Lagrangian Diagnostic of Ventilation Pathways: Case Study of an Idealized Tropical Cyclones in Vertical Wind Shear

The presentation will discuss a novel 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(θe)-temperature space. The mass flux vector in this space succinctly 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 will be on the classification and quantitative depiction of different pathways of environmental interaction, so-called ventilation, based on thermodynamic properties of trajectories at initial and end time. Confirming results from previous work, vertical shear significantly increases the intrusion of low-θe air into the eyewall through the frictional inflow layer. In contrast to previous ideas, vertical shear decreases mid-level ventilation in our (specific) experiments. Consequently, the difference in eyewall θe 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.