Monitoring Tropical-Cyclone Intensity Using Environmental Wind Fields Derived from Short-Interval Satellite Images

Abstract

Rapid-scan visible images from the Visible Infrared Spin Scan Radiometer (VISSR) sensor on board SMS-2 and GOES-1 have been used to derive high-resolution upper and lower tropospheric environmental wind fields around three western Atlantic tropical cyclones (Caroline, August 1975; Anita, August and September 1977; and Ella, September 1978). These wind fields were used to derive the local change of the net relative angular momentum (RAM), upper and lower tropospheric areal mean relative vorticity and their difference, and the storm's transverse circulation. The local change of the storm's net RAM was investigated for the purpose of predicting future storm intensification while the areal mean relative vorticity and transverse circulation were investigated to better understand how the storm's environmental circulation was affecting its intensification. The three cases studied suggested that storm intensification could be predicted from the analyses of the storm's local change of net RAM and that there is an ?6 h lag between the local change in the net RAM and storm intensification. In addition, it was found that changes in the local change of net RAM wore being affected primarily by the net horizontal transport of relative angular momentum caused either by the convergence of cyclonic vorticity in the lower troposphere or by the divergence of anticyclonic vorticity in the upper troposphere. For the three cases studied, the upper tropospheric environmental circulation helped to influence the local change of net RAM and, therefore, changes of storm intensity by hindering or enhancing the storm's outflow and by weakening or strengthening the environmental anticyclonic circulation.