A Numerical Study on a Clear-Air Turbulence (CAT) Encounter near Bohai Sea on 13 February 2013

At 0247 UTC 13 February 2013, a Korean commercial aircraft, which was en route from Incheon, South Korea, to Tianjin, China, encountered serious moderate-intensity turbulence at ~24,000-feet (~7.3 km) altitude over about 187 miles southeast of the Tianjin airport. Two flight attendants got serious injuries, and this accident is currently investigated by the Korean government. The turbulence event occurred at 121.25° longitude and 38.55° latitude near Bohai Sea between China and Korea, in which strong upper-jet stream generally flows over the region in winter. Based on the Multifunctional Transport Satellite (MTSAT) images, there were thin stratiform clouds around the region, and the event can be considered as clear-air turbulence (CAT). In this study, we investigate the possible mechanisms of the CAT event based on the numerical simulation of the case using ARW-WRF model. Three domains are considered in the simulation with horizontal grid spacings of 12, 4, and 1.3 km. In synoptic features from the simulation results, strong jet stream with maximum wind speed over 60 m s^-1 bifurcates above the China, and one of two seperated jet streams passes through the Bohai Sea. In the simulation results, the tropopause is folded down deeply in the encounter location. Because stratospheric air comes down to the mid-troposphere through the deep tropopause folding, the stability is relatively strong in the region of the encounter. This tropopause folding region located on the cyclonic shear side (cold air side) and below the jet stream core has strong vertical wind shear due to large vertical gradient of the meridional and zonal velocity. Consequently, because of the strong vertical wind shear, despite the relatively strong stability, the local Richardson number becomes smaller near the tropopause folding region. The more detailed simulation results for the generation mechanism of the CAT related to jet stream will be presented in the conference.