An Observational Analysis of Tropical Cyclone Recurvature

Abstract

Data for 21 years (1957?77) of North Pacific rawinsondes were examined to investigate the interaction between the synoptic-scale circulation 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 related to tropical cyclone motion prior to, and during, recurvature. For tropical cyclones that recurve, significant changes in the upper-tropospheric zonal wind fields were observed 1?2 days prior to beginning recurvature in the environmental sector northwest of the storm. Cyclones actually began to recurve when positive zonal winds (westerlies) penetrated the middle and upper troposphere to within 6° of the cyclone's center. Tropical cyclones that did not recurve consistently showed negative zonal winds at this radius. From the results of this study, a recurvature forecasting scheme was developed that uses environmental wind field data for the region northwest of the cyclone. This recurvature scheme was tested on 55 tropical cyclones that developed in the northwest Pacific during 1984?86. In general, the movement of these cyclones was found to be fairly well related to the mid- and upper-tropospheric wind fields in areas north, northwest, and west of the cyclone. This recurvature scheme was also applied in real time as part of the Tropical Cyclone Motion Experiment during the summer of 1990 in the northwest Pacific, and was found to show promising results.