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RESEARCH PRODUCT

Intraseasonal descriptors and rainfall extremes in austral summer over South Africa : Observations and Meso-scale modelling

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Rainfall extremes are of major and increasing importance in semi-arid countries and their variability has strong implications for water resource and climate impacts on the local societies and environment. Here, we examine intraseasonal descriptors (ISDs) and wet extremes in austral summer rainfall (November−February) over South Africa (SA). Using daily observations from 225 rain gauges and ERA5 reanalysis between 1979 and 2015, we propose a novel typology of wet extreme events based on their spatial fraction, thus differentiating large- and small-scale extremes. Long-term variability of both types of extreme rainfall events is then extensively discussed in the context of ISDs. Following the definition of a novel typology of rainfall extremes, disentangling large- and small-scale events we further examine the relationship between these two types of rainfall extremes and different modes of climate variability at different timescales. At low-frequencies, rainfall extremes are assessed at interannual (IV: 2−8 years) and quasi-decadal (QDV: 8−13 years) timescales, which are primarily associated with the El Niño Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO), respectively. At sub-seasonal timescales, the typology of rainfall extremes is analysed depending on the synoptic configurations, as inferred by seven convective regimes including Tropical Temperate Troughs (TTTs: 3–7 days), and the intraseasonal variability associated with the Madden-Julien Oscillation (MJO: 30–60 days). To identify potentially high-impact rainfall spells, we introduce duration into the definition of extreme rainfall typology. Large-scale longest-lived events are then considered as case studies of potentially high-impact rainfall spells and are selected for meso-scale modelling. To that end, we use state-of-the-art Weather Research and Forecasting (WRF version 4.2.1) model which is widely used for both atmospheric research and operational forecasting applications. ERA5 reanalysis is first used to drive WRF simulations with several experimental setups on different case studies to obtain the finest WRF configuration. The optimal experimental design is then used to drive WRF simulations using coarse resolution global forecast i.e., Re-forecast version 2 (RF2) developed by National Oceanic and Atmospheric Administration/Earth System Research Laboratory (NOAA/ESLR) having a resolution of 25 kms. Here, we attempt to investigate if the properties of such major events can be improved using convection-permitting downscaling of these global grids in terms on intensity and location.The results demonstrate that using 7% of spatial fraction simultaneously exceeding the local threshold of the 90th percentile produces remarkable results in characterizing rainfall extremes into large- and small-scale extremes.