The effect of trough interactions on tropical cyclone intensity change

The objective of this dissertation is to understand how interactions with upper-level troughs affect the intensity change of tropical cyclones (TCs). A trough interaction is defined to occur when eddy momentum flux convergence (EFC) calculated over a 300--600 km radial range is greater than 10 m s-1 d-1 and when a potential vorticity anomaly is within 1000 km of the TC centre at the time of intensity change. This definition is applied to all named Atlantic tropical cyclones during 1985--1996 to create a climatology of interaction cases. To minimize other factors, subcritical sea surface temperature and landfall times are removed.;The resulting climatology is separated into three categories, based on whether the interaction resulted in intensification or weakening of the TC and the distance of the trough from the TC centre at the time of interaction. These cases are used to create the following composites: (1) superposition (upper tropospheric PV anomaly within 400 km of storm centre), (2) distant interaction (PV anomaly between 400 and 1000 km from the centre), and (3) unfavourable interaction (weakening with enhanced EFC). Additional composites created include: (4) extratropical transition (intensification during a trough interaction after transition) and (5/6) favourable/unfavourable no trough (intensification/weakening with EFC less than 5 m s-1 d-1 for at least three consecutive periods, respectively).;Examination of the composites as well as selected case studies presented indicate that the scale of the approaching PV anomaly is important in determining whether an interaction will be favourable. A trough undergoing wave-breaking or a pre-existing small-scale PV anomaly is more likely to result in a favourable interaction via superposition arguments. Larger troughs may also result in intensification if the TC moves into the entrance region of the upper-level jet ahead of the trough and the trough remains far enough away from the TC centre to keep the vertical shear relatively small. Unfavourable interactions often result when an approaching trough is too large in scale and produces strong shear over the TC. The use of water vapour satellite imagery is suggested to be a promising tool in forecasting TC-trough interactions.