Case Analysis of Generation Mechanism for a Clear-Air Turbulence (CAT) Encounter near Tokyo on 30 October 2018

Generation mechanism of Clear-Air Turbulence (CAT) occurred near the Narita airport, Japan, on 30 October 2018 is analyzed by using aircraft observation data and the European Centre for Medium-range Weather and Forecast (ECMWF)’s Re-Analysis data version 5 (ERA5). Turbulence was observed at altitudes of 14,098 ft (z = 4.3 km) when the aircraft is approaching the Narita airport at 1025 UTC with a displacement of vertical acceleration (g = 9.8 m/s²) more than 1 g per second. Calculations of Turbulence Kinetic Energy (TKE) and EDR (cube root of the eddy dissipation rate; m^2/3/s²) showed that their peak values are 46 m²/s² and 0.57 m^2/3/s², respectively at that time, which corresponds to the moderate turbulence criteria (0.4 m^2/3/s²) of the World Meteorological Organization /International Civil Aviation Organization (WMO/ICAO).

The analysis of the ERA5 data indicated the enhancement of the jet stream in a vicinity of the accident area with a confluence of the polar jet and subtropical jet streams at 300 hPa. In addition, the cold advection and the negative vorticity advection around the accident area generate the descending motion in the upper middle and upper troposphere. The descending motion can also be confirmed by the divergence of the Q vector in vicinity of the accident area. This led to the secondary ageostophic flow of the center of the enhanced polar jet stream with deep tropopause folding down to about 600 hPa. Therefore, vertical wind shear and the turbulence index related to frontogenesis were found to have two to three times higher than the surrounding areas at the accident area. The results suggest that CAT near Narita Airport was occurred by development of the upper-level front with strengthened jet stream and secondary ageostrophic flow responsible for deep tropopause folding, which is suggested as a mechanism of this CAT encounter near the Narita airport.