The evolution of ventilation pathways, i.e. distinct pathways how environmental air with low moist entropy enters the inner-core convection, will be discussed. Trajectories contributing to inner-core convection are identified as the subset of trajectories that exhibit diabatic heating with large values of helicity at some point during their life time. Following a recently-derived diagnostic, these trajectories are mapped from the three-dimensional, physical space to the (two-dimensional) entropy-temperature space. The mass flux vector in this space succinctly subsumes the thermodynamic evolution of the trajectories. In thermodynamic space, a hierarchal cluster algorithm can be used to identify distinct thermodynamic pathways. The evolution of these clusters indicates a transition in the characteristics of the ventilation pathways: from low-level ventilation to more prominent upper-level ventilation as extratropical transition progresses.
To provide a more complete description of the general air stream characteristics in the storm's vicinity, we compress the complete trajectory information to a small number of key parameters. Our preliminary set of parameters comprises maxima in helicity and theta_e and total change of pressure and of theta. Clustering in this reduced phase space provides good indication of expected sets of trajectories: inner-core ascent, environmental air passively circling around the storm center, and dry-slot and warm conveyor belt trajectories developing as extratropical transition progresses. We conclude with a discussion of the sensitivities and general difficulties of using an automated approach to identify distinct air streams.
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