Momentum transports associated with tropical cyclone recurvature
Student thesis: Master's Thesis
This study attempts to investigate the linear momentum budget responsible for tropical cyclone (TC) recurvature. Typhoon Flo, a typical right-turning TC that occurred during the Tropical Cyclone Motion Experiment (TCM-90), was chosen as a preliminary case study. Results of the single case were then verified by compositing similar recurving TCs over western North Pacific for the years 1991-1995 using the analyses from the UK Meteorological Office (UKMO) global model. Much consistency exists between the single case and the composite studies, except for a small difference at the upper troposphere. Both studies identify an eastward-retreating subtropical ridge (STR) and an approaching westerly trough as the main features during the recurvature. While the southeasterly flow associated with the STR changed to southerly in the northeast quadrant of the TC in the mid to upper troposphere, the southwesterly flow associated with the trough penetrated into the northwest quadrant of the TC especially in the upper troposphere. These changes of the environmental flow throughout the recurvature altered the linear momentum of the TC from southeasterly to southwesterly. A positive absolute zonal momentum tendency was also found. To understand the physical processes responsible for the linear momentum changes, individual absolute momentum tendency terms were calculated. For the absolute zonal momentum tendency terms, it is found that prior to recurvature, the southeasterly flow of the STR led to an increase in the zonal momentum through the earth momentum advection and the zonal component of the Coriolis force. During and after recurvature, the strong southerly component from the environment enhanced the earth momentum advection and the zonal component of the Coriolis force. Even though a certain amount of zonal momentum was exported through the meridional advection by the southerly flow, the net zonal momentum tendency was still positive. For the meridional tendency component, the net positive meridional force appeared to be the main contributor. The negative meridional momentum advection term during and after recurvature only weakened the net meridional momentum tendency at the later stage.
- Tropics, Cyclones, Cyclone forecasting