Clear-Air Turbulence (CAT) Encounters on 13 November 2019 over Central and Eastern China: Numerical Simulation and Generation Mechanism

Abstract

On 13 November 2019, seven commercial aircraft of China Eastern Airlines encountered nine severe-or-greater clear-air turbulence (CAT) events over central and eastern China within 12 hours (0000 to 1200 UTC). These events mainly occurred at altitudes between 6.0 and 6.7 km. A high-resolution nested numerical simulation is carried out using the Weather Research and Forecasting (WRF) model to investigate the generation mechanism of these CAT events, with a horizontal resolution of 1 km over the inner domain. In addition, seven CAT diagnostics with outstanding performances are employed for the mechanism analysis. The WRF model can reasonably reproduce both synoptic-scale systems (Siberian high and upper-level jet stream) and local vertical structures (temperature, dewpoint temperature, and wind field). The simulation indicates that an upper-level front-jet system with a remarkable meridional temperature gradient intensifies over central and eastern China, with the maximum wind speed increasing from 59.0 to 67.3 m s−1. The intensification of the front-jet system induces the tropopause folding, and nine localized CAT events occur in the region with large vertical wind shear (VWS) (1.55×10−2−2.53×10−2 s−1) and small Richardson numbers (Ri) (0.42-0.85) below the cyclonic side of the jet stream. Diagnostic analysis reveals that Kelvin-Helmholtz instability plays an important role in CAT generation, while convective and inertial instability is not directly associated with CAT generation in this study. A typical flight case with continuous CAT events also suggests that large VWS (greater than 1.3×10−2 s−1) accompanied with small Ri (less than 1) favors CAT generation in a front-jet system environment.

A Numerical Study of Clear-Air Turbulence over North China on 6 June 2017

“…Ri was less than 0.25 for event 2 in D2 and D3, and Ri was greater or equal than 1 for event 3 in D1 and D2 (discussed below). This result is inconsistent with the research results of Hu et al [42] (0.25 < Ri < 1 for CAT events), and those of Kim et al [39] and Lee et al [40] (Ri < 0.25 for CAT events). This suggests that 0.25 or 1 cannot be used simply as a criterion for the occurrence of CAT in real cases.…”