Clear Air Turbulence event during HALO flight: a case study

PAOLA RODRIGUEZ IMAZIO; NICOLAS RIBAVEN; ALEJANDRO GODOY; ANDREAS DORNBRACK

Workshop; Workshop on Antarctic Meteorology and Climate (WAMC); 2021

A synoptic review of a Clear-Air Turbulence (CAT) event occurred during HALO aircraft flight on12 November 2019 was performed using ERA5 reanalysis (ECMWF), and GFS (NCEP) and PWRFforecast models. Results were compared with in situ measurements obtained from a radiosondelaunched from Marambio base in Antarctica at 17:45 UTC, and measurements from the Basic HALOMeasurement and Sensor System (BAHAMAS). Synoptic analysis reveal a strong and well-definedpolar low located at West of the Antarctic Peninsula over the Bellingshausen Sea in the morningof 12 November. At the north of the polar low, a small and deep perturbation at 300hPa levelmoved quickly towards the north-west of the Antarctic peninsula at 18 UTC, favouring a cold airadvection at low levels over Bellingshausen Sea and a stratospheric air intrusion between 450hPaand 300 hPa levels near 62◦S 􀀀 60◦W. These conditions favoured a strong upper level frontogenesis,associated with a tropopause folding in that region. Additionally, a strong jet streak greaterthan 100knots at 300hPa was observed at 60◦S, with horizontal and vertical wind shear. Strongtemperature gradients and high Thermal Front Parameter (TFP) were observed in the NWP modelat 200 􀀀 350hPa. Ellrod Turbulent Index (1 and 2) and Brown Index indicate moderate to severeturbulence conditions between 300hPa and 250hPa. Radiosondes data from Marambio stationshow a sharp temperature inversion with an amplitude of 4K between 250hPa and 200hPa(nearly from 9800m to 11000m), for times close to the critical occurrence of the synoptic pattern.Vertical pattern of the Richardson number shows values between 0:25 and 10 above the identifiedtropopause folding, indicating buoyancy driven turbulence. These conditions are reproduced inBAHAMAS measurements, where large peak to peak fluctuations can be observed in all meteorologicalparameters around 18 UTC. Horizontal and vertical energy spectra show a well definedenergy cascade towards small scales, with Kolmogorov scaling. Structure and autocorrelation functionsfor the wind speed were also calculated, supporting the evidence of turbulent atmosphericconditions, with signatures of anisotropy generated by stratification. Scales involved are found tobe between the buoyancy length scale LB 1300m and the Ozmidov scale LO 140m. Turbulentparameters TKE and EDR indicate a localized event of moderate to severe turbulence around62◦S 􀀀 58◦W, with the aircraft flying nearly 2000m above the tropopause folding.