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A4 - Evolution and predictability of storm structure during extratropical transition of tropical cyclones

Principal Investigators: Dr. Michael Riemer, Prof. Dr. Elmar Schömer

Other researchers: Christian Euler (PhD), Tobias Kremer (PhD), Isabel Urbich (Master)

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 trains and may affect a near-hemispheric region. This project will investigate a specific part of this upscale growth, namely how uncertainties in the deep convection of the ET system amplify and project onto structural changes of the storm system, including the upstream trough. A basic understanding of the highly-nonlinear processes that govern this upscale growth is lacking.

The central hypothesis of this project is that systematic intrusion of environmental low-entropy air into the ET system’s convection, dubbed ''ventilation'' in the tropical-cyclone community, plays a key role in the overall evolution. The ET system’s convection has been shown to be crucial in a) maintaining the ET system against the detrimental impact of vertical shear of the environmental winds and b) modifying the upstream trough, potentially alleviating the vertical shear on the system. Arguably, an important feedback loop exists between ventilation, reduced convection within the ET system, less modification of the upstream trough and associated vertical shear, and further enhanced ventilation. A detailed analysis of ventilation, and its relation to the structural evolution of the ET system and the modification of the upstream trough is thus at the heart of this project.

We will perform convection-permitting COSMO simulations with online trajectories for a small number of ET cases that span the broad spectrum of possible ET scenarios. An accurate ventilation diagnostic will be developed based on the identification of coherent bundles within the extensive set of trajectories. Special attention will be given to the identification of distinct ventilation pathways. Several such pathways have been hypothesized in the literature. However, a truly Lagrangian description of these pathways, let alone an understanding of what processes govern different pathways, is lacking. A less data demanding, yet approximate ventilation diagnostic based on Lagrangian coherent structures will be developed and compared to the trajectory bundles. Eulerian diagnostic tools developed during PANDOWAE and before will be adopted and applied to describe the ET system's structural evolution and the modification of the upstream trough. A synopsis of the results from our Lagrangian and Eulerian diagnostics will yield first important advances in our understanding of the complex dynamics that govern the growth of uncertainty in ET systems.

Finally, ensemble COSMO simulations will be designed, performed and evaluated. The ensemble will include initial condition uncertainties pertaining to ventilation, as well as uncertainties associated with unresolved processes within the boundary layer and the interaction with the ocean surface. The evolution of ensemble spread will be used to diagnose the growth of uncertainty in more detail. Made possible by collaborations within W2W, we will make first steps into an important, but largely unexplored problem: the relation between the key kinematic information contained in the Lagrangian coherent structures and the dynamics of the underlying time-dependent flow. Future work will continue this exploration and will extend the analysis of the upscale growth in the ensemble framework.