A Lagrangian Trajectory Analysis of Azimuthally Asymmetric Equivalent Potential Temperature in the Outer Core of Sheared Tropical Cyclones

In this study, the characteristics of azimuthally asymmetric equivalent potential temperature ( θ e ) distributions in the outer core of tropical cyclones (TCs) encountering weak and strong vertical wind shear are examined using a Lagrangian trajectory method. Evaporatively forced downdrafts in the outer rainbands can transport low-entropy air downward, resulting in the lowest θ e in the downshear-left boundary layer. Quantitative estimations of θ e recovery indicate that air parcels, especially those originating from the downshear-left outer core, can gradually revive from a low entropy state through surface enthalpy fluxes as the parcels move cyclonically. As a result, the maximum θ e is observed in the downshear-right quadrant of a highly sheared TC. The trajectory analyses also indicate that parcels that move upward in the outer rainbands and those that travel through the inner core due to shear make a dominant contribution to the midlevel enhancement of θ e in the downshear-left outer core. In particular, the former plays a leading role in such θ e enhancements, while the latter plays a secondary role. As a result, moist potential stability occurs in the middle-to-lower troposphere in the downshear-left outer core.