Anthropogenic impacts on convective storms over the South African Highveld

Abstract

Convective events over the South African Highveld are a frequent and often dangerous summer weather phenomenon. Features such as aerosols, land-use, and thermodynamics, which humans can unintentionally modify, change the nature of convective events. Radar data indicates that convective weather has a strong diurnal signal. The Highveld experiences the most severe convective weather in the late afternoon, and November has the highest number convective events. Media reports indicate that rainfall is the most impactful convective weather event over the region followed by hail. The number of reported heavy rainfall events suggests an increase in frequency of these events from the 1980's, while hail reports for the same time have remained fairly consistent. Objective clustering methods indicate that both events are directly linked to characteristic mid- and early summer circulation patterns respectively. During mid-summer, tropical systems displace the westerlies and rainfall is the most significant event. During early summer, the presence of the westerlies at 500 hPa enhances conditional instability and wind shear, and favours hail formation. Simulations show that under high CCN loads, storms are less severe and less hail and precipitation reaches the surface. A high CCN environment results in a persistent stable layer that inhibits convection. A green city scenario simulates the most severe convective storms with the highest persistent CAPE and accumulated hail. Increased latent heat and local moisture could enhance precipitation processes under this scenario. Under low vegetation scenarios, there is a decrease in surface hail and rainfall and events are characteristics of tropical, short-lived, isolated thunderstorms. Trends show that the Highveld is favouring tropical circulation types more frequently. The thermodynamic environment shows signs of increasing instability and decreasing wind shear, characteristic of tropical storms. These observed changes are consistent with current projections of anthropogenic climate change.