The dynamics of cut-off lows and their vertical extension to the surface

Abstract

Cut-off low (COL) pressure systems are critical weather systems which can result in resulting various severe weather events on South Africa. COLs are often associated with surface cyclones which can enhance weather-related impacts on affected communities. This is shown by a case study of Cape Storm, a COL extension associated with explosive cyclogenesis that had dire effects on the Western Cape of South Africa. The stratospheric effects that determine a COLs extension to the surface and eventual impact on South Africa are studied from a potential vorticity (PV) perspective in terms of a case study, a climatology and an idealised framework. Deep COLs (which extend to the surface) are most frequent in the autumn months, are longer lasting, are more mobile and found most frequently in the higher latitudes. Stratospheric, high-PV intrusions that are associated with COLs are studied in detail with respect to a COLs extension. PV intrusions which extend closer to the surface more frequently result in surface cyclogenesis. This is independent on the size (or intensity) of the intrusion as PV intrusions which extend from higher tropopause heights tend to result in shallow COL development. COLs, have in the past, been viewed as upper-level systems that systematically extend to the surface. Composite analysis however shows that deep COLs associated with deep PV intrusions result in the simultaneous amplification of troughs throughout the troposphere, with the surface cyclone developing within a day of the COL.