the HALO research aircraft flying at 28 FL450 detected large peak-to-peak variations in all meteorological parameters. The 29 computed EDR values (cubic root of the eddy dissipation rate) indicate moderate to 30 severe turbulence. The location of this turbulence encounter was well-predicted by the 31 CAT indices derived from the NWP results. The enhanced CAT indices emphasize 32 the large shear and horizontal deformation of the airflow as the cause of the turbu-33 lence. Horizontal and vertical energy spectra calculated from the 10 Hz BAHAMAS 34 data show a well-defined energy cascade towards small scales with Kolmogorov scal-35 ing. Maximum EDR values of about 0.5 derived both from the spectra and structure 36 functions for the wind speed agree quantitatively very well. In addition, the structure 37 functions support the detection of turbulent atmospheric conditions with signatures of 38 flow anisotropy generated by enhanced thermal stratification in the UTLS. The scales 39 involved are between the buoyancy length scale L B ≈ 1500 m and the Ozmidov scale 40 L O ≈ 111 m. 41 Plain Language Summary 42 This case study documents an aircraft encounter with clear-air turbulence (CAT) 43 that occurred over the remote Drake Passage. Exceptional atmospheric conditions are 44 known to exist there as low pressure systems regularly pass through the passage. Dur-45 ing a research flight conducted as part of the SOUTHTRAC campaign in November 46 2019, moderate to severe turbulence was detected using flight-level observations of 47 wind and temperature with a frequency of 10 Hz. The vertical distribution of atmo-48 spheric variables is provided by a high-resolution radiosonde profile launched from the 49 Argentinian research station Marambio. In addition, the results of global and regional 50 numerical weather prediction models are used to provide the meteorological context 51 and to derive indices commonly used to predict CAT for aviation. Two different anal-52 ysis techniques are used to quantify the eddy dissipation rates associated with the 53 CAT event. Both methods show that the event exhibits the characteristic feature of 54 the bursty and intermittent nature of turbulence in the upper troposphere and lower 55 stratosphere. Flow anisotropy is large outside the mixing regions of the CAT patch. 56