Characteristics and structure of an upper air cold vortex in the subtropics of South America

Summary The horizontal and vertical structures of an upper air cold core vortex over the subtropical latitudes of South America in the winter of 1999 reveal many features that are different from tropical upper tropospheric cold core vortices. The vortex in the present study is observed poleward of the subtropical jetstreak. In the middle troposphere, the center of the vortex is cooler than the periphery by 6 °C. The relative vorticity is greatest just below the tropopause and the vortex presents a slight eastward tilt in the vertical. The CPTEC Eta regional model simulation dataset has been used to study the life cycle of the vortex. Intensification and movement of the system are well simulated by the combined effect of divergence and vorticity advection at 500 hPa. The Lifted Index and CAPE values at the mature stage of the system are not sufficiently high to explain the precipitation associated with the vortex. However, Q-vector analysis of the model simulation datasets support the observed precipitation. The precipitation pattern closely follows the 700 hPa vertical velocity. These results indicate that the rain associated with the vortex has been forced by dynamical lifting rather than by thermodynamic processes. Upper tropospheric stable conditions are responsible for the shallowness of convection. The vortex presented here is different from subtropical cyclones, which develop over oceans and have higher intensities. The present vortex development somewhat resembles lee development.