Secondary Circulation of Tropical Cyclones in Vertical Wind Shear: Lagrangian Diagnostic and Pathways of Environmental Interaction

This study introduces a Lagrangian diagnostic of the secondary circulation of tropical cyclones (TCs), here defined by those trajectories that contribute to latent heat release in the region of high inertial stability of the TC core. This definition accounts for prominent asymmetries and transient flow features. Trajectories are mapped from the three-dimensional physical space to the (two dimensional) entropy-temperature space. The mass flux vector in this space subsumes the thermodynamic characteristics of the secondary circulation. The Lagrangian diagnostic is then employed to further analyze the impact of vertical wind shear on TCs in previously published idealized numerical experiments. One focus of this analysis is the classification and quantitative depiction of different pathways of environmental interaction based on thermodynamic properties of trajectories at initial and end times. Confirming results from previous work, vertical shear significantly increases the intrusion of low-equivalent potential temperature ( [GRAPHICS] ) air into the eyewall through the frictional inflow layer. In contrast to previous ideas, vertical shear decreases midlevel ventilation in these experiments. Consequently, the difference in eyewall [GRAPHICS] between the no-shear and shear experiments is largest at low levels. Vertical shear, however, significantly increases detrainment from the eyewall and modifies the thermodynamic signature of the outflow layer. Finally, vertical shear promotes the occurrence of a novel class of trajectories that has not been described previously. These trajectories lose entropy at cold temperatures by detraining from the outflow layer and subsequently warm by 10-15 K. Further work is needed to investigate in more detail the relative importance of the different pathways for TC intensity change and to extend this study to real atmospheric TCs.