Ventilation Pathways and Air Stream Evolution during the Extratropical Transition of Typhoon Malakas (2010)

Tuesday, 19 April 2016
Plaza Grand Ballroom (The Condado Hilton Plaza)
Michael Riemer, Johannes Gutenberg Univ., Mainz, Germany; and T. Kremer, C. Euler, E. Schömer, C. M. Grams, and A. Miltenberger

We will present a Lagrangian description of the evolution of Malakas during its tropical phase of extratropical transition. During this time, Malakas interacted with vertical wind shear, developed significant asymmetries, increased its translation speed, and its circulation started to impinge on the baroclinic zone polewards of the storm. The regional numerical weather model COSMO has been used to simulate the evolution of Malakas. A total of approx. 3 million online trajectories over a time period of two days have been calculated to provide a Lagrangian description of the storm evolution.

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.

- Indicates paper has been withdrawn from meeting
- Submission entered in competition