Dust storms over South Africa : synoptic weather patterns and evolution of meteorological variables associated with significant dust storm events

Abstract

Dust storms (DS) are common in semi-arid and arid regions, with the majority of the natural dust source regions located in the Northern Hemisphere. Although there are some significant source regions in the Southern Hemisphere, South Africa is not a major contributor to dust emissions. Consequently, very little research has been conducted on DS in South Africa. However, there has been an increase in anthropogenic source regions linked to agricultural activities over the central parts of South Africa. With the expected increase in DS frequency over South Africa, it is therefore important to understand the meteorological factors associated with these DS. The study aims to first identify the main synoptic weather systems that produce DS over the country, separating the findings according to DS along the coast and DS over the interior of the country. Secondly, the study aims to identify how meteorological variables evolve before, during, and after a DS. Ten DS are selected for analysis, five from coastal regions and five from the interior. The weather systems associated with coastal DS are similar for all five DS: a surface trough extending along the west or adjacent interior. This often results in offshore flow along the west coast, also known as berg wind conditions. For interior DS, three are a result of thunderstorm activity, with one DS associated with a cut-off low. Two DS were triggered by cold fronts, one being a prefrontal dust storm and the other a post-frontal dust storm. The meteorological variables analysed are temperature, focussing on the diurnal temperature range (ΔT) and the maximum temperature (Tmax), wind speed and direction, relative humidity (RH), dew point temperature (Td), rainfall, and soil moisture. All coastal DS had an easterly component to the wind direction, which is caused by the location weather systems that caused the DS. The DS that lasted the longest, did not have the lowest RH, Td, or temperature, but had the strongest wind gusts and strongest high-pressure system. The results indicate how DS are associated with drier atmospheric conditions, which are depicted by low RH and Td. This is observed with synoptic scale-induced DS, where the RH and Td gradually decrease, being the lowest on the day of the DS. However, the opposite was observed for mesoscale-induced DS. The presence of thunderstorms associated with the mesoscale-induced DS meant that there was enough moisture in the atmosphere for their development, indicated by higher Td and RH values. The results also show that the behaviour of the meteorological variables is influenced by several factors such as whether it is a synoptic-scale or mesoscale weather system, the duration, and strength of the weather system associated with the DS, the season in which the DS occurred, as well as the location or source region of the DS. Overall, this study is a base study for DS over South Africa, to contribute to the research necessary for a decision tree for forecasting DS in the future.