Dynamical downscaling of prevailing synoptic-scale winds over the complex terrain of Mariepskop, South Africa

Abstract

Mariepskop (direct translation: “Marieps hill”) forms part of the northernmost edge of the Drakensberg Mountain range in the east of South Africa, and is known for its complex topography associated with meso-scale atmospheric circulation, and therefore its numerous climatic zones. As a result the mountain hosts a high degree of biodiversity. The peak of Mariepskop lies at approximately 1900m Above Mean Sea Level (AMSL), which is higher than the surrounding escarpment to the east bordering the Highveld. Its foothills also extend well into the Lowveld at about 700m AMSL. Mariepskop is therefore ideal for studying airflow exchange between the industrialized Highveld and the Lowveld with its diversity of natural resources. It is also ideal for detecting global warming signals on altitudinal gradients extending from the Lowveld to altitudes above the Highveld escarpment. In this study, long-term National Centre for Atmospheric Research / National Centre for Environmental Prediction (NCAR/NCEP) reanalysis wind data at two atmospheric pressure levels (850hPa and 700hPa), as well as reanalysis near-surface temperature data, were obtained for the Mariepskop region for the austral summer (and winter) seasons. The data was used to force a Computational Fluid Dynamics (CFD) model (also known as STAR-CCM+) across its lateral boundaries with the dominant synoptic flow in order to generate mesoscale simulation output over the complex terrain of Mariepskop. Wind speed and direction modelled results were then correlated to observations measured by three weather stations on Mariepskop. Modelled wind flow results for the summer simulation were also validated against aerial photographs in order to infer whether the model could accurately capture areas with high rainfall, which are related to denser vegetation.