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

PAOLA RODRIGUEZ IMAZIO; ANDREAS DORNBRACK; RODRIGO DELGADO URZUA; NICOLAS RIBAVEN; ALEJANDRO GODOY

Workshop; SouthTRAC GW WORKSHOP; 2021

Deutsches Zentrum für Luft- und Raumfahrt (DLR)

An aircraft turbulence encounter over the Drake Passage is investigated by combining uniquehigh-frequency flight level data, vertical profiles of a near-simultaneous radiosonde profile and numericalresults from global and regional numerical weather prediction (NWP) models. Meteorological analysis revealsan intense polar low propagating from the Bellinghausen Sea toward the Drake Passage. A small and deepstratospheric intrusion formed a tropopause fold that promoted strong upper-level frontogenesis and enhancedshear and horizontal deformation of the upper tropospheric and lower stratospheric (UTLS) airflow. In thisregion, the Basic HALO Measurement and Sensor System (BAHAMAS) aboard the HALO research aircraftflying at FL450 detected large peak-to-peak variations in all meteorological parameters. The computed Energydissipation 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 derivedfrom the NWP results. The enhanced CAT indices emphasize the large shear and horizontal deformation ofthe airflow as the cause of the turbulence. Horizontal and vertical energy spectra calculated from the 10 HzBAHAMAS data show a well-defined energy cascade toward small scales with Kolmogorov scaling. MaximumEDR values of about 0.35 derived both from the spectra and structure functions for the wind speed agreequantitatively very well. In addition, the structure functions support the detection of turbulent atmosphericconditions with signatures of flow anisotropy generated by enhanced thermal stratification in the UTLS. Thescales involved are between the buoyancy length scale LB ≈ 1,500 m and the Ozmidov scale LO ≈ 111 m.