Mechanisms responsible for structural changes during the extratropical transition of Typhoon Sinlaku (2008): a model study

Extratropical transition (ET) of tropical cyclones (TCs) has been investigated in several studies. General structural changes and processes involved in the evolution are known. Still, a lot of uncertainty can be found while forecasting ET.

During the T-PARC field campaign (autumn, 2008) one objective was observing ET in order to better understand its mechanisms, which was achieved with Typhoon Sinlaku. Other than during characteristic ETs, Sinlaku re-intensified as a tropical cyclone in the course of the first ET phase before continuing its ET. Measurements could be performed during the entire evolution and in different regions of the storm, so that an extensive dataset is available. Sinlaku is therefore an interesting case to get a better understanding of the development of ET and its influence on the midlatitude flow.

This study investigates the mechanisms that cause structural changes during Sinlaku's ET. To do so, a reasonable representation of the storm is needed. Piecewise PV inversion, in combination with ECMWF re-analysis data, dropsonde and airborne radar data, is used to create a most realistic analysis that captures the intensity of the typhoon. This analysis is used to initialize the non-hydrostatic regional forecast model COSMO (COnsortium for Small-scale MOdeling). Sinlaku is simulated for a period of two days on a 2.8 km horizontal grid and compared to airborne lidar data. The evolution of ET is evaluated from convective to synoptic scale and the relative influence of the baroclinic zone and of the jet stream on the ET process is analyzed. Furthermore, piecewise PV inversion will be used to produce a simulation run without storm and simulations with a relocated storm to investigate the influence of the TC on the midlatitude flow.