Cut-off lows over South Africa and their contribution to the total rainfall of the Eastern Cape Province

Abstract

Cut-Off Lows (COLs) are certainly amongst the most important synoptic-scale rain producing weather systems in South Africa. Rainfall associated with COLs is usually widespread, while about 20% of COLs are associated with heavy rainfall. Both these attributes of rainfall associated with COLs are important to agriculture. Widespread rainfall secures good grazing potential, while heavy rainfall not only contributes largely to maintaining dam levels that are needed for irrigation, but also to flooding and erosion. Agriculture in the Eastern Cape Province of South Africa is predominantly rain-fed, while agricultural activities that depend on irrigation are mostly located along the western coastal belt of the province. Despite of the fact that all COLs do not pass directly over the Eastern Cape Province, most of them appear to have a direct or indirect influence on the rainfall of the province, and in turn, impact on agricultural production and even the economy. In this study, the contribution of COLs to rainfall over the Eastern Cape Province is investigated. In order to achieve this, a climatology of COLs for the period 1979 to 2009 (31 years) was constructed by utilizing the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data to create 6-hourly contour images of geopotential heights and air-temperatures at the 500 hPa pressure level. All COLs that occurred over South Africa bounded by 200S to 400S and 00E to 500E from the day they started (at the formation of a closed low pressure system) until the day they ended (at the disappearance of the closed low pressure system), and that were cold cored, were considered as potential COLs in this study. In addition, low level circulation maps obtained from the South African Weather Service’s (SAWS’s) daily weather bulletins were used to ensure that the defined COLs were indeed extending from the 500 hPa pressure level to the land surface. Daily rainfall totals from 22 well-distributed weather stations over the Eastern Cape Province were used to determine the contribution of COLs to the rainfall over the province. It was found that 64% of COLs that lasted for more than 24-hours over the study domain had an influence on the total rainfall over the Eastern Cape Province. Monthly frequency distribution of COLs reveal that April and May had the highest occurrences, while December and January have the least occurrence. Long-term seasonal frequencies distributions of COLs show the highest occurrence during March-April-May (MAM) with the least occurrences during December-January-February (DJF). Most COLs lasted for 2-4 days over South Africa and the Eastern Cape Province for the study period of 31-years. The contribution of rainfall associated with the occurrence of COLs is found to be approximate 37-38% annually along the coastal areas, while it is less than 10% annually over the interior of the Eastern Cape Province.