Abstract/Summary

This study examines in detail the three dimensional structure and evolution of the 200 most intense Cut off Lows (COLs) in the subtropical Southern Hemisphere. This is done using feature tracking and cyclone centred compositing based on the ERA Interim reanalysis. For the upper level features, composites confirm the existence of a well defined tropospheric cold core colocated with warmer air in the lower stratosphere. Such cores are surrounded by regions of strong temperature gradients (frontal zones) which move downstream throughout the life cycle. The stratospheric air intrusion into the troposphere is identified in the vertical cross sections of potential vorticity and ozone, a process referred to as tropopause folding. Precipitation occurs ahead of the COLs because of the low (high) level convergence (divergence) and strong upward motion. The occurrence of maximum precipitation is observed in the decay stage of the COLs, indicating a possible link between COLs and surface cyclones. The implications for precipitation are quantitatively analysed by producing composites conditioned on vorticity and precipitable water suggesting these variables may be related to precipitation. The study finds that moisture is an important factor controlling the precipitation area, while the intensity of COLs seems to influence the magnitude of the precipitation. The COLs also exhibit a westward tilt during their early stages but they change to a barotropic state in the mature stage. Finally, an objective classification of COLs is proposed indicating there are significant differences in the intensity and vertical structure of COLs with consequences for precipitation. These efforts aim to provide new insights into the development of COLs which could aid in identifying and forecasting the various types of COLs and their associated precipitation patterns which may be useful.