Aerosol impacts on squall lines over the South African highveld

Abstract

South Africa is highly vulnerable to the occurrences of squall lines, particularly during the austral summer season, posing significant threats to both socio-economic activities and safety of the general public. Despite their known socio-economic impacts, squall lines are not extensively studied in South Africa. This study therefore investigates the meteorological characteristics of squall line events and assesses the performance of the Weather Research and Forecasting (WRF) model in simulating their key features. The WRF coupled with Chemistry (WRF-Chem) is later applied to simulate impacts of aerosols on rainfall for selected squall line events with a focus on the Highveld region. Using data from the SAWS such as rainfall and weather radar imagery, the European Centre for Medium-Range Weather Forecasts ERA5 reanalyses, and Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth (AOD) 550 nm observations, selected high-impact squall lines were examined to identify their synoptic and mesoscale drivers. Results indicate that these squall line events are not confined to the austral summer but can also occur in other seasons as was the case for the 30 May 2023 event which was observed in autumn. From a diurnal perspective, the events initiate in the late afternoon and often persist into the evening or early morning hours. The analysis revealed that all studied events were associated with surface troughs interacting with mid-tropospheric westerly waves, while an autumn case developed in conjunction with a cut-off low system. Of the selected events, the early summer events exhibit a strong vertically development, occurring in highly unstable environments with narrow trailing stratiform regions, whereas convection of the late summer and autumn events were not as intense and developed under lower Convective Available Potential Energy (CAPE) but with strong wind shear. The WRF model, both without and with chemistry (WRF-Chem), was used to evaluate the ability of numerical simulations to reproduce features of these squall lines and to examine the influence of aerosols on rainfall patterns. All the current simulations were performed on the Council for Scientific and Industrial Research (CSIR), Centre for High performance Computing (CHPC) dell cluster. WRF effectively captured synoptic-scale circulations, such as large-scale pressure systems, surface wind patterns, and moisture transport. The models are also able to reproduce thermodynamic variables such as CAPE, K-index, totals total index and 2m-dewpoint. With regards to rainfall, WRF overestimated large-scale stratiform rainfall while underestimating localised convective rainfall. The inclusion of aerosols in WRF-Chem improved the simulation of rainfall timing though challenges remained in reproducing storm intensity and precise spatial locations. These findings highlight the importance of both synoptic and mesoscale processes, as well as aerosol interactions, in understanding the behavior of squall lines. This research study provides vii new insights into the dynamics of high-impact thunderstorms over South Africa which can inform strategies for improving forecast accuracy and disaster preparedness