Generation Mechanisms of Near-Cloud Turbulence Events in the Upper-Level Outflow of Tropical Cyclone Hagibis

Mechanisms of near-cloud turbulence (NCT) occurring at two different altitudes within the upper-level outflow of tropical cyclone (TC) “Hagibis” in the northwestern Pacific Ocean on 11 October 2019 are investigated using observations and high-resolution numerical simulations. Two NCT regions located several hundred kilometers from the TC center are the focus of this study. These include the 1) top half of cirrus anvil cloud near z 5 14 km at 0330 UTC and 2) the anticyclonic outflow at a lower altitude within the TC anvil at z 5 11 km later at 0850 UTC. In region 1, the simulated NCT occurred within banded cirrus, which was observed by a geostationary satellite, was organized by horizontal convective rolls resulting from differential thermal advection and enhanced vertical shear within the radial outflow of the TC. In region 2, simulated NCT collocated with observed light-to-moderate turbulence occurred in the radial outflow in the northwestern quadrant of the northward-moving TC that was located along the anticyclonic shear side of the midlatitude jet. Here, strong radial accelerations within the TC outflow toward areas of inertial instability generated the Kelvin–Helmholtz billows, which resulted in localized convective instability with 1-km deep convective overturning in the half bottom of anvil cloud. Simulated turbulence intensities represented by the eddy dissipation rate (EDR) derived from both subgrid-scale turbulent kinetic energy (TKE) and resolved TKE from model domains with subkilometer horizontal grid spacing were in agreement with observed EDR from the aircraft data.