Wednesday, 13 January 2016
On 13 February 2013, a Korean Airline (Airbus A330) encountered a moderate-intensity turbulence at ~24,000-feet (~7.3 km) near Bohai Sea, China (121.25°E, 38.55°N) during the flight from Incheon, South Korea to Tianjin, China. Two flight attendants got serious injuries due to this case, and this accident is currently under investigation. From 1147 LST, vertical acceleration is perturbed strongly and the variation of the vertical acceleration continued for about 79 second. During this variation, the aircraft experienced two strong variations of 0.89g and 0.72g, which correspond to moderate intensity of turbulence. Based on the Multifunctional Transport Satellite (MTSAT) images, there were thin stratiform clouds, not intense convective activity, around the incident region, and thus the event is considered as a clear-air turbulence (CAT). In this study, we investigate possible mechanisms of this event based on a high-resolution numerical simulation using the Weather Research Forecast (WRF) model version 3.6.1. For the simulation, three domains with different horizontal grid spacings of 12, 4, and 1.3 km are used with the two-way nesting interaction. All domains have 107 vertical layers with a model top of 50 hPa, and high vertical resolution (100 m) is applied to heights between 6 and 9.2 km. In synoptic-scale flow features from the simulation, one of two bifurcated strong jet streams passes through the Bohai Sea. Because the aircraft flew along the northern edge of the jet stream in which strong wind gradient occurred, strong horizontal deformation, strong vertical wind shear, and strong horizontal wind shear co-locate near the incident region. Due to the strong vertical wind shear, the region of small Richardson number, less than 0.5, is also coincide with the region of the turbulence occurred, regardless of relatively strong stability associated with tropopause folding that extended down to 8 km. The strong horizontal wind shear and vertical wind shear satisfy the necessary conditions for the barotropic instability and baroclinic instability in the region of the event encountered. Considering that the barotropic instability is a typical mechanism in transferring kinetic energy from the large-scale flow to small-scale disturbances and that the baroclinic instability is a mechanism in conversion from potential energy to kinetic energy, these two instabilities are responsible for the turbulence in the present case. Although mountain waves with a horizontal wavelength about 9 km in the west of flight passage lead to weak fluctuations before the turbulence encounter, they are not directly related to the encountered turbulence.
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