Ventilation and Storm Structure during Extratropical Transition of Tropical Cyclones

Predictability of midlatitude weather systems is frequently compromised by tropical cyclones that undergo extratropical transition (ET) in the far upstream region. This loss in predictability has its origin in uncertainties in the evolution of the ET system itself. These uncertainties ultimately project onto midlatitude Rossby wave packets and may affect a near-hemispheric region. This study focuses on the processes that arguably govern important sources of these uncertainties: inner-core deep convection and the changes of storm structure during extratropical transition. The basic hypothesis here is that the intrusion of environmental, low moist-entropy (Θ_e) air into the ET system’s convection plays a key role in the overall evolution of the storm and its structure. To gain a better insight into this highly nontrivial ventilation processes, we apply a Lagrangian trajectory analysis to identify the main pathways of cold and dry ambient air into the inner-core convection of the storm. We analyze convection-permitting simulations with trajectories, using the operational local-area model of the German weather service (COSMO), for two contrasting ET cases. One case is the ET of Karl (2016). Karl was a tropical storm at the onset of ET that was sampled during a recent field campaign. The other case is the ET of Katia (2011), a major Hurricane at the onset of ET. We identify the main air streams associated with an ET system and differentiate between trajectories that enter the inner-core convection through the boundary layer and trajectories that intrude from above the boundary layer. The storm ventilation is characterized by the evolution of the thermodynamic properties along these air streams. A concomitant analysis of the storm structure in terms of vortex tilt, convective asymmetries and incipient low-level frontogenesis links storm ventilation to structure change. First results of the comparison of both storms show an overall similar evolution of the storm structure and the massflux during ET. Both storms show a loss in vertical extent and a shift towards lower Θ_e values. For Karl, intrusion of low-entropy air is mostly through the boundary layer, whereas for Katia, intrusion occurs more prominently above the boundary layer. Interestingly, at the onset of transition, Karl exhibits lower outflow temperatures then Katia. During ET, however, Karl’s outflow temperature increases significantly while the increase of Katia’s outflow temperature is much less prominent.