On the 3D Structure and Formation of UTLS Jetlets Associated with Potential Vorticity Dipoles in Tropical Cyclones

This study explores the 3D structure and formation of mesoscale jets associated with potential vorticity (PV) dipoles in the upper troposphere and lower stratosphere (UTLS) near deep convective clusters in tropical cyclones (TCs). Simulations of Hurricane Ita (2014), TC Talas (2011) and Hurricane Edouard (2014) with the University of Wisconsin Non-Hydrostatic Modeling System (UWNMS) were investigated. Deep convective clusters embedded in TCs commonly exhibit PV dipoles in the UTLS, with a horizontal “jetlet” between each dipole. In each case, a vertically-continuous horizontal wind speed maximum is evident in the updraft, extending from the boundary layer into the UTLS. During extratropical transition, jetlets often extend poleward and merge with the subtropical westerly jet.

Theory of the interaction between cumuli and their environment is applied to the formation of jetlets, their confinement to the UTLS, momentum exchange, and tilt in shear. In these cases, the cyclonic azimuthal velocity within the updraft exceeds that of the surrounding air during ascent through the troposphere, due to thermodynamic acceleration. This horizontal momentum maximum is transported upward in the updraft and ejected downstream near the tropopause, forming a vorticity dipole in the UTLS. Since the momentum surges are dynamically active, local wind profiles inside and outside of an updraft are needed to properly represent drag and mixing, or “cumulus friction”. A jetlet/PV dipole is expected to form whenever an updraft carries horizontal momentum from the troposphere into the base of the stratosphere, where ambient winds are significantly slower.