Clear Air Turbulence event during HALO flight: a case study

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

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 2022

0148-0227

An aircraft turbulence encounter over the Drake Passage is investigated bycombining unique high-frequency fight level data, vertical proles of a nearsimultaneousradiosonde probe and numerical results from global and regionalnumerical weather prediction (NWP) models. Meteorological analysis revealsan intense polar low propagating from the Bellinghausen Sea toward the DrakePassage. A small and deep stratospheric intrusion formed a tropopause foldthat promoted strong upper-level frontogenesis and enhanced shear andhorizontal deformation of the upper tropospheric and lower stratospheric(UTLS) airow. Generation of mountain waves due to the gradually changingwind direction in the lower troposphere associated with the approaching lowpressure system is observed in the NPM. At higher levels, the phase lines ofthe mountain waves become stretched and elongated in the direction of thesouthwesterly airflow and perturb the airflow near the position of theturbulence encounter. In this region, the Basic HALO Measurement and SensorSystem (BAHAMAS) aboard the HALO research aircraft system at FL450 detectedlarge peak-to-peak variations in all meteorological parameters. The computedEDR values (cubic root of the eddy dissipation rate) indicate moderate tosevere turbulence. The location of this turbulence encounter was wellpredictedby the CAT indices derived from the NWP results. The enhanced CATindices emphasize the large shear and horizontal deformation of the airow asthe cause of the turbulence. Horizontal and vertical energy spectracalculated from the 10Hz BAHAMAS data show a well-dened energy cascadetowards small scales with Kolmogorov scaling. Maximum EDR values of about0.5 derived both from the spectra and structure functions for the wind speedagree quantitatively very well. In addition, the structure functions supportthe detection of turbulent atmospheric conditions with signatures of owanisotropy generated by enhanced thermal stratication in the UTLS. Thescales involved are between the buoyancy length scale LB~1500m and theOzmidov scale LO~111m.