The Annual Rainfall Cycle Over the Central Interior of South Africa

Abstract

Satellite imagery and surface rainfall records have been used to develop statistics of Orange Free State rainfall for a number of rain-bearing synoptic systems. Two major categories of system were included in the analysis. Tropical systems are distinguished by a connecting cloud link to the tropical cloud masses. This link is absent for temperate systems. Tropical systems accounted for about 60% of the rainfall over the two areas of the Orange Free State during the period 1967 to 1977. In January this contribution exceeded 80%. Cloud bands linking the tropical and temperate circulations were the major individual contributors. The December rainfall hiatus appears to have its origins in the circulation over tropical Africa. Harmonic analysis reveals that cloud band rainfall was primarily responsible for the annual rainfall cycle over the Orange Free State. Temperate systems made an important contribution to the late season rainfall maximum over the western Orange Free State. Mean atmospheric circulation patterns during wet and dry spells may be interpreted in terms of changes in the frequency of passage of the cloud bands across South Africa.