The role of vertical shear on aviation turbulence within cirrus bands of a simulated western Pacific cyclone

At 0300 UTC 9 September 2010, commercial aircraft traveling between Tokyo and Hawaii encountered regions of moderate and severe intensity turbulence at about 12-km elevation in or just above banded structures in the cirrus anvil associated with an oceanic cyclone located off the east coast of Japan. The generation mechanisms of the cirrus bands and turbulence are investigated using the Advanced Research Weather Research and Forecasting Model with five nested domains having a finest horizontal grid spacing of 370 m. The simulation reproduces the satellite-observed patterns of cloud brightness, including the bands, and suggests that synoptic-scale vertical shear within the anvil cloud layer and radiative effects, including longwave cooling at cloud top and warming at cloud base, act together to produce banded structures within the southern edge of the cirrus cloud shield. The character of the bands within the nearly neutral or convectively unstable layer of the cirrus shield is similar to boundary layer rolls in that the vertical wind shear vectors are nearly parallel to the cirrus bands. The strong vertical shear aligned with the banded convection leads to flow deformations and mixing near the cloud top, resulting in localized moderate and severe turbulence. The estimated maximum value of the cube root of eddy dissipation rate within the bands is ~0.7 m²/³ s⁻¹, consistent with severe turbulence levels experienced by large aircraft.