T-PARC: Ensemble Simulation and Clustering of the Evolving Cyclone Structure Through Extratropical Transition (ET)

This research addresses multiple scientific goals of the THe Observing system Research and Predictability EXperiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) including (i) advancing knowledge of the dynamics governing North Pacific Extratropical cyclone Transitions (ET) as the storms move poleward, (ii) improving the interpretation and utility of ensemble forecast systems, and (iii) testing the improvement in local and downstream forecast skill afforded by high-resolution, non-hydrostatic modeling of ET. It is also synergistic with the goals of the THORPEX Integrated Grand Global Ensemble to (i) develop new methods to combine ensembles, and (ii) enhance collaboration on development of ensemble prediction, internationally and between operational centers and universities.

Tropical cyclones undergoing extratropical transition (ET) experience rapid structural changes as they transform from a deep, warm core cyclone to a cold core cyclone. This presents a major forecasting problem and a numerical modeling challenge. It remains an open question whether the hybrid structure of the transitioning cyclone is sufficiently well resolved to capture the complex process of transition, but the PI's assessment of the forecast skill of two global operational models at simulating ET timing and structure evolution finds the deterministic forecasts from these operational models to be lacking. Ensemble prediction systems offer the potential for assessing confidence in numerical guidance during cases of ET by exploring the uncertainties in the initial vortex and environment structures. In this project, European Centre for Medium-Range Weather Forecasting (ECMWF) ensemble forecasts of storm structure are evaluated within the framework of the cyclone phase space (CPS).

This collaborative effort between meteorology and statistics will encompass: (1) development of cyclone structure-based sub-ensembles for track and intensity forecasts of ET; (2) extension of the objective partitioning of storm structure evolution to mixture-based clustering of CPS tracks; and (3) quantification of the ECMWF ensemble forecast spread in terms of initial vortex structure.

Intellectual merit of the research lies in both the creative exploitation of novel statistical and numerical modeling methods to investigate the factors forcing cyclone structure evolution through the ET of initially tropical cyclones. Broader impacts of this research include the application of novel statistics techniques to synoptic meteorology and testing the effectiveness of the Weather Research and Forecast Model configuration for simulating ET. Societal impacts include the potential for improvements in local severe weather forecasts associated with ET, as these are critically dependent on the cyclone structure.