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Rainfall responses to climate change in a convective-permitting model over Western Cape (HighResWC)

Rainfall responses to climate change in a convective-permitting model over Western Cape (HighResWC)

Cities formed where resources such as safe harbours, mining and freshwater are easily accessible. While many cities developed due to ready access to water, population growth, economic development, and competing demands for water from industry and agriculture have increased the demand for water resources and made cities more vulnerable to rainfall variability and change. There is growing evidence that climate change is already impacting water resources globally and climate variability and change is almost certain to continue to change the distribution and quantity of freshwater resources posing serious socio-economic challenges with negative impacts on the way of life. The city of Cape Town experienced a water crisis with an acute phase that started in early 2017. This was considered the worst water shortage in 113 years (Botai el al 2017) and was due to a long-term deficit in rainfall. Such events are likely to occur more frequently in the future due to climate change (Otto et al 2018). While the slow changes in rainfall patterns are detectable, their temporal structure and spatial patterns are complex, which makes understanding their causes and linkages to regional and global variability and change difficult (Wolski et al. 2020). Fundamental dynamics governing changes in this topographically diverse terrain, especially those in response to increases in greenhouse gases is therefore important for better planning.

Currently, bespoke climate information products and services are produced using data from global models (usually 100s km resolution) and regional models (usually 25km to 50km). However, higher resolution modeling that captures local processes such as orographic enhancement of rainfall and convection is important in this complex region. The use of convection permitting modeling (CPM, horizontal grid spacing <4km) has emerged recently (e.g. Prein et al 2015, Coppola et al 2018), which allows us to understand and simulate processes not resolved in global models. The motivation has been fueled by the gap between climate information provided by current state-of-the-art climate models and the local-scale, actionable information required by stakeholders and policy makers (eg., Gutowski et al 2020). The role of local topography in altering the regional response to climate change has been a standing question for 20+ years and we propose using CPM to reduce the associated uncertainty.  Therefore, this study will make the use of high-resolution modelling (~4km) over the Western Cape region in South Africa and to use idealized pseudo-global warming (PGW, Schär et al 1996) experiments to represent a 2-degree global warming level. One advantage of the PGW method is to estimate the effect of changes in temperature on a specific year, which allows construction of easy to communicate storylines. This approach also simplifies the estimate of the climate difference between current and future climate without costly ensemble simulations and to attribute the changes.

The study aims at understanding drivers of mean and extreme rainfall in a region with complex topography (e.g., meso-scale processes, convection-subsidence over and around the sea-plains-mountains-interior plateau system). The changing characteristics of precipitation and related processes under human-induced climate change is of particular importance, and it is also timely.  They are also a priority under the WCRP Grand Challenge on climate extremes (GSQ#3), because they carry both society-relevant and scientific challenges that can be tackled in the coming years.


Contact person:
Izidine Pinto and Piotr Wolski

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