AN OBSERVATIONAL ANALYSIS OF TROPICAL CYCLONE RECURVATURE

Data for 21 years ( 1957-77) of North Pacific rawinsondes were,examine d to investigate the interaction between the synoptic-scale circulatio n and tropical cyclones prior to, and during, the recurvature process. This study is believed to be the first to quantitatively examine how the environmental wind fields at all levels of the troposphere are rel ated to tropical cyclone motion prior to, and during, recurvature. For tropical cyclones that recurve, significant changes in the upper-trop ospheric zonal wind fields were observed 1-2 days prior to beginning r ecurvature in the environmental sector northwest of the storm. Cyclone s actually began to recurve when positive zonal winds (westerlies) pen etrated the middle and upper troposphere to within 6-degrees of the cy clone's center. Tropical cyclones that did not recurve consistently sh owed negative zonal winds at this radius. From the results of this stu dy, a recurvature forecasting scheme was developed that uses environme ntal wind field data for the region northwest of the cyclone. This rec urvature scheme was tested on 55 tropical cyclones that developed in t he northwest Pacific during 1984-86. In general, the movement of these cyclones was found to be fairly well related to the mid- and upper-tr opospheric wind fields in areas north, northwest, and west of the cycl one. This recurvature scheme was also applied in real time as part of the Tropical Cyclone Motion Experiment during the summer of 1990 in th e northwest Pacific, and was found to show promising results.