Dynamical processes in the evolution of Cut Off Lows over South Africa

Abstract

This study presented deviations in the evolution of Cut-off lows (COLs) through four COLs events over the South African domain from a case study point of view. Furthermore, this study demonstrated COLs as cold-cored low-pressure systems estranged from westerly wind belts above . COLs are hazardous as they are mostly associated with unbearable precipitations and winds. The main aim of this study is to develop an understanding of dynamical processes during COLs. ERA5 data sets were used throughout the study to achieve objectives. Mean seal level pressure surfaces along with the 250 hPa geopotential heights were used to link the existence of COLs with ridging highs, the results shows that COLs that develop north of the subdivided study region (C and D) are mostly associated with Type-N ridging highs, whereas those that develop south (A and B) are associated with Type-S ridging highs. COLs were identified using 500 hPa geopotential height and the 330K potential vorticity. The results showed that high negative pool of potential vorticity is one of the principal factors during COL genesis. The small-scale jet associated with COLs tends to disappear as the COL dissipates. Some COLs extend to 850 hPa and further towards the surface and while others may not. COLs that extend towards the surface are associated with deeper PV intrusions. Cold cores of COLs are usually found at 500 hPa and uplifting of air during COLs occur further east from its centre which is filled with cold air mass. COLs tend to propagate east as soon as they are formed, but the speed (distance/time) is different from one COL to the next. The mechanism involved in COL development was investigated in detail using the eddy kinetic energy (EKE) budget applied to data from ERA5 reanalysis. This approach considers the most important processes involved such as the Ageostrophic flux convergence (AFC), Baroclinic convergence (BRC) and the Barotropic convergence (BRT). The results showed that the AFC together with the BRC are important for the EKE growth. Furthermore, AFC is responsible for the COL genesis, BRC is necessary for system maintenance and the BRT affects COLs negatively. The operational forecasting of these weather systems across the nation can be enhanced by the application of the knowledge about the evolution in the characteristics of COL events from this study.