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.