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Improving rainfall simulations over the Drakensberg on weak-synoptic days

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Abstract

Reanalysis is well known for reliable reproduction of weather and climate. However, there is a dearth of information on the reliability of reanalysis in reproducing rainfall distribution over mountains during weak-synoptic conditions, when the deep convections are mostly triggered by small-scale circulations that are not usually explicitly resolved in reanalysis models. The present study examines the capability of the Climate Forecast System Reanalysis (CFRS) to represent rainfall patterns over the Drakensberg mountains on weak-synoptic days (WSDs) and investigates the extent to which the dynamic downscaling of the CFRS simulation with the Weather Research and Forecasting (WRF) model can improve the quality of the simulated rainfall fields. The self-organizing map (SOM) analysis was used to identify the major rainfall patterns over the Drakensberg and the WRF model was used to downscale the atmospheric condition associated with each pattern. The results reveal that the WSDs are induced by the presence or ridging of high-pressure system over the mountains. In agreement with observation, CFSR simulation showed that wet WSDs contributes up to 5% of the annual rainfall over the Drakensberg, but the simulation underestimates the frequency of the wet WSDs. It also underestimates the magnitude of the major WSD rainfall patterns over the Drakensberg: (1) heavy precipitation over the whole Drakensberg domain; (2) weak precipitation over the domain; (3) moderate precipitation over the southwest part; and (4) moderate precipitation over the north-eastern part of the domain. The dynamic downscaling of CFSR simulation with WRF improves the quality of CFSR rainfall products, although the magnitude of the improvement does not increase linearly with higher horizontal grid resolution in the WRF simulations. The results of the study have application in improving rainfall prediction over the Drakensberg.

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Acknowledgements

This work is a part of the first author’s PhD thesis carried out at the University of Cape Town. Computational resources are provided by the South African Center for High-Performance Computing, the University of Cape Town High-Performance Computing, and the Climate Systems Analysis Group. The datasets used for the study are available on request.

Funding

This work was supported by the University of Cape Town Faculty of Science Ph.D. Fellowship, the International Students and Refugees Scholarship of the Postgraduate Funding Office, South African Water Research Commission, South African National Research Foundation.

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  1. Climate System Analysis Group, Department of Environmental and Geographical Science, University of Cape Town, Cape Town, South Africa

    Ridick Roland Takong & Babatunde J. Abiodun

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  1. Ridick Roland Takong
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Correspondence to Babatunde J. Abiodun.

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Takong, R.R., Abiodun, B.J. Improving rainfall simulations over the Drakensberg on weak-synoptic days. Model. Earth Syst. Environ. 9, 3503–3516 (2023). https://doi.org/10.1007/s40808-022-01686-y

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