A Numerical Simulation of Convectively Induced Turbulence above Deep Convection

At 1034 UTC 2 September 2007, a commercial aircraft flying from Jeju, South Korea, to Osaka, Japan, at an altitude of approximately 11.2 km encountered severe turbulence above deep convection. To investigate the characteristics and generation mechanism of this event, the real atmosphere is simulated using the Weather Research and Forecasting model with six nested domains, the finest of which is a horizontal grid spacing of 120 m. The model reproduces well the observed large-scale flows and the location and timing of the turbulence along the evolving deep convection. Three hours before the incident, isolated deep convection with two overshooting tops develops in a warm area ahead of the cold front in the southwestern region of the turbulence. As the deep convection moves with the dominant southwesterly flow toward the incident region, its thickness shrinks significantly because of weakening of upward motions inside the convection. Twenty minutes before the incident, the dissipating convection disturbs the southwesterly flow at the incident altitude, enhancing local vertical wind shear above the dissipating convection. The leading edge of the cloud stretches toward the lee side because of shear-inducedyvorticity, finally overturning. This activates turbulence and vertical mixing at the cloud boundary through convective instability in the entrainment process. While the dissipating convection, its thickness still shrinking, continues to move toward the observed turbulence region, the turbulence generated at the cloud interface is advected by the dominant southwesterly flow, emerging about 1–2 km above the dissipating convection and intersecting the aircraft’s flight route at the incident time.